The Various Contrivances by which Orchids are Fertilised by Insects/Chapter 7

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Catasetidæ, the most remarkable of all Orchids—The mechanism by which the pollinia of Catasetum are ejected to a distance and are transported by insects—Sensitiveness of the horns of the rostellum—Extraordinary difference in the male, female, and hermaphrodite forms of Catasetum tridentatum—Mormodes ignea, curious structure of the flowers; ejection of the pollinia—Mormodes luxata—Cycnoches ventricosum, manner of fertilisation.

I have reserved for separate description one sub-family of the Vandeæ, namely, the Catasetidæ, which must, I think, be considered as the most remarkable of all Orchids.

I will begin with Catasetum. A brief inspection of the flower shows that here, as with most other Orchids, some mechanical aid is requisite to remove the pollen-masses from their cells, and to carry them to the stigmatic surface. We shall, moreover, presently see that Catasetum is exclusively a male form; so that the pollen-masses must be transported to the female plant, in order that seed should be produced. The pollinium is furnished with a viscid disc of huge size; but this, instead of being placed in a position likely to touch and adhere to an insect visiting the flower, is turned inwards and lies close to the upper and back surface of a chamber, which must be called the stigmatic chamber, though functionless as a stigma. There is nothing in this chamber to attract insects; and even if they did enter it, the viscid surface of the disc could not possibly come into contact with them.

How then does Nature act? She has endowed these plants with, what must be called for want of a better term, sensitiveness, and with the remarkable power of forcibly ejecting their pollinia even to a considerable distance. Hence, when certain definite points of the flower are touched by an insect, the pollinia are shot forth like an arrow, not barbed however, but having a blunt and excessively adhesive point. The insect, disturbed by so sharp a blow, or after having eaten its fill, flies sooner or later away to a female plant, and, whilst standing in the same position as before, the pollen-bearing end of the arrow is inserted into the stigmatic cavity, and a mass of pollen is left on its viscid surface. Thus, and thus alone, can the five species of Catasetum which I have examined be fertilised.

In many Orchideæ, as in Listera, Spiranthes, and Orchis, the surface of the rostellum is so far sensitive, that, when touched or when exposed to the vapour of chloroform, it ruptures in certain defined lines. So it is in the tribe of the Catasetidæ, but with this remarkable difference, that in Catasetum the rostellum is prolonged into two curved tapering horns, or, as I shall call them, antennæ, which stand over the labellum where insects alight. If these are touched even very lightly, they convey some stimulus to the membrane which surrounds and connects the disc of the pollinium with the adjoining surface, causing it instantly to rupture; and as soon as this happens the disc is suddenly set free. We have also seen in several Vandeæ that the pedicels of the pollinia are fastened flat down in a state of tension, and are highly elastic, so that, when freed, they immediately spring up, apparently for the sake of detaching the pollen-masses from the anther-cells. In the genus Catasetum, on the other hand, the pedicels are fastened down in a curved position; and when freed by the rupture of the attached edges of the disc, they straighten themselves with such force, that not only do they drag the balls of pollen together with the anther-cells from their places of attachment, but the whole pollinium is jerked forward, over and beyond the tips of the so-called antennæ, to the distance sometimes of two or three feet. Thus, as throughout nature, pre-existing structures and capacities are utilised for new purposes.

Catasetum saccatum.[1]—I will first describe the male forms, belonging to five species, which are included under the generic name of Catasetum. The general appearance of the present species is represented in the following woodcut, fig. 28. A side view of the flower, with all the petals and sepals excepting the labellum cut off, is shown by B; and A gives a front view of the column. The upper sepal and two upper petals surround and protect the column; the two lower sepals project out at right angles. The flower stands more or less inclined to either side, but with the labellum downwards, as represented in the drawing. The dull coppery and orange-spotted tints,—the yawning cavity in the great fringed labellum,—the one antenna projecting with the other hanging down—give to these flowers a strange, lurid, and almost reptilian appearance.

In front of the column, in the middle, the deep stigmatic chamber (fig. 28, A, s), may be seen; but this is best shown in the section (fig. 29, C, s), in which all the parts are a little separated from each other, in order that the mechanism may be intelligible. In the middle of the roof of the stigmatic chamber, far back (d, in A, fig. 28), the upturned anterior edge of the viscid disc can just be seen. The upper membranous surface of the disc, before it is ruptured, is continuous with the fringed bases of the two antennæ between which it lies. The rostellum projects over the disc and stigmatic chamber (see section C, fig. 29), and is prolonged on each side so as to form the two antennæ; the middle part is covered by the ribbon-like, pedicel (ped.) of the pollinium. The lower end of the pedicel is attached to the disc, and the upper end to the two pollen-masses (p) within the anther-cell. The pedicel in its natural position is held much bowed round the protuberant rostellum; when freed it forcibly straightens itself, and at the same time its lateral edges curl inwards. At an early period of growth, it is continuous with the rostellum, but subsequently becomes separated from it by the solution of a layer of cells.

The pollinium when set free and after it has straightened itself, is represented at D, fig. 29. Its under surface, which lies in contact with the rostellum, is shown at E, with the lateral edges of the pedicel now curled inwards. In this latter view, the clefts in the under sides of the two pollen-masses are shown. Within these clefts, near their bases, a layer of strong extensible tissue is attached, forming the caudicles, by which the pollen-masses are united to the pedicel. The lower end of the pedicel is joined to the disc by a flexible hinge, which occurs in no other genus, so that the pedicel can play backwards and forwards, as far as the upturned end (fig. D) of the disc permits. The disc is large and thick ; it consists of a strong upper

Fig. 28.

Darwin - The various contrivances by which orchids are fertilized by insects (1877) - Fig 28.png

Catasetum saccatum.


Fig. 29.

Darwin - The various contrivances by which orchids are fertilized by insects (1877) - Fig 29.png

Catasetum saccatum.

an.a. anther.
an. antennæ of the rostellum.
an.d. disc of pollinium.
an.f. filament of anther.
an.g. germen or ovarium.
an.l. labellum.
an.p. pollen-masses.
pd. or ped. pedicel of pollinium.
an.s. stigmatic chamber.
A. Front view of column.
B. Side view of flower, with all the sepals and petals removed except the labellum.
C. Diagrammatic section through the column, with all the parts a little separated.
D. Pollinium, upper surface.
E. Pollinium, lower surface, which before removal lies in close contact with the rostellum.

membrane, to which the pedicel is united, with an inferior cushion of great thickness, of pulpy, flocculent, and viscid matter. The posterior margin is much the most viscid part, and this necessarily first strikes any object when the pollinium is ejected. The viscid matter soon sets hard. The whole surface of the disc is kept damp before ejection, by resting close against the roof of the stigmatic chamber; but in the section (fig. C) it is represented, like the other parts, a little separated from the roof.

The connective membrane of the anther (a in all the figures) is produced into a spike, which adheres loosely to the pointed end of the column; this pointed end (f, fig. C) is homologically the filament of the anther.

The anther has this peculiar shape apparently for the sake of leverage, so that it may be easily torn off by a pull at its lower end, when the pollinium is jerked out by the elasticity of the pedicel.

The labellum stands at right angles to the column, or hangs a little downwards; its lateral and basal lobes are turned under the middle portion, so that an insect can stand only in front of the column. In the middle of the labellum there is a deep cavity, bordered by crests. This cavity does not secrete nectar, but its walls are thick and fleshy, with a slightly sweet nutritious taste; and it will presently be shown that they are gnawed by insects. The extremity of the left-hand antenna stands immediately over the cavity, and would infalliby be touched by an insect visiting this part of the labellum for any purpose.

The antennæ are the most singular organs of the flower, and occur in no other genus. They form rigid, curved horns, tapering to a point. They consist of a narrow ribbon of membrane, with the edges curled inwards so as to touch; each horn therefore is tubular, with a slit down one side, like an adder's fang. They are composed of numerous, much elongated, generally hexagonal cells, pointed at both ends; and these cells (like those in most of the other tissues of the flower) have nuclei with nucleoli. The antennæ are prolongations of the sides of the anterior face of the rostellum. As the viscid disc is continuous with a little fringe of membrane on each side, and as this fringe is continuous with the bases of the antennæ, these latter organs are put into direct connection with the disc. The pedicel of the pollinium passes, as already stated, between the bases of the two antennæ. The antennæ are not free for their whole length; but their exterior edges are firmly united to and blend for a considerable space with the margins of the stigmatic chamber.

In all the flowers which I examined, taken from three plants, the two antennæ which are alike in structure occupied the same relative position. The extreme part of the left-hand antenna bends upwards (see B, fig. 28, in which the position is shown plainer than in A), and at the same time a little inwards, so that its tip is medial and guards the entrance into the cavity of the labellum. The right-hand antenna hangs down, with its tip turned a little outwards; and as we shall immediately see, is almost paralysed, so as to be functionless.

Now for the action of the parts. When the left-hand antenna of this species (or either of the antennæ in three of the following species) is touched, the edges of the upper membrane of the disc, which are continuously united with the surrounding surface, instantly rupture, and the disc is set free. The highly elastic pedicel then instantly flirts the heavy disc out of the stigmatic chamber with such force, that the whole pollinium is ejected, bringing away with it the two balls of pollen, and tearing the loosely attached spike-like anther from the top of the column. The pollinium is always ejected with its viscid disc foremost. I imitated the action with a minute strip of whalebone, slightly weighted at one end to represent the disc; this was then bent half round a cylindrical object, the upper end being at the same time gently held by the smooth head of a pin, to represent the retarding action of the anther, the lower end was then suddenly set free, and the whalebone was pitched forward, like the pollinium of the Catasetum, with the weighted end foremost.

That the disc is first jerked out of the stigmatic chamber, I ascertained by pressing the middle of the pedicel; and when I touched the antenna the disc instantly sprung forth, but, owing to the pressure on the pedicel, the pollinium was not dragged out of the anther-cell. Besides the spring from the straightening of the pedicel, elasticity in a transverse direction comes into play: if a quill be split lengthways, and the half be forced longitudinally on a too thick pencil, immediately the pressure is removed the quill jumps off; and an analogous action takes place with the pedicel of the pollinium, owing to the sudden inward curling of its edges, when set free. These combined forces suffice to eject the pollinium with considerable force to the distance of two or three feet. Several persons have told me that, when touching the flowers of this genus in their hothouses, the pollinia have struck their faces. I touched the antennæ of C. callosum whilst holding the flower at about a yard's distance from a window, and the pollinium hit the pane of glass, and stuck by its adhesive disc, to the smooth vertical surface.

The following observations on the nature of the excitement which causes the disc to separate from the surrounding parts, include some made on the following species. Several flowers were sent me by post and by the railroad, and must have been much jarred, but they had not exploded. I let two flowers fall from a height of two or three inches on the table, but the pollinia were not ejected. I cut off with a crash with a pair of scissors the thick labellum and ovarium close beneath the flower; but this violence produced no effect. Nor did deep pricks in various parts of the column, even within the stigmatic chamber. A blow, sufficiently hard to knock off the anther, causes the ejection of the pollinium, as occurred to me once by accident. Twice I pressed rather hard on the pedicel, and consequently on the underlying rostellum, without any effect. Whilst pressing on the pedicel, I gently removed the anther, and then the pollen-bearing end of the pollinium sprang up from its elasticity, and this movement caused the disc to separate. M. Ménière,[2] however, states that the anther-case sometimes detaches itself, or can be gently detached, without the disc separating; and that then the upper end of the pedicel, bearing the pollen-masses, swings downwards in front of the stigmatic chamber.

After trials made on fifteen flowers of three species, I find that no moderate degree of violence on any part of the flower, except on the antennæ, produces any effect. But when the left-hand antenna of C. saccatum, or either antenna of the three following species, is touched, the pollinium is instantly ejected. The extreme tip and the whole length of the antennæ are sensitive. In one specimen of C. tridentatum a touch from a bristle sufficed; in five specimens of C. saccatum a gentle touch from a fine needle was necessary; but in four other specimens a slight blow was requisite. In C. tridentatum a stream of air and of cold water from a small pipe did not suffice; nor in any case did a touch from a human hair; so that the antennæ are less sensitive than the rostellum of Listera. Such extreme sensitiveness would indeed have been useless to the plant, for, as is now known, the flowers are visited by powerful insects.

That the disc does not separate owing to the simple mechanical movement of the antennæ is certain; for they adhere firmly for a considerable space to the sides of the stigmatic chamber, and are thus immovably fixed near their bases. If a vibration is conveyed along them, it must be of some special nature, for ordinary jars of manifold greater strength do not excite the act of rupture. The flowers in some cases, when they first arrived, were not sensitive, but after the cut-off spikes had stood for a day or two in water they became sensitive. Whether this was owing to fuller maturity or to the absorption of water, I know not. Two flowers of C. callosum, which were completely torpid, were immersed in tepid water for an hour; and then the antennæ became highly sensitive; this indicates either that the cellular tissue of the antennæ must be turgid in order to receive and convey the effects of a touch, or, as is more probable, heat increases their sensitiveness. Two other flowers placed in hot water, but not so hot as to scald my fingers, spontaneously ejected their pollinia. A plant of C. tridentatwm had been kept for some days in a rather cool house, and the antennæ were consequently in a torpid condition; a flower was cut off and placed in water at a temperature of 100° F. (37.7° C.), and no effect was immediately produced; but when it was looked at after an interval of 1h. 30m. the pollinium was found ejected. Another flower was placed in water at 90° F. (32.2° C.), and after 25m. the pollinium was found ejected: two other flowers left for 20m. in water at 87° F. (30.5° C.) did not explode, though they were afterwards proved to be sensitive to a slight touch. Lastly, four flowers were placed in water at 83° F. (28.3° C.); two of these did not eject their pollinia in 45m., and were then found to be sensitive; whereas the other two, when looked at after 1h. 15m., had spontaneously ejected their pollinia. These cases show that immersion in water raised to a temperature only a little higher than that to which the plant had been exposed, causes the membrane by which the discs are attached to rupture. A thin stream of almost boiling water was allowed to fall through a fine pipe on the antennæ of some flowers on the above plant; these were softened and killed but the pollinia were not ejected. Nor did sulphuric acid, dropped on the tips of the antennæ, cause any action; though their upper parts which had not been injured by the acid were afterwards found to be sensitive to a touch. In these two latter cases, I presume that the shock was so sudden and violent that the tissue was instantly killed. Considering the above several facts, we may infer that it must be some molecular change which is conveyed along the antennæ, causing the membrane round the discs to rupture. In C. tridentatum the antennæ were one inch and a tenth in length, and a gentle touch from a bristle on the extreme tip was conveyed, as far as I could perceive, instantaneously throughout this length. I measured several cells in the tissue composing the antennæ of this species, and on a rough average it appeared that the stimulus must travel through no less than from seventy to eighty cells.

We may, at least, safely conclude that the antennæ, which are characteristic of the genus Catasetum, are specially adapted to receive and convey the effects of a touch to the disc of the pollinium. This causes the membrane to rupture, and the pollinium is then ejected by the elasticity of its pedicel. If we required further proof, nature affords it in the case of the so-called genus Monachanthus, which, as we shall presently see, is the female of Catasetum tridentatum, and it does not possess pollinia which can be ejected, and the antennæ are here entirely absent.

I have stated that in C. saccatum the right-hand antenna invariably hangs down, with the tip turned slightly outwards, and that it is almost paralysed. I ground my belief on five trials, in which I violently hit, bent, and pricked this antenna, and this produced no effect; but when immediately afterwards the left-hand antenna was touched with much less force, the pollinium was shot forth. In a sixth case a forcible blow on the right-hand antenna did cause the act of ejection, so that it is not completely paralysed. As this antenna does not guard the labellum, which in all Orchids is the part attractive, that is to insects, its sensitiveness would be useless.

From the large size of the flower, more especially of the viscid disc, and from its wonderful power of adhesion, I formerly inferred that the flowers were visited by large insects, and this is now known to be the case. The viscid matter sticks so firmly after it has set hard, and the pedicel is so strong (though very thin and only one-twentieth of an inch in breadth at the hinge), that to my surprise a pollinium attached to an object supported for a few seconds a weight of 1262 grains, or nearly three ounces; and it supported for a considerable time a slightly less weight. When the pollinium is shot forth, the large spike-like anther is generally carried with it. If the disc strikes a flat surface like a table, the momentum from the weight of the anther often carries the pollen-bearing end beyond the disc, and the pollinium is thus affixed in a wrong direction for the fertilisation of another flower, supposing it to have been attached to an insect's body. The flight of the pollinium is often rather crooked.[3] But it must not be forgotten that under nature the ejection is caused by the antennæ being touched by a large insect standing on the labellum, which will thus have its head and thorax placed near to the anther. A rounded object thus held is always accurately struck in the middle, and when removed with the pollinium adhering to it, the weight of the anther depresses the hinge of the pollinium; and in this position the anther-case readily drops off, leaving the balls of pollen free, in a proper position for fertilising the female flower. The utility of so forcible an ejection no doubt is to drive the soft and viscid cushion of the disc against the hairy thorax of the large hymenopterous insects which frequent the flowers. When once attached to an insect, assuredly no force which the insect could exert would remove the disc and pedicel; but the caudicles are ruptured without much difficulty, and thus the balls of pollen might readily be left on the adhesive stigma of the female flower.

Catasetum callosum.—The flowers of this species[4] are smaller than those of the last, but resemble them in most respects. The edge of the labellum is covered with papillæ; the cavity in the middle is small, and behind it there is an elongated anvil-like projection,—facts which I mention from the resemblance in some of these points between the labellum of this species and that of Myanthus barbatus, the hermaphrodite form of Catasetum tridentatum, presently to be described. When either antenna is touched, the pollinium is ejected with much force. The yellow-coloured pedicel is much bowed, and is joined by a hinge to the extremely viscid disc. The two antennæ stand symmetrically on each side of the anvil-like projection, with their tips lying within the small cavity of the labellum. The walls of this cavity have a pleasant nutritious taste. The antennæ are remarkable, from their whole surface being roughened with papillæ. The plant is a male, and the female form is at present unknown.

Catasetum tabulare.—This species belongs to the same type as C. saccatum, but differs greatly from it in appearance. The central portion of the labellum consists of a narrow, elongated, table-like projection, of an almost white colour and formed of a thick mass of succulent tissue, having a sweetish taste. Towards the base of the labellum there is a large cavity, which externally resembles the nectary of an ordinary flower, but apparently never contains nectar. The pointed extremity of the left-hand antenna lies within this cavity, and would infallibly be touched by an insect gnawing the bilobed and basal end of the medial projection of the labellum. The right-hand antenna is turned inwards, with the extreme part bent at right angles and pressed against the column; therefore I do not doubt that it is paralysed as in C. saccatum; but the flowers examined by me had lost almost all their sensitiveness.

Catasetum planiceps (?).—This species does not differ much from the following one, so I will describe it briefly. The green and spotted labellum stands on the upper side of the flower; it is jar-shaped, with a small orifice. The two elongated and roughened antennæ lie coiled up some little way apart and parallel to one another, within the labellum. They are both sensitive to a touch.

Catasetum tridentatum.—The general appearance of this species, which is very different from that of C. saccatum, callosum and tabulare, is represented in fig. 30, with a sepal on each side cut off.

The flower stands with the labellum uppermost, that is, in a reversed position compared with most Orchids. The labellum is helmet-shaped, its distal portion being reduced to three small points. It cannot hold nectar from its position; but the walls are thick, and have, as in the other species, a pleasant nutritious taste. The stigmatic chamber, though functionless as a stigma, is of large size. The summit of the column, and the spike-like anther, are not so much elongated as in C. saccatum. In other respects there is no important difference. The antennæ are of greater length; their tips for about one-twentieth of their length are roughened by cells produced into papillæ.

Fig. 30.

Darwin - The various contrivances by which orchids are fertilized by insects (1877) - Fig 30.png

Catasetum tridentatum.

pd.a. anther.
pd. pedicel of pollinium. antennæ.
pd.l. labellum.
A. Side view of flower in its natural position, with two of the sepals cut off.
B. Front view of column, in position reverse of fig. A.

The pedicel of the pollinium is articulated as before by a hinge to the disc; it can move freely only in one direction owing to one end of the disc being upturned, and this restricted power of movement apparently comes into play when the pollinium is carried by an insect to the female flower. The disc is, as in the other species, of large size, and the end which when ejected first strikes any object, is much more viscid than the rest of the surface. This latter surface is drenched with a milky fluid, which, when exposed to the air, rapidly turns brown, and sets into a cheesy consistence. The upper surface of the disc consists of strong membrane formed of polygonal cells, resting on and adhering to a thick cushion, formed of irregular rounded balls of brown matter, separated from each other and embedded in a transparent, structureless, highly elastic substance. This cushion towards the posterior end of the disc graduates into viscid matter, which when consolidated is brown, translucent, and homogeneous. Altogether the disc of Catasetum presents a much more complex structure than in the other Vandeæ.

I need not further describe the present species, except as to the position of the antennæ. They occupied exactly the same position in all the many flowers which were examined. Both lie curled within the helmet-like labellum; the left-hand one stands higher up, with its inwardly bowed extremity in the middle; the right-hand antenna lies lower down and crosses the whole base of the labellum, with the tip just projecting beyond the left margin of the base of the column. Both are sensitive, but apparently the one which is coiled within the middle of the labellum is the more sensitive of the two. From the position of the petals and sepals, an insect visiting the flower would almost certainly alight on the crest of the labellum; and it could hardly gnaw any part of the great cavity without touching one of the two antennæ, for the left-hand one guards the upper part, and the right-hand one the lower part. When either of these is touched the pollinium is ejected and the disc will strike the head or thorax of the insect.

The position of the antennæ in this Catasetum may be compared with that of a man with his left arm raised and bent so that his hand stands in front of his chest, and with his right arm crossing his body lower down so that the fingers project just beyond his left side. In Catasetum callosum both arms are held lower down and are extended symmetrically. In C. saccatum the left arm is bowed and held in front, as in C. tridentatum, but rather lower down; whilst the right arm hangs downwards paralysed, with the hand turned a little outwards. In every case notice will be given in an admirable manner, when an insect visits the labellum, and the time has arrived for the ejection of the pollinium, so that it may be transported to the female plant.

Catasetum tridentatum is interesting under another point of view. Botanists were astonished when Sir R. Schomburgk[5] stated that he had seen three forms, believed to constitute three distinct genera, namely, Catasetum tridentatum, Monachanthus viridis, and Myanthus barbatus, all growing on the same plant. Lindley remarked[6] that "such cases shake to the foundation all our ideas of the stability of genera and species." Sir R. Schomburgk affirms that he has seen hundreds of plants of C. tridentatum in Essequibo without ever finding one specimen with seeds;[7] whereas he was surprised at the gigantic seed-vessels of the Monachanthus; and he correctly remarks that "here we have traces of sexual difference in Orchideous flowers." Dr. Crüger also informs me that in Trinidad he never saw capsules naturally produced by the flowers of this Catasetum;[8] nor when they were fertilised by him with their own pollen, as was done repeatedly. On the other hand, when he fertilised the flowers of the Monachanthus viridis with pollen from the Catasetum, the operation never failed. The Monachanthus also commonly produces fruit in a state of nature.

From what I had myself observed, I was led to examine carefully the female organs of C. tridentatum, callosum, and saccatum. In no case was the stigmatic surface viscid, as it is in all other Orchids (except as we shall hereafter see in Cypripedium), and as is indispensable for securing the pollen-masses by the rupture of the caudicles. I carefully looked to this point both in young and old flowers of C. tridentatum. When the surface of the stigmatic chamber and of the stigmatic canal of the above-named three species is scraped off, after having been kept in spirits, it is found to be composed of utriculi (including nuclei of the proper shape), but not nearly so numerous as with ordinary Orchids. The utriculi cohere more together and are more transparent; I examined for comparison those of many kinds of Orchids which had been kept in spirits, and in all found them much less transparent. In C. tridentatum, the ovarium is shorter, much less deeply furrowed, narrower at the base, and internally more solid than in Monachanthus. Again, in all three species of Catasetum the ovule-bearing cords are short; and the ovules present a considerably different appearance, in being thinner, more transparent, and less pulpy than in the numerous other Orchids examined for the sake of comparison. Perhaps these bodies hardly ought to be called ovules, although they correspond closely in general appearance and position with true ovules, for I was unable in any case to make out the opening of the testa and the included nucleus; nor were the ovules ever inverted.

From these several facts, namely,—the shortness, smoothness, and narrowness of the ovarium, the shortness of the ovule-bearing cords, the state of the ovules themselves, the stigmatic surface not being viscid, the transparent condition of the utriculi,—and from neither Sir R. Schomburgk nor Dr. Crüger having ever seen C. tridentatum producing seed in its native home, or when artificially fertilised, we may confidently look at this species, as well as the other species of Catasetum, as male plants.

With respect to Monachanthus viridis, and Myanthus barbatus, the President of the Linnean Society has kindly permitted me to examine the spike bearing these two so-called genera, preserved in spirits, which was sent home by Sir E. Schomburgk. The flower of the Monachanthus (A, fig. 31) resembles pretty closely in external appearance that of Catasetum tridentatum (fig. 30). The labellum, which holds the same relative position to the other parts, is not nearly so deep, especially on the sides, and its edge is crenated. The other petals and sepals are all reflexed, and are not so much spotted as in the Catasetum. The bract at the base of the ovarium is much larger. The whole column,

Fig. 31.

Darwin - The various contrivances by which orchids are fertilized by insects (1877) - Fig 31.png

B. Myanthus barbatus.

sep.a. anther. antennæ.
sep.l. labellum.
sep.p. pollen-mass, rudimentary.
sep.s. stigmatic cleft.
sep. two lower sepals.

A. Monachanthus viridis.

A. Side view of Monachanthus viridis in its natural position. (The shading in both drawings has been added from Mr. Reiss' drawing in the 'Linnean Transactions.')
B. Side view of Myanthus barbatus in its natural position.

especially the filament and the spike-like anther, are much shorter; and the rostellum is much less protuberant. The antennæ are entirely absent, and the pollen-masses are rudimentary. These are interesting facts, from corroborating the view taken of the function of the antennæ; for as there are no pollinia to eject, an organ adapted to convey the stimulus from the touch of an insect to the rostellum would be useless. I could find no trace of a viscid disc or pedicel, and no doubt they had been lost; for Dr. Crüger says[9] that "the anther of the female flower drops off immediately after the opening of the same, i. e. before the flower has reached perfection as regards colour, size, and smell. The disc does not cohere, or very slightly, to the pollen-masses, but drops off about the same time, with the anther;" leaving behind them the rudimentary pollen-masses.

Instead of a large stigmatic chamber, there is a narrow transverse cleft close beneath the small anther. I was able to insert one of the pollen-masses of the male Catasetum into this cleft, which from having been kept in spirits was lined with coagulated beads of viscid matter, and with utriculi. The utriculi, differently from those in Catasetum, were charged (after having been kept in spirits) with brown matter. The ovarium is longer, thicker near the base, and more plainly furrowed than in Catasetum; the ovule-bearing cords are also much longer, and the ovules more opaque and pulpy, as in all common Orchids. I believe that I saw the opening at the partially inverted end of the testa, with a large projecting nucleus; but as the specimens had been kept many years in spirits and were somewhat altered, I dare not speak positively. From these facts alone it is almost certain that Monachanthus is a female plant; and as already stated. Sir R. Schomburgk and Dr. Crüger have both seen it seeding abundantly. Altogether the flower differs in a most remarkable manner from that of the male Catasetum tridentatum, and it is no wonder that the two plants were formerly ranked as distinct genera.

The pollen-masses offer so curious and good an illustration of a structure in a rudimentary condition, that they are worth description; but I must first recur to the perfect pollen-masses of the male Catasetum. These may be seen at D and E, fig. 29, attached to the pedicel: they consist of a large sheet of cemented or waxy pollen-grains, folded over so as to form a sack, with an open slit along the lower surface, within which at the lower and produced end, a layer of highly elastic tissue, forming the caudicle, is attached; the other end being attached to the pedicel of the rostellum. The exterior grains of pollen are more angular, have thicker walls, and are yellower than the interior grains. In the early bud the two pollen-masses are enveloped in two conjoined membranous sacks, which are soon penetrated by the two produced ends of the pollen-masses and by their caudicles; and afterwards the extremities of the caudicles adhere to the pedicel. Before the flower expands the membranous sacks including the two pollen-masses open; and the pollen-masses are left resting naked on the back of the rostellum.

In Monachanthus, on the other hand, the two membranous sacks containing the rudimentary pollen-masses never open; but they easily separate from each other and from the anther. The tissue of which they are formed is thick and pulpy. Like most rudimentary parts, the pollen-masses vary much in size and form; they are only about one-tenth of the bulk of those of the male; they are flask-shaped (p, fig. 31), with the lower end greatly produced so as almost to penetrate the exterior or membranous sack. There is no fissure along their lower surfaces for the protrusion of the caudicles. The exterior pollen-grains are square and have thicker walls than the interior grains, just as in the proper male pollen; and, what is very curious, each cell has its nucleus. Now, E. Brown states[10] that in the early stages of the formation of the pollen-grains of ordinary Orchids (as with other plants) a minute nucleus is often visible; so that the rudimentary pollen-grains of Monachanthus apparently have retained—as is so general with rudiments in the animal kingdom—an embryonic character. Lastly, at the base, within each flask-shaped pollen-mass, there is a little mass of brown elastic tissue,—that is, a vestige of a caudicle,—which runs far up the pointed end of the flask, but does not (at least in some of the specimens) come to the surface, and could never be attached to any part of the pedicel. These rudimentary and enclosed caudicles are, therefore, utterly useless. Notwithstanding the small size and almost aborted condition of the female pollen-masses, when they were placed by Dr. Crüger within the stigma of a female plant they emitted "here and there a rudimentary tube." The petals then faded and the ovarium enlarged, but after a week it turned yellow and finally dropped off without bringing any seeds to perfection. This appears to me a very curious instance of the slow and gradual manner in which structures are modified; for the female pollen-masses, which can never be naturally removed or applied to the stigma, still partially retain their former powers and function.

Thus every detail of structure which characterises the male pollen-masses is represented in the female plant in a useless condition. Such cases are familiar to every naturalist, but can never be observed without renewed interest. At a period not far distant, naturalists will hear with surprise, perhaps with derision, that grave and learned men formerly maintained that such useless organs were not remnants retained by inheritance, but were specially created and arranged in their proper places like dishes on a table (this is the simile of a distinguished botanist) by an Omnipotent hand "to complete the scheme of nature."

The third form, Myanthus barbatus (fig. 31, B), is sometimes borne on the same plant together with the two preceding forms. The flowers differ greatly in external appearance, but not in essential structure, from those of both the other forms. They generally stand in a reversed position, compared with those of Catasetum tridentatum and of Monachanthus viridis, that is, with the labellum downwards. The labellum is fringed in an extraordinary manner with long papillæ; it has a quite insignificant medial cavity, at the hinder margin of which a curious curved and flattened horn projects, which represents the anvil-like projection on the labellum of the male C. callosum. The other petals and sepals are spotted and elongated, with the two lower sepals alone reflexed. The antennæ are not so long as in the male C. tridentatum; they project symmetrically on each side of the horn-like process at the base of the labellum, with their tips, which are not roughened with papillæ, almost entering the medial cavity. The stigmatic chamber is of nearly intermediate size between that of the male and female forms; it is lined with utriculi charged with brown matter. The straight and well-furrowed ovarium is nearly twice as long as that of the female Monachanthus, but not so thick where it joins the flower; the ovules are opaque and pulpy after having been kept in spirits, and resemble those of the female in all respects, but are not so numerous. I believe that I saw the nucleus projecting from the testa, but dare not, as in the case of the Monachanthus, speak positively. The pollinia are about a quarter of the size of those of the male Catasetum, but have a perfectly well developed disc and pedicel. The pollen-masses were lost in the specimens examined by me; but Mr. Reiss has given, in the Linnean Transactions, a drawing of them, showing that they are of due proportional size and have the proper folded or cleft structure, within which the caudicles are attached. Thus as both the male and female organs are in appearance perfect, Myanthus barbatus may be considered as an hermaphrodite form of the same species, of which the Catasetum is the male and Monachanthus the female. Nevertheless, the intermediate forms, which are common in Trinidad, and which resemble more or less closely the above described Myanthus, have never been seen by Dr. Crüger to produce seed-capsules.

It is a highly remarkable fact, that this sterile hermaphrodite form resembles in its whole appearance and structure the males of two other species, namely, C. saccatum and more especially C. callosum, much more closely than it does either the male or female form of the same species. As all orchids, with the exception of a few in the present small sub-family, as well as all the members of several allied groups of plants, are hermaphrodites, there can be no doubt that the common progenitor of the Orchideæ was an hermaphrodite. We may therefore attribute the hermaphrodite condition and the general appearance of Myanthus to reversion to a former state; and if so, the ancestors of all the species of Catasetum must have resembled the males of C. saccatum and callosum, for as we have just seen, it is to these two plants that Myanthus presents so many striking resemblances.[11]

Lastly I may be permitted to add that Dr. Crüger, after having carefully observed these three forms in Trinidad, fully admits the truth of my conclusion that Catasetum tridendatum is the male and Monachanthus viridis the female of the same species. He further confirms my prediction that insects are attracted to the flowers for the sake of gnawing the labellum, and that they carry the pollen-masses from the male to the female plant. He says "the male flower emits a peculiar smell about twenty-four hours after opening, and the antennæ assume their greatest irritability at the same time. A large humble-bee, noisy and quarrelsome, is now attracted to the flowers by the smell, and a great number of them may be seen every morning for a few hours disputing with each other for a place in the interior of the labellum, for the purpose of gnawing off the cellular tissue on the side opposite to the column, so that they turn their backs to the latter. As soon as they touch the upper antenna of the male flower, the pollen-mass, with its disc and gland, is fixed on their back, and they are often seen flying about with this peculiar-looking ornament on them. I have never seen it attached except to the very middle of the thorax. When the bee walks about, the pollen-mass lies flat on the back and wings; but when the insect enters a female flower, always with the labellum turned upwards, the pollinium, which is hinged to the gland by elastic tissue, falls back by its own weight and rests on the anterior face of the column. When the insect returns backwards from the flower, the pollinia are caught by the upper margin of the stigmatic cavity, which projects a little beyond the face of the column; and if the gland be then detached from the back of the insect, or the tissues which connect the pollinia with the caudicle, or this with the gland, break, fecundation takes place." Dr. Crüger sent me specimens of the humble-bees which he caught gnawing the labellum, and these consist of Euglossa nov. spec., cajennensis and piliventris.

Catasetum mentosum and a Monachanthus, according to Fritz Müller,[12] grow in the same district of South Brazil; and he easily succeeded in fertilising the latter with pollen from the former. The pollen-masses could be inserted only partially into the narrow stigmatic cleft; but when this was done, a process of deglutition, as described under Cirrhæa, commenced and was slowly completed. On the other hand, Fritz Müller entirely failed in his attempts to fertilise the flowers of this Catasetum with its own pollen or with that from another plant. The pollinia of the female Monachanthus are very small; the pollen-grains are variable both in size and shape; the anther never opens, and the pollen-masses are not attached to the caudicle. Nevertheless, when these rudimentary pollen-masses, which can never naturally be removed from their cells, were placed on the slightly viscid stigma of the male Catasetum, they emitted their tubes.

The genus Catasetum is interesting to an unusual degree in several respects. The separation of the sexes is unknown amongst other Orchids, except perhaps in the allied genus Cycnoches. In Catasetum we have three sexual forms, generally borne on separate plants, but sometimes mingled together on the same plant; and these three forms are wonderfully different from one another, much more different than, for instance, a peacock is from a peahen. But the appearance of these three forms now ceases to be an anomaly, and can no longer be viewed as an unparalleled instance of variability.

This genus is still more interesting in its manner of fertilisation. We see a flower patiently waiting with its antennæ stretched forth in a well-adapted position, ready to give notice whenever an insect puts its head into the cavity of the labellum. The female Monachanthus, not having true pollinia to eject, is destitute of antennæ. In the male and hermaphrodite forms, namely Catasetum tridentatum and Myanthus barbatus, the pollinia lie doubled up, like a spring, ready to be instantly shot forth when the antennæ are touched. The disc end is always projected foremost, and is coated with viscid matter which quickly sets hard and affixes the hinged pedicel firmly to the insect's body. The insect flies from flower to flower, till at last it visits a female plant: it then inserts one of the pollen-masses into the stigmatic cavity. As soon as the insect flies away the elastic caudicle, made weak enough to yield to the viscidity of the stigmatic surface, breaks, and leaves behind a pollen-mass; then the pollen-tubes slowly protrude, penetrate the stigmatic canal, and the act of fertilisation is completed. Who would have been bold enough to have surmised that the propagation of a species depended on so complex, so apparently artificial, and yet so admirable an arrangement?

I have examined three other genera placed by Lindley in the small sub-family of Catasetidæ, namely, Mormodes, Cycnoches and Cyrtopodium. The latter plant was purchased by me under this name, and bore a flower-stem about four feet in height with yellowish bracts spotted with red; but the flowers presented none of the remarkable peculiarities of the three other genera, with the exception that the anther was hinged to a point projecting from the summit of the column, as in Catasetum.

Mormodes ignea.—To show how difficult it sometimes is to understand the manner in which an Orchid is fertilised, I may mention that I carefully examined twelve flowers,[13] trying various experiments and recording the results, before I could at all make out the meaning and action of the several parts. It was plain that the pollinia were ejected, as in Catasetum, but how each part of the flower played its proper part I could not even conjecture. I had given up the case as hopeless, until summing up my observations, the explanation presently to be given, and subsequently proved by repeated experiments to be correct, suddenly occurred to me.

The flower presents an extraordinary appearance, and its mechanism is even more curious than its appearance (fig. 32). The base of the column is bent backwards, at right angles to the ovarium or footstalk, and then resumes an upright position to near its summit, where it is again bent. It is, also, twisted in a unique manner, so that its front surface, including

Fig. 32.

Darwin - The various contrivances by which orchids are fertilized by insects (1877) - Fig 32.png

Mormodes ignea.

Lateral view of flower, with the upper sepal and the near upper petal cut off.

N.B. The labellum in the drawing is a little lifted up, to show the depression on its under surface, which ought to be pressed close down on the bent summit of the column.

l. s.a. anther.
l. s.pd. pedicel of pollinium.
l. s.s. stigma.
l. s.l. labellum.
l. s. lateral sepal.

the anther, rostellum, and the upper part of the stigma faces one side of the flower; this being either to the right or left, according to the position of the flower on the spike. The twisted stigmatic surface extends down to the base of the column and is hollowed out into a deep cavity at its upper end. The large viscid disc of the pollinium is lodged in this cavity close beneath the rostellum; and the rostellum is seen in the drawing (pd.) covered by the bowed pedicel.

The anther-case (a in the figure) is elongated and triangular, closely resembling that of Catasetum; but it does not extend up to the apex of the column. The apex consists of a thin flattened filament, which from the analogy of Catasetum I suppose to be the produced filament of the stamen; but it may be a prolongation of some other element of the column. In the bud-state it is straight, but before the flower expands, it becomes much bent by the pressure of the labellum. A group of spiral vessels runs up the column as far as the summit of the anther-case; they are then reflexed and run some way down the anther-case. The point of reflexion forms a short thin hinge by which the top of the anther-case is articulated to the column beneath its bent summit. The hinge, although smaller than a pin's head in size, is of paramount importance; for it is sensitive and conveys the stimulus from a touch to the disc of the pollinium, causing it to separate from its place of attachment. The hinge also serves to guide the pollinium during its ejection. As it has to convey the necessary stimulus to the disc, one may suspect that a portion of the rostellum, which lies in close contact with the filament of the anther, runs up to this point; but I could not here detect any difference in structure on comparing these parts with those of Catasetum. The cellular tissue round the hinge is gorged with fluid, and a large drop exudes when the anther is torn from the column during the ejection of the pollinium. This gorged condition may perhaps facilitate the rupture of the hinge.

The pollinium does not differ much from that of Catasetum (see fig. 29, D, p. 183); and it lies in like manner curved round the rostellum, which is less protuberant than in that genus. The upper and broad end of the pedicel, however, extends beneath the pollen-masses within the anther; and these are attached by rather weak caudicles to a medial crest on its upper surface.

The viscid surface of the large disc lies in contact with the roof of the stigmatic cavity, so that it cannot be touched by an insect visiting the flower. The anterior end of the disc is furnished with a small dependent curtain (dimly shown in fig. 32); and this, before the act of ejection, is continuously joined on each side to the upper margins of the stigmatic cavity. The pedicel is united to the posterior end of the disc; but when the disc is freed, the lowermost part of the pedicel becomes doubly bent, so that it then appears as if attached by a hinge to the centre of the disc.

The labellum is a highly remarkable structure: it is narrowed at its base into a nearly cylindrical foot-stalk, and its sides are so much reflexed as almost to meet at the back, forming a folded crest on the summit of the flower. After rising up perpendicularly it arches over the apex of the column, against which it is firmly pressed down. The labellum at this point is hollowed out (even in the bud) into a slight cavity, which receives the bent summit of the column. This slight depression manifestly represents the large cavity, with thick fleshy walls, which insects gnaw, on the anterior surface of the labellum in the several species of Catasetum. Here by a singular change of function, the cavity serves to keep the labellum in its proper position on the summit of the column, but is, perhaps, likewise attractive to insects. In the drawing (fig. 32) the labellum has been forcibly raised a little up, so as to show the depression and the bent filament. In its natural position it may almost be compared to a huge cocked-hat, supported by a footstalk and placed on the head of the column.

The twisting of the column, which I have seen in no other Orchid, causes all the important organs of fructification in the flowers on the left side of the spike to face to the left, and in all those on the right side to face to the right. So that two flowers taken from opposite sides of the same spike and held in the same relative position are seen to be twisted in opposite directions. One single flower, which was crowded by the others, was barely twisted, so that its column faced the labellum. The labellum is also slightly twisted: for instance, in the flower figured, which faced to the left, the midrib of the labellum was first twisted to the right-hand, and then to the left, but in a less degree, and being bent over it pressed on the posterior surface of the crooked summit of the column. The twisting of all the parts of the flower commences in the bud.

The position thus acquired by the several organs is of the highest importance; for if the column and labellum had not been twisted laterally, the pollinia, when shot forth, would have struck the overarching labellum and have then rebounded, as actually occurred with the single abnormal flower having a nearly straight column. If the organs had not been twisted in opposite directions on the opposite sides of the same crowded spike, so as always to face to the outside, there would not have been a clear space for the ejection of the pollinia and their adhesion to insects.

When the flower is mature the three sepals hang down, but the two upper petals remain nearly upright. The bases of the sepals, and especially of the two upper petals, are thick and swollen and have a yellowish tint; when quite mature, they are so gorged with fluid, that, if punctured by a fine glass tube, the fluid rises by capillary attraction to some height in it. These swollen bases, as well as the footstalk of the labellum, have a decidedly sweet and pleasant taste; and I can hardly doubt that they are attractive to insects, for no free nectar is secreted.

I will now endeavour to show how all the parts of the flower are co-ordinated and act together. The pedicel of the pollinium is bowed round the rostellum, as in Catasetum; in this latter genus, when freed, it merely straightens itself with force, in Mormodes something more takes place. If the reader will look forward to fig. 34 (p. 223), he will see a section of the flower-bud of the allied genus of Cycnoches, which differs only in the shape of the anther and in the viscid disc having a much deeper dependent curtain. Now let him suppose the pedicel of the pollinium to be so elastic that, when freed, it not only straightens itself, but suddenly bends back on itself with a reversed curvature, so as to form an irregular hoop. The curved surface which was before in contact with the protuberant rostellum now forms the outside of the hoop. The exterior surface of the curtain, which depends beneath the disc, is not viscid; and it now lies on the anther-case, with the viscid surface of the disc on the outside. This is exactly what takes place with Mormodes. But the pollinium assumes with such force its reversed curvature (aided, apparently, by a transverse curling outwards of the margins of the pedicel), that it not only forms itself into a hoop, but suddenly springs away from the protuberant face of the rostellum. As the two pollen-masses adhere, at first, rather firmly to the anther-case, the latter is torn off by the rebound; and as the thin hinge at the summit of the anther-case does not yield so easily as the basal margin, the pollinium together with the anther-case is instantly swung upwards like a pendulum. But in the course of the upward swing the hinge yields, and the whole body is projected perpendicularly up in the air, an inch or two above and close in front of the terminal part of the labellum. If no object is in the way, as the pollinium falls down, it generally alights and sticks, though not firmly, on the folded crest of the labellum, directly over the column. I witnessed repeatedly all that has been here described.

The curtain of the disc, which, after the pollinium has formed itself into a hoop, lies on the anther-case, is of considerable service in preventing the viscid edge of the disc from adhering to the anther, and thus permanently retaining the pollinium in the form of a hoop. This would have been fatal, as we shall presently see, to a subsequent movement of the pollinium which is necessary for the fertilisation of the flower. In some of my experiments, when the free action of the parts was checked, this did occur, and the pollinium, together with the anther-case, remained permanently glued together in the shape of an irregular hoop.

I have already stated that the minute hinge by which the anther-case is articulated to the column, a little way beneath its bent filamentary apex, is sensitive to a touch. I tried four times and found that I could touch with some force any other part; but when I gently touched this point with the finest needle, instantly the membrane which unites the disc to the edges of the stigmatic cavity where it is lodged, ruptured, and the pollinium was shot upwards and fell on the crest of the labellum as just described.

Now let us suppose an insect to alight on the folded crest of the labellum, and no other convenient landing-place is afforded, and then to lean over the front of the column so as to gnaw or suck the bases of the petals swollen with sweet fluid. The weight and movements of the insect would disturb the labellum and the bent underlying summit of the column; and the latter, pressing on the hinge in the angle, would cause the ejection of the pollinium, which would infallibly strike the head of the insect and adhere to it. I tried by placing my gloved finger on the summit of the labellum, with the tip just projecting beyond its margin, and then gently moving my finger it was really beautiful to see how instantly the pollinium was projected upwards, and how accurately the viscid surface of the disc struck my finger and firmly adhered to it. Nevertheless, I doubt whether the weight and movements of an insect would suffice to thus act indirectly on the sensitive point; but look at the drawing and see how probable it is that an insect leaning over would place its front legs over the edge of the labellum on the summit of the anther-case, and thus touch the sensitive point. The pollinium would then be ejected, and the viscid disc would certainly strike and adhere to the insect's head.

Before proceeding, it may be worth while to mention some of the early trials which I made. I pricked deeply the column in different parts, including the stigma, and cut off the petals, and even the labellum, without causing the ejection of the pollinium; this, however, once happened when I cut rather roughly through the thick footstalk of the labellum, the filamentary summit of the column no doubt having been thus disturbed. When I gently prised up the anther-case at its base or on one side, the pollinium was ejected, but then the sensitive hinge would necessarily have been bent. When the flower has long remained expanded and is nearly ready for spontaneous ejection, a slight jar on any part of the flower causes the action. Pressure on the thin pedicel of the pollinium, and therefore on the underlying protuberant rostellum, is followed by the ejection of the pollen-masses; but this is not surprising, as the stimulus from a touch on the sensitive hinge has to be conveyed through this part of the rostellum to the disc. In Catasetum slight pressure on this point does not cause the act of ejection; but in this genus the protuberant part of the rostellum does not lie in the course along which the stimulus has to be conveyed from the antennæ to the disc. A drop of chloroform, of spirits of wine, or of boiling water placed on this part of the rostellum produced no effect; nor, to my surprise, did exposure of the whole flower to vapour of chloroform.

Seeing that this part of the rostellum was sensitive to pressure, and that the flower was widely open on one side, and being pre-occupied with the case of Catasetum, I at first felt convinced that insects entered the lower part of the flower and touched the rostellum. Accordingly I pressed the rostellum with variously-shaped objects, but the viscid disc never once adhered in a proper manner to the object. If I used a thick needle, the pollinium, when ejected, formed a hoop round it with the viscid surface outside; if I used a broad flat object, the pollinium struggled against it and sometimes coiled itself up spirally, but the disc either did not adhere at all, or very imperfectly. At the close of the twelfth trial I was in despair. The strange position of the labellum, perched on the summit of the column, ought to have shown me that here was the place for experiment. I ought to have rejected the notion that the labellum was thus placed for no good purpose. This plain guide was overlooked, and for a long time I completely failed to understand the structure of the flower.

We have seen that when the pollinium is ejected and swings upwards, it adheres by the viscid surface of the disc to any object projecting beyond the edge of the labellum directly over the column. When thus attached, it forms an irregular hoop, with the torn-off anther-case still covering the pollen-masses which are close to the disc, but protected from adhering to it by the dependent curtain. Whilst in this position the projecting and bowed part of the pedicel would effectually prevent the pollen-masses from being placed on the stigma, even supposing the anther-case to have fallen off. Now let us suppose the pollinium to be attached to an insect's head, and observe what takes place. The pedicel, when first separated from the rostellum, is damp; as it dries, it slowly straightens itself, and when perfectly straight the anther-case readily drops off. The pollen-masses are now naked, and they are attached to the end of the pedicel by easily ruptured caudicles, at the right distance and in a proper position for their insertion into the adhesive stigma, as soon as the insect visits another flower. Thus every detail of structure is now perfectly adapted for the act of fertilisation.

When the anther-case drops off, it has performed its triple function; namely, its hinge as an organ of sense, its weak attachment to the column as a guide causing the pollinium at first to swing perpendicularly upwards, and its lower margin, together with the curtain of the disc, as a protection to the pollen-masses from being permanently glued to the viscid disc.

From observations made on fifteen flowers, it was ascertained that the straightening of the pedicel does not occur until from twelve to fifteen minutes have elapsed. The first movement causing the act of ejection is due to elasticity, and the second slow movement to the drying of the outer and convex surface; but this latter movement differs from that observed in the pollinia of so many Vandeæ and Ophreæ, for, when the pollinium of this Mormodes was placed in water, it did not recover the hoop-like form which it had at first acquired by elasticity.

The flowers are hermaphrodites. The pollinia are perfectly developed. The elongated stigmatic surface is extremely viscid and abounds with innumerable utriculi, the contents of which shrink and become coagulated after immersion for less than an hour in spirits of wine. When placed in spirits for a day, the utriculi were so acted on that they disappeared, and this I have not noticed in any other Orchid. The ovules, after exposure to spirits for a day or two, presented the usual semi-opaque, pulpy appearance common to all hermaphrodite and female Orchids. From the unusual length of the stigmatic surface I expected that, if the pollinia were not ejected from the excitement of a touch, the anther-case would have detached itself, and the pollen-masses would have swung downwards and fertilised the stigma of the same flower. Accordingly, I left four flowers untouched; after they had remained expanded from eight to ten days, the elasticity of the pedicel conquered the force of attachment and the pollinia were spontaneously ejected, but they did not fall on the stigma and were consequently wasted.

Although Mormodes ignea is an hermaphrodite, yet it must be as truly diœcious in function as Catasetum; for as it takes from twelve to fifteen minutes before the pedicel of an ejected pollinium straightens itself and the anther-case drops off, it is almost certain that within this time an insect with a pollinium attached to its head would have left one plant and flown to another.

Mormodes luxata.—This rare and fine species is fertilised in the same manner as Mormodes ignea, but differs in several important points of structure. The right and left sides of the same flower differ from one another even in a greater degree than in the last species. One of the petals and one of the sepals project at right angles to the column, while the corresponding ones stand upright and surround it. The upturned and twisted labellum is furnished with two large lateral lobes: of these one embraces the column, while the other stands partly open on the side where the one petal and sepal lie flat. Insects can thus easily enter the flower on this latter side. All the flowers on the left side of the spike are open on their left sides, while those on the right side are open on this side. The twisted column with all the important accessory parts, together with the rectangularly bent apex, closely resemble the corresponding parts in M. ignea. But the under side of the labellum does not rest on and press against the rectangularly bent apex of the column. This stands free in the middle of a cup formed by the extremity of the labellum.

I did not obtain many flowers fit for examination, as three had ejected their pollinia owing to the shocks received during their journey. I pricked deeply the labellum, column and stigma of some of the flowers without any effect; but when I lightly touched with a needle, not the anther-hinge as in the last species, but the apex of the column of one flower, the pollinium was instantly ejected. The bases of the petals and sepals are not swollen and succulent like those of M. ignea; and I have little doubt that insects gnaw the labellum, which is thick and fleshy, with the same peculiar taste as in Catasetum. If an insect were to gnaw the terminal cup, it could hardly fail to touch the apex of the column, and then the pollinium would swing upwards and adhere to some part of the insect's body. The pedicels of the pollinia straighten themselves and the anther-cases are cast off, in about fifteen minutes after the act of ejection. We may therefore confidently believe that this species is fertilised in the same peculiar manner as Mormodes ignea.

Cycnoches ventricosum.—Mr. Veitch was so kind as to send me on two occasions several flowers and flower-buds of this extraordinary plant. A sketch of a flower in its natural position, with one sepal cut off, is shown at fig. 33 (p. 222), and a longitudinal section through a young bud at fig. 34 (p. 223).

The labellum is thick and fleshy, with the usual taste of this organ in the Catasetidæ; it resembles in shape a shallow basin turned upside down. The two other petals and the three sepals are reflexed. The column is almost cylindrical, thin, flexible, elastic and of extraordinary length. It curves round so as to bring the stigma and anther opposite to and beneath the convex surface of the labellum. The apex of the column is not nearly so much produced as in Mormodes and Catasetum. The pollinia closely resemble those of Mormodes; but the disc is larger, and its curtain, which is fringed, is so large that it covers the whole entrance into the stigmatic chamber. The structure of these parts is best seen in the section, fig. 34; in which the pedicel of the pollinium has not as yet become separate from the rostellum, but the future line of separation is shown by a line (dotted in the figure) of hyaline tissue. The filament of the anther (f, fig. 34) has not as yet grown to its full length. When fully developed it bears two little leaf-like appendages which lie oh the anther. Lastly, on the sides of the stigma there are two slight protuberances (fig. 33), which apparently represent the antennæ of Catasetum, but have not the same function.

Neither the labellum nor the protuberances on the sides of the stigma are at all sensitive; but when on three occasions I momentarily touched the filament, between the little leaf-like appendages, the pollinium was ejected in the same manner and through the same mechanism as in Mormodes; but it was thrown only to the distance of about an inch. If the filament had been touched by an object which had not been quickly removed, or if by an insect, the viscid disc would certainly have adhered to it. Mr. Veitch informs me that he has often touched the end of the column, and the pollinium has adhered to his finger. When the pollinium is ejected, the pedicel forms a hoop, with the exterior surface of the curtain of the disc resting on and covering the anther. In about fifteen minutes the pedicel straightens itself, and the anther-case drops off; and now the pollinium is in a right position for fertilising another flower. As soon as the viscid matter on the under surface of the disc is exposed to the air it quickly changes colour and sets hard. It then adheres with surprising force to any object. From these various facts and from the analogy of the other Catasetidæ, we may conclude that insects visit the flowers for the sake of gnawing the labellum: but it cannot be predicted whether they alight on the surface which is uppermost in the drawing (fig. 33) and

Fig. 33.

Darwin - The various contrivances by which orchids are fertilized by insects (1877) - Fig 33.png

Cycnoches ventricosum.

Flower viewed in its natural dependent position.

sep.c. column, after the ejection of the pollinium together with the anther.
sep.f. filament of anther.
sep.s. stigmatic cavity.
sep.L. labellum. the two lateral petals.
sep. sepals.

then crawl over the margin so as to gnaw the convex surface, and in doing so touch with their abdomens the extremity of the column, or whether they first alight on this part of the column; but in either case they would cause the ejection of the pollinia, which would adhere to some part of their bodies.

The specimens which I examined were certainly

Fig. 34.

Darwin - The various contrivances by which orchids are fertilized by insects (1877) - Fig 34.png

Diagrammatic Section of a Flower-bud, the column placed upright.

pd.a. anther.
pd.f. filament of anther.
pd.p. pollen-mass.
pd. pedicel of pollinium, barely separated as yet from the rostellum.
pd.d. disc of pollinium with the dependent curtain.
pd.s. stigmatic chamber.
pd.g. stigmatic canal leading to the ovarium.

male plants, for the pollinia were well developed. The stigmatic cavity was lined with a thick layer of pulpy matter which was not adhesive. But as the flowers cannot possibly be fertilised until the pollinia have been ejected, together with the great curtain which covers the whole stigmatic surface, it may be that this surface becomes at a later period adhesive so as to secure the pollen-masses. The ovules when kept for some time in alcohol were filled with brownish pulpy matter, as is always the case with perfect ovules. Therefore it appears that this Cycnoches must be an hermaphrodite; and Mr. Bateman, in his work on the Orchideæ, says that the present species produces seeds without being, as I understand, artificially fertilised; but how this is possible is unintelligible to me. On the other hand, Beer says[14] that the stigma of Cycnoches is dry, and that the plant never sets seeds. According to Lindley C. ventricosum produces on the same scape flowers with a simple labellum, others with a much segmented and differently coloured labellum (viz., the so-called C. egertonianum), and others in an intermediate condition. From the analogous differences in the flowers of Catasetum, we are tempted to believe that we here have male, female, and hermaphrodite forms of the same species of Cycnoches.[15]

I have now finished my description of the Catasetidæ as well as of many other Vandeæ. The study of these wonderful and often beautiful productions, with all their many adaptations, with parts capable of movement, and other parts endowed with something so like, though no doubt different from, sensibility, has been to me most interesting. The flowers of Orchids, in their strange and endless diversity of shape, may be compared with the great vertebrate class of Fish, or still more appropriately with tropical Homopterous insects, which appear to us as if they had been modelled in the wildest caprice, but this no doubt is due to our ignorance of their requirements and conditions of life.

  1. I am much indebted to Mr. James Veitch of Chelsea for the first specimen which I saw of this Orchid; subsequently Mr. S. Rucker, so well known for his magnificent collection of Orchids, generously sent me two fine spikes, and has aided me in the kindest manner with other specimens.
  2. 'Bull, de la Soc. Bot. de France,' tom. i. 1854, p. 367.
  3. M. Baillon ('Bull. de la Soc. Bot. de France,' tom. i. 1854, p. 285) states that Catasetum luridum ejects its pollinia always in a straight line, and in such a direction that it sticks fast to the bottom of the concavity of the labellum; and he imagines that in this position it fertilises the flower in a manner not clearly explained. In a subsequent paper in the same volume (p. 367) M. Ménière justly disputes M. Baillon's conclusion. He remarks that the anther-case is easily detached, and sometimes naturally detaches itself; the pollinia then swing downwards by the elasticity of the pedicel, the viscid disc still remaining attached to the roof of the stigmatic chamber. M. Ménière hints that, by the subsequent and progressive retraction of the pedicel, the pollen-masses might be carried into the stigmatic chamber. This is not possible in the three species which I have examined, and would be useless. But M. Ménière himself then goes on to show how important insects are for the fertilisation of Orchids; and apparently infers that their agency comes into play with Catasetum, and that this plant does not fertilise itself. Both M. Baillon and M. Ménière correctly describe the curved position in which the elastic pedicel lies before it is set free. Neither of these botanists seems to be aware that the species of Catasetum (at least the five which I have examined) are exclusively male plants.
  4. A fine spike of flowers of this species was kindly sent me by Mr. Rucker, and was named for me by Dr. Lindley.
  5. 'Transactions of the Linnean Soc.' vol. xvii. p. 522. Another account by Dr. Lindley appeared in the 'Botanical Register,' fol. 1951, of a distinct species of Myanthus and Monachanthus appearing on the same scape: he alludes also to other cases. Some of the flowers in these cases were in an intermediate condition, which is not surprising, seeing that in diœcious plants we sometimes have a partial resumption of the characters of both sexes. Mr. Endgers of Riverhill informs me that he imported from Demerara a Myanthus, and that when it flowered a second time it was metamorphosed into a Catasetum. Dr. Carpenter ('Comparative Physiology,' 4th edit. p. 633) alludes to an analogous case which occurred at Bristol. Lastly Dean Herbert informed me many years ago that Catasetum luridum flowered and kept true for nine years in the Botanic Garden at York; it then threw up a scape of a Myanthus, which as we shall presently see is hermaphrodite, intermediate in form between the male and female. M. Duchartre has given a full historical account of the appearance of those forms on the same plant, in 'Bull. de la Soc. Bot. de France,' vol. ix. 1862, p. 113.
  6. The 'Vegetable Kingdom,' 1853, p. 178.
  7. Brongniart states ('Bull. de la Soc. Bot. de France,' tom. ii. 1855, p. 20) that M. Neumann, a skilful fertiliser of Orchids, could never succeed in fertilising Catasetum.
  8. Dr. Hance writes to me that he has in his collection a plant of Catasetum tridentatum from the West Indies bearing a fine capsule; but it does not appear to have been ascertained that this particular flower was that of Catasetum, and there is no great improbability in a single flower of Monachanthus being produced by a plant of Catasetum, as well as a whole scape, which we know has often occurred. J. G. Beer says (quoted by Irmisch, 'Beiträge zu Biologie der Orchideen,' 1853, p. 22) that during three years he tried in vain to fertilise Catasetum, but on one occasion, by placing the viscid disc of a pollinium within the stigma, a ripe fruit was produced; but it may be asked, Did the seeds contain embryos?
  9. 'Journ. Linn. Soc. Bot.' vol. viii. 1864, p. 127.
  10. 'Transactions of the Linnean Soc.' vol. xvi. p. 711.
  11. The male of the Indian antelope (A. bezoartica) after castration produces horns of a widely different shape from those of the perfect male; and larger and thicker than those occasionally produced by the female. We see something of the same kind in the horns of the common ox. I have remarked in my 'Descent of Man' (2nd edit. p. 506), that such cases may probably be attributed to reversion to a former slate of the species; for we have good reason to believe that any cause which disturbs the constitution leads to reversion. Myanthus, though having the organs of both sexes apparently perfect, is sterile; it has therefore had its sexual constitution disturbed, and this seems to have caused it to revert in character to a former state.
  12. 'Bot. Zeitung,' 1868, p. 630.
  13. I must express my cordial thanks to Mr. Rucker, of West Hill, Wandsworth, for having lent me a plant of this Mormodes with two fine spikes, bearing an abundance of flowers, and for having allowed mo to keep the plant for a considerable time.
  14. Quoted by Irmisch, 'Beiträge zur Biologie der Orchideen,' 1853, p. 22.
  15. Lindley's 'Vegetable Kingdom,' 1853, p. 177. He has also published in the 'Botanical Register,' fol. 1951, a case of two forms appearing on the same scape of another species of Cycnoches. Mr. Bateman also says that C. egertonianum has been known to produce in Guatemala and once in England scapes of a purple-flowered and widely different species of Cycnoches; but that it generally produces in England scapes of the common yellow C. ventricosum.