The Various Contrivances by which Orchids are Fertilised by Insects/Chapter 6

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Structure of the column and pollinia—Importance of the elasticity of the pedicel; its power of movement—Elasticity and strength of the caudicles—Calanthe with lateral stigmas, manner of fertilisation—Angræcum sesquipedale, wonderful length of nectary—Species with the entrance into the stigmatic chamber much contracted, so that the pollen-masses can hardly be inserted—Coryanthes, extraordinary manner of fertilisation.

We now come to the immense tribe of the Vandeæ, which includes many of the most magnificent productions of our hothouses, but like the Epidendreæ has no British representative. I have examined twenty-nine genera. The pollen consists of waxy masses, as in the two last tribes, and each ball of pollen is furnished with a caudicle, which becomes, at an early period of growth, united to the rostellum. The caudicle is seldom attached directly to the viscid disc, as in most of the Ophreæ, but to the upper and posterior surface of the rostellum; and this part is removed by insects, together with the disc and pollen-masses. The sectional diagram (fig. 23), with the parts separated, will best explain the type-structure of the Vandeæ. As in the rest of the Orchideæ there are three confluent pistils; of these the dorsal one (2) forms the rostellum arching over the two others (3) which unite to form a single stigma. On the left hand we have the filament (1) bearing the anther. The anther opens at an early period, and the tips of the two caudicles (but only one caudicle and one pollen-mass are represented in the diagram) protrude in a not fully-hardened condition through a small slit, and adhere to the back of the rostellum. The upper surface of the rostellum is generally hollowed out for the reception of the pollen-masses; it is represented as smooth in the diagram, but is really often furnished with crests

Fig. 23.

Darwin - The various contrivances by which orchids are fertilized by insects (1877) - Fig 23.png

Imaginary Section, illustrative of the structure of the column in the Vandeæ.

(1.) The filament, bearing the anther with its pollen-masses; the anther is represented after it has opened along its whole under surface, so that the section shows only the dorsal surface.
(2.) The upper pistil, with the upper part modified into the rostellum.
(3.) The two lower confluent pistils, bearing the two confluent stigmas.

or knobs for the attachment of the two caudicles. The anther afterwards opens more widely along its under surface, and leaves the two pollen-masses unattached, excepting by their caudicles to the rostellum.

During an early period of growth, a remarkable change has been going on in the rostellum: either its extremity or its lower surface becomes excessively viscid (forming the viscid disc), and a line of separation, at first appearing as a zone of hyaline tissue, is gradually formed, which sets free the disc, as well as the whole upper surface of the rostellum, as far back as the point of attachment of the caudicles. If any object now touches the viscid disc, it, together with the whole back of the rostellum, the caudicles and pollen-masses, can all be readily removed together. In botanical works the whole structure between the disc or viscid surface (generally called the gland) and the balls of pollen is designated as the caudicle; but as these parts play an essential part in the fertilisation of the flower, and as they are fundamentally different in their origin and in their minute structure, I shall call the two elastic ropes, which are developed strictly within the anther-cells, the caudicles; and the portion of the rostellum to which the caudicles are attached (see diagram), and which is not viscid, the pedicel. The viscid portion of the rostellum I shall call, as heretofore, the viscid surface or disc. The whole may be conveniently spoken of as the pollinium.

In the Ophreæ we have (except in O. pyramidalis and a few other species), two separate viscid discs. In the Vandeæ, with the exception of Angræcum, we have only one disc. The disc is naked, or is not enclosed in a pouch. In Habenaria the discs, as we have seen, are separated from the two caudicles by short drum-like pedicels, answering to the single and generally much more largely developed pedicel in the Vandeæ. In the Ophreæ the caudicles of the pollinia, though elastic, are rigid, and serve to place the packets of pollen at the right distance from the insect's head or proboscis, so as to reach the stigma. In the Vandeæ this end is gained by the pedicel of the rostellum. The two caudicles in the Vandeæ are embedded and attached within a deep cleft in the pollen-masses, and until stretched are rarely visible, for the pollen-masses lie close to the pedicel of the rostellum. These caudicles answer both in position and function to the elastic threads, by which the packets of pollen are tied together in the Ophreæ, at the point where they become confluent; for the function of the true caudicle in the Vandeæ is to break when the masses of pollen, transported by insects, adhere to the stigmatic surface.

In many Vandeæ the caudicles are easily ruptured, and the fertilisation of the flower, as far as this point is concerned, is a simple affair; but in other cases their strength, and the length to which they can be stretched before they break, are surprising. I was at first perplexed to understand what purpose these qualities could serve. The explanation probably is that the pollen-masses in this tribe are very precious objects; in most of the genera a flower produces only two, and judging from the size of the stigma both are generally left adhering to it. In other genera, however, the orifice leading into the stigma is so small that probably only one pollen-mass is left on it, and in this case the pollen from one flower would suffice to fertilise two flowers, but never a greater number. From the large size of the flowers of many of the Vandeæ, they no doubt are fertilised by large insects, and these whilst flying about would be likely to brush away and lose the pollinia attached to them, unless the caudicles were very strong and highly elastic. So again, when an insect thus provided visited a flower either too young, with its stigma not yet sufficiently adhesive, or one already impregnated, with its stigma beginning to dry, the strength of the caudicle would prevent the pollen-masses from being uselessly removed and lost.

Although the stigmatic surface is astonishingly adhesive at the proper period in many of these Orchids, for instance, in Phalænopsis and Saccolabium, yet when I inserted their pollinia attached to a rough object into the stigmatic chamber, they did not adhere with sufficient force to prevent their removal from the object. I even left them for some little time in contact with the adhesive surface, as an insect would do whilst feeding; but when I pulled the pollinia straight out of the stigmatic chamber, the caudicles, though they were stretched to a great length, did not rupture, nor did their attachment to the object yield so that the balls of pollen were withdrawn. It then occurred to me that an insect in flying away would not pull the pollinia straight out of the chamber, but would pull at nearly right angles to its orifice. Accordingly I imitated the action of a retreating insect, and dragged the pollinia out of the stigmatic chamber at right angles to its orifice; and now the friction on the caudicles thus caused, together with the adhesiveness of the stigmatic surface, generally sufficed to rupture them; the pollen-masses being left on the stigma. Thus, it seems that the great strength and extensibility of the caudicles, which, until stretched, lie embedded within the pollen-masses, serve to protect the pollen-masses from being accidentally lost by an insect whilst flying about, and yet, by friction being brought into play, allow them at the proper time, to be left adhering to the stigmatic surface; the fertilisation of the flower being thus safely effected.

The discs and pedicels of the pollinia present great diversities in shape, and an apparently exhaustless number of adaptations. Even in species of the same genus, as in Oncidium, these parts difier greatly. I here give a few figures (fig. 24), taken almost at hazard. The pedicel generally consists, as far as I have seen, of a thin ribbon-shaped membrane (fig. A); sometimes it is almost cylindrical (fig. C) but often of the most diversified shapes. The pedicel is generally nearly straight, but in Miltonia clowesii it is naturally curved; and in some cases, as we shall immediately see, it assumes, after removal, various shapes. The extensible and elastic caudicles, by which the pollen-masses are attached to the pedicel, are barely or not at all visible, being embedded in a cleft or hollow within each pollen-mass. The disc, which is viscid on the under side, consists of a piece of thin or thick membrane of

Fig. 24.

Darwin - The various contrivances by which orchids are fertilized by insects (1877) - Fig 24.png

Pollinia of Vandeæ.

ped.d. viscid disc.
ped. pedicel.
ped.p. pollen-masses.
The caudicles, being embedded within the pollen-masses, are not shown.
A. Pollinium of Ondicium grande after partial depression.
B. Pollinium of Brassia maculata (copied from Bauer).
C. Pollinium of Stanhopea saccata after depression.
D. Pollinium of Sarcanthus teretifolius after depression.

varied forms. In Acropera it is like a pointed cap; in some cases it is tongue-shaped, or heart-shaped (fig. C), or saddle-shaped, as in some Maxillarias, or like a thick cushion (fig. A), as in many species of Oncidium, with the pedicel attached at one end, instead of, as is more usual, nearly to the centre. In Angræcum distichum and sesquipedale the rostellum is notched, and two separate, thin, membranous discs can be removed, each carrying by a short pedicel a pollen-mass. In Sarcanthus teretifolius the disc (fig. D) is very oddly shaped; and as the stigmatic chamber is deep and likewise curiously shaped, we are led to believe that the disc is fastened with great precision to the square projecting head of some insect.[1]

In most eases there is a plain relation between the length of the pedicel and the depth of the stigmatic chamber, into which the pollen-masses have to be inserted. In some few cases, however, in which a long pedicel and a shallow stigma co-exist, we shall presently meet with curious compensating actions. After the disc and pedicel have been removed, the shape of the remaining part of the rostellum is of course altered, being now slightly shorter and thinner, and sometimes notched. In Stanhopea, the entire circumference of the extremity of the rostellum is removed, and a thin, pointed, needle-like process alone is left, which originally ran up the centre of the disc.

If we now turn to the diagram (fig. 23, p. 150), and suppose the rectangularly bent rostellum to be thinner and the stigma to lie closer beneath it than is there represented, we shall see that, if an insect with a pollinium attached to its head were to fly to another flower and occupy exactly the same position which it held whilst the attachment was effected, the pollen-masses would be in the right position for striking the stigma, especially if, from their weight, they were to become in the least degree depressed. This is all that takes place in Lycaste skinnerii, Cymbidiwm giganteum, Zygopetalum mackai, Angræcum eburneum, Miltonia clowesii, in a Warrea, and, I believe, in Galeandra funkii. But if in our diagram we suppose, for instance, the stigma to be seated at the bottom of a deep cavity, low down in the column, or the anther to be seated higher up, or the pedicel of the rostellum to slope more upwards, &c.—all of which contingencies occur in various species,—in such cases, an insect with a pollinium attached to its head, if it flew to another flower, would not place the pollen-masses on the stigma, unless their position had become greatly changed after attachment.

This change is effected in many Vandeæ in the same manner as is so general with the Ophreæ, namely, by a movement of depression in the pollinium in the course of about half a minute after its removal from the rostellum. I have seen this movement conspicuously displayed, generally causing the pollinium to rotate through about a quarter of a circle, in several species of Oncidium, Odontoglossum, Brassia, Vanda, Aerides, Sarcanthus, Saccolabium, Acropera, and Maxillaria. In Rodriguezia suaveolens the movement of depression is remarkable from its extreme slowness; in Eulophia viridis from its small extent. Mr. Charles Wright, in a letter to Professor Asa Gray, says that he observed in Cuba a pollinium of an Oncidium attached to a humble-bee, and he concluded at first that I was completely mistaken about the movement of depression; but after several hours it moved into the proper position for fertilising the flower. In some of the cases above specified in which the pollinia apparently undergo no movement of depression, I am not sure that there was not a very slight one after a time. In the various Ophreæ the anther-cells are sometimes seated exteriorly and sometimes interiorly with respect to the stigma; and there are corresponding outward and inward movements in the pollinia: but in the Vandeæ the anther-cells always lie, as far as I have seen, directly over the stigma, and the movement of the pollinium is always directly downwards. In Calanthe, however, the two stigmas are placed exteriorly to the anther-cells, and the pollinia, as we shall see, are made to strike them by a peculiar mechanical arrangement of the parts.

In the Ophreæ the seat of contraction, which causes the act of depression, is in the upper surface of the viscid disc, close to the point of attachment of the caudicles: in most of the Vandeæ the seat is likewise in the upper surface of the disc, but at the point where the pedicel is united to it, and therefore at a considerable distance from the point of attachment of the true caudicles. The contraction is hygrometric, but to this subject I shall return in the ninth chapter; therefore the movement does not take place until the pollinium has been removed from the rostellum, and the point of union between the disc and pedicel has been exposed for a few seconds or minutes to the air. If, after the contraction and consequent movement of the pedicel, the whole body be placed into water, the pedicel slowly moves back and resumes its former position with respect to the viscid disc. When taken out of water, it again undergoes the movement of depression. It is of importance to notice these facts, as we thus get a test by which this movement can be distinguished from certain other movements.

In Maxillaria ornithorhyncha, we have a unique case. The pedicel of the rostellum is much elongated, and is entirely covered by the produced front lip of the anther, and is thus kept damp. When removed it bends quickly backwards on itself, at about its central point, and thus becomes only half as long as it was before. When placed in water it resumes its original straight form. If the pedicel had not been in some manner shortened, it is hardy possible that the flower could have been fertilised. After this movement, the pollinium attached to any small object can be inserted into the flower, and the balls of pollen readily adhere to the stigmatic surface. Here we have an instance of one of those compensating actions in the pollinia, before alluded to, in relation to the shallowness of the stigma.

In some cases, besides hygrometric movements, elasticity comes into play. In Aerides odorata and virens, and in an Oncidium (roseum?), the pedicel of the rostellum is fastened down in a straight line, at one extremity by the disc, and at the other by the anther; it has, however, a strong elastic tendency to spring up at right angles to the disc. Consequently, if the pollinium, attached by its viscid disc to some object, is removed from the anther, the pedicel instantly springs up and stands at nearly right angles to its former position, with the pollen-masses carried aloft. This has been noticed by other observers; and I agree with them that the object gained is to free the pollen-masses from the anther-cells. After this upward elastic spring, the downward hygrometric movement immediately commences, which, oddly enough, carries the pedicel back again into almost exactly the same position, relatively to the disc, which it held whilst forming part of the rostellum. In Aerides the end of the pedicel, to which the pollen-masses are attached by short dangling caudicles, after springing up, remains a little curved upwards; and this curvature seems well adapted to drop the pollen-masses into the deep stigmatic cavity over the ledge in front. The difference between the first elastic and the second or reversed hygrometric movement, was well shown by placing the pollinium of the above Oncidium into water, after both movements had taken place; and the pedicel then moved into the same position which it had at first assumed through its elasticity; this movement not being in any way affected by the water. When taken out of water the hygrometric movement of depression soon commenced for the second time.

In Rodriguezia secunda there was no hygrometric movement of depression in the pedicel as in the before-mentioned R. suaveolens, but there was a rapid downward movement, due to elasticity, and of this I have seen no other instance; for when the pedicel was put into water it showed no tendency to recover its original position, as occurred in many other cases.

In Phalænopsis grandiflora and amabilis the stigma is shallow and the pedicel of the rostellum long. Some compensating action is therefore requisite, which, differently from that in Maxillaria ornithorhyncha is effected by elasticity. There is no movement of depression; but, when the pollinium is removed, the straight pedicel suddenly curls up in the middle, thus ( ·–⌢–- ): the full-stop on the left hand may represent the balls of pollen, and the thick hyphen to the right may be supposed to represent the triangularly shaped disc. The pedicel does not straighten itself when placed in water. The end carrying the balls of pollen is a little raised up after this elastic movement, and the pedicel, with one end raised, and with the middle part upwardly bowed, is well adapted to drop the pollen-masses into the deep stigmatic cavity, over a ledge in front. Fritz Müller informs me of a case in which the shortening of a very long pedicel is effected partly by elasticity and partly by a hygrometric movement. A small Ornithocephalus, growing in South Brazil, has a very long pedicel, which is shown closely attached to the rostellum in the accompanying figure A.

Fig. 25.

Darwin - The various contrivances by which orchids are fertilized by insects (1877) - Fig 25.png

Pollinium of Ornithocephalus. (From a sketch by Fritz Müller.)

A. Pollinium still attached to the rostellum with the pollen-mass still lying in the clinandrum on the summit of the column.
B. Pollinium in the position which it first assumes from the elasticity of the pedicel.
C. Pollinium in the position ultimately assumed from the hygrometric movement.

The pedicel when freed suddenly bends into the form represented at B, and soon afterwards owing to the hygrometric contraction curls up into the odd figure shown at C. When placed in water it resumes the form represented at B.

In Calanthe masuca and the hybrid C. dominii the structure is very different to what it is in most other Vandeæ. We here have two oval, pit-like stigmas on each side of the rostellum (fig. 26). The viscid disc is oval (fig. B), and has no pedicel, but eight masses of pollen are attached to it by very short and

Fig. 26.

Darwin - The various contrivances by which orchids are fertilized by insects (1877) - Fig 26.png

Calanthe masuca.

s s.p. pollen-masses.
s s. the two stigmas.
s s.n. mouth of nectary.
s s.l. labellum.
s s.d. viscid disc.
s in fig. C, clinandrum the pollen-masses being removed.
A. Flower viewed from above, with the anther-case removed, showing the eight pollen-masses in their proper position within the clinandrum. All the sepals and petals have been cut away except the labellum.
B. Pollen-masses attached to the viscid disc, seen from the under side.
C. Flower in same position as in A, but with the disc and pollen-masses removed, and now showing the deeply notched rostellum and the empty clinandrum in which the pollen masses lay. Within the left-hand stigma two pollen-masses may be seen adhering to its viscid surface.

easily ruptured caudicles. These pollen-masses radiate from the disc like the leaves of a fan. The rostellum is broad, and its sides slope on each side towards the lateral pit-like stigmas. When the disc is removed the rostellum is seen (fig. C) to be deeply notched in the middle. The labellum is united to the column almost up to its summit, leaving a passage (n, A) to the long nectary close beneath the rostellum. The labellum is studded with singular, wartlike, globular excrescences.

If a thick needle be inserted into the mouth of the nectary (fig. A), and then withdrawn, the viscid disc is removed, bearing with it the elegant fan of radiating pollen-masses. These undergo no change in position. But if the needle be now inserted into the nectary of another flower, the ends of the pollen-masses necessarily hit the upper and laterally sloping sides of the rostellum, and, glancing off both ways, strike down into the two lateral pit-like stigmas. The thin caudicles being easily ruptured, the pollen-masses are left adhering like little darts to the viscid surface of both stigmas (see left-hand stigma in fig. C), and the fertilisation of the flower is completed in a simple manner pleasing to behold.

I should have stated that a narrow transverse rim of stigmatic tissue, beneath the rostellum, connects the two lateral stigmas; and it is probable that some of the middle pollen-masses may be inserted through the notch in the rostellum, so as to adhere to this rim. I am the more inclined to this opinion from having found in the elegant Calanthe vestita the rostellum extending so widely over the two lateral stigmas, that apparently all the pollen-masses must be inserted beneath its surface.

The Angræcum sesquipedale, of which the large six-rayed flowers, like stars formed of snow-white wax, have excited the admiration of travellers in Madagascar, must not be passed over. A green, whip-like nectary of astonishing length hangs down beneath the labellum. In several flowers sent me by Mr. Bateman I found the nectaries eleven and a half inches long, with only the lower inch and a half filled with nectar. What can be the use, it may be asked, of a nectary of such disproportionate length? We shall, I think, see that the fertilisation of the plant depends on this length, and on nectar being contained only within the lower and attenuated extremity. It is, however, surprising that any insect should be able to reach the nectar. Our English sphinxes have proboscides as long as their bodies; but in Madagascar there must be moths with proboscides capable of extension to a length of between ten and eleven inches! This belief of mine has been ridiculed by some entomologists, but we now know from Fritz Müller[2] that there is a sphinx-moth in South Brazil which has a proboscis of nearly sufficient length, for when dried it was between ten and eleven inches long. When not protruded it is coiled up into a spiral of at least twenty windings.

The rostellum is broad and foliaceous, and arches rectangularly over the stigma and over the orifice of the nectary: it is deeply notched by a cleft enlarged or widened at the inner end. Hence the rostellum nearly resembles that of Calanthe after the disc has been removed (see fig. 26, C). The under surfaces of both margins of the cleft, near their ends, are bordered by narrow strips of viscid membrane, easily removed; so that there are two distinct viscid discs. A short membranous pedicel is attached to the middle of the upper surface of each disc; and the pedicel carries a pollen-mass at its other end. Beneath the rostellum a narrow, ledge-like, adhesive stigma is seated.

I could not for some time underhand how the pollinia of this Orchid were removed, or how the stigma was fertilised. I passed bristles and needles down the open entrance into the nectary and through the cleft in the rostellum with no result. It then occurred to me that, from the length of the nectary, the flower must be visited by large moths, with a proboscis thick at the base; and that to drain the last drop of nectar, even the largest moth would have to force its proboscis as far down as possible. Whether or not the moth first inserted its proboscis by the open entrance into the nectary, as is most probable from the shape of the flower, or through the cleft in the rostellum, it would ultimately be forced in order to drain the nectary to push its proboscis through the cleft, for this is the straightest course; and by slight pressure the whole foliaceous rostellum is depressed. The distance from the outside of the flower to the extremity of the nectary can be thus shortened by about a quarter of an inch. I therefore took a cylindrical rod one-tenth of an inch in diameter, and pushed it down through the cleft in the rostellum. The margins readily separated, and were pushed downwards together with the whole rostellum. When I slowly withdrew the cylinder the rostellum rose from its elasticity, and the margins of the cleft were upturned so as to clasp the cylinder. Thus the viscid strips of membrane on each under side of the cleft rostellum came into contact with the cylinder, and firmly adhered to it; and the pollen-masses were withdrawn. By this means I succeeded every time in withdrawing the pollinia; and it cannot, I think, be doubted that a large moth would thus act; that is, it would drive its proboscis up to the very base through the cleft of the rostellum, so as to reach the extremity of the nectary; and then the pollinia attached to the base of its proboscis would be safely withdrawn.

I did not succeed in leaving the pollen-masses on the stigma so well as I did in withdrawing them. As the margins of the cleft rostellum must be upturned before the discs adhere to a cylindrical body, during its withdrawal, the pollen-masses become affixed some little way from its base. The two discs did not always adhere at exactly opposite points. Now, when a moth with the pollinia adhering to the base of its proboscis, inserts it for a second time into the nectary, and exerts all its force so as to push down the rostellum as far as possible, the pollen-masses will generally rest on and adhere to the narrow, ledge-like stigma which projects beneath the rostellum. By acting in this manner with the pollinia attached to a cylindrical object, the pollen-masses were twice torn off and left glued to the stigmatic surface.

If the Angræcum in its native forests secretes more nectar than did the vigorous plants sent me by Mr. Bateman, so that the nectary ever becomes filled, small moths might obtain their share, but they would not benefit the plant. The pollinia would not be withdrawn until some huge moth, with a wonderfully long proboscis, tried to drain the last drop.[3] If such great moths were to become extinct in Madagascar, assuredly the Angræcum would become extinct. On the other hand, as the nectar, at least in the lower part of the nectary, is stored safe from the depredation of other insects, the extinction of the Angræcum would probably be a serious loss to these moths. We can thus understand how the astonishing length of the nectary had been acquired by successive modifications. As certain moths of Madagascar became larger through natural selection in relation to their general conditions of life, either in the larval or mature state, or as the proboscis alone was lengthened to obtain honey from the Angræcum and other deep tubular flowers, those individual plants of the Angræcum which had the longest nectaries (and the nectary varies much in length in some Orchids), and which, consequently, compelled the moths to insert their proboscides up to the very base, would be best fertilised. These plants would yield most seed, and the seedlings would generally inherit long nectaries; and so it would be in successive generations of the plant and of the moth. Thus it would appear that there has been a race in gaining length between the nectary of the Angræcum and the proboscis of certain moths; but the Angræcum has triumphed, for it flourishes and abounds in the forests of Madagascar, and still troubles each moth to insert its proboscis as deeply as possible in order to drain the last drop of nectar.

I could add descriptions of many other curious structures in the Vandeæ, more especially from the letters of Fritz Müller with respect to those of Brazil; but the reader would be wearied. I must, however, make a few remarks on certain genera, the fertilisation of which remains a mystery, chiefly on account of the narrowness of the mouth of the stigma, as this renders the insertion of the pollen-masses extremely difficult. Two closely allied species or varieties of Acropera, viz., A. luteola and loddigesii have been observed by me during several seasons, and every detail of their structure seems as if specially adapted to render their fertilisation almost impossible. I have met with hardly any other such case, not that I fully understand the contrivances in any Orchid, for new and admirable ones become apparent, the longer I study even one of our commonest British species.

The thin and elongated rostellum of Acropera projects at right angles to the column (see diagram, fig. 23, p. 150); and the pedicel of the pollinium is of course equally long and much thinner. The disc consists of an extremely small cap, viscid within, which fits on the extremity of the rostellum. The viscid matter sets hard but slowly. The upper sepal forms a hood enclosing and protecting the column. The labellum is an extraordinary organ, baffling all description: it is articulated to the column by a thin strap, so elastic and flexible that a breath of wind sets it vibrating. It hangs downwards; and the retention of this position seems to be of importance, for the footstalk (ovarium) of each flower is curved into a semicircle, so as to compensate for the pendulous habit of the plant. The two upper petals and the lateral lobes of the labellum serve as guides leading into the hood-like upper sepal.

The pollinium, when adhering by its disc to an object, undergoes the common movement of depression; and this seems superfluous, for the stigmatic cavity lies (see diagram, fig. 23) high up at the base of the rectangularly projecting rostellum. But this is a comparatively trifling difficulty; the real difficulty lies in the orifice of the stigmatic chamber being so narrow that the pollen-masses, though consisting of thin sheets, can hardly be forced in. I repeatedly tried, and succeeded only three or four times. Even after leaving them to dry for four hours before a fire, and thus to shrink a little, I rarely succeeded in forcing them into the stigma. I examined quite young flowers and almost withered ones, for I imagined that the mouth of the chamber might be of larger size at some period of growth; but the difficulty of insertion remained the same. Now when we observe that the viscid disc is extraordinarily small, and consequently its power of attachment not so firm as with Orchids having a large disc, and that the pedicel is very long and thin, it would seem almost indispensable that the stigmatic chamber should be unusually large for the easy insertion of the pollinium, instead of being much contracted. Moreover, the stigmatic surface, as Dr. Hooker has likewise observed, is singularly little adhesive!

The flowers when ready for fertilisation do not secrete nectar;[4] but this is no difficulty, for as Dr. Crüger has seen humble-bees gnawing the projections on the labellum of the closely allied Gongora maculata, there can be little doubt that the distal cup-shaped part of the labellum of Acropera offers a similar attraction to insects. After numberless trials in many ways, I have found that the pollinia can be removed with certainty only by pushing the rostellum a little upwards with a camel-hair brush, held in such a position that the tip slides along the under side of the rostellum, so as to brush off the little viscid cap on its extremity, into which the hairs enter and are glued fast. I further find that if the brush with a pollinium thus attached to its tip is pushed into and then withdrawn from the stigmatic cavity, the mouth of which is furnished with a sharp ridge, the end of the pedicel which bears the viscid cap is often left sticking within the chamber, with the pollen-masses close outside. Many flowers were thus treated, and three of them produced fine capsules. Mr. Scott also succeeded in fertilising two flowers in the same apparently unnatural manner, as he likewise did on one occasion by placing a pollen-mass, moistened with the viscid matter from a distinct kind of Orchis, at the mouth of the stigmatic chamber. These facts lead me to suspect that an insect with the extremity of its abdomen produced into a sharp point alights on the flower, and then turns round to gnaw the distal portion of the labellum. In doing so it removes the pollinium, the viscid cap of which adheres to the extremity of its abdomen. The insect then visits another flower, by which time the movement of depression will have caused the pedicel to lie flat on its back; and from occupying the same position as before, the insect will be apt to insert the end of its abdomen into the stigmatic chamber, and the viscid cap will then be scraped off by the ledge in front, and the pollen-masses will be left close outside, as in the above experiments. The whole operation would probably be aided by the oscillatory movement of the labellum whilst gnawed by an insect. This whole view is very improbable, but it is the only one, as far as I can see, which explains the fertilisation of the flower.

The allied genera Gongora, Acineta, and Stanhopea present nearly the same difficulty from the narrowness of the entrance into the stigmatic chamber. Mr. Scott tried repeatedly but in vain to force the pollen-masses into the stigma of Gongora atro-purpurea and truncata; but he readily fertilised them by cutting off the clinandrum and placing pollen-masses on the now exposed stigma; as he likewise did in the case of Acropera. Dr. Crüger says[5] that Gongora maculata "often bears fruit in Trinidad. It is visited, exclusively during the day, as far as I can see, by a splendid bee, probably a Euglossa, but with the tongue nearly twice as long as the body. The tongue passes out behind the abdomen, and is there curved upwards. As these bees only come for biting and gnawing the anterior side of the labellum, the protruding tongue touches or approaches the gland (i. e., viscid disc) at every retrograde movement of the insect. By this it can hardly fail to be loaded sooner or later with the pollen-masses, which are then easily inserted into the stigmatic cleft. I have, however, not as yet observed this fact." I am surprised that Dr. Crüger should speak of the pollen-masses being easily inserted, and I suppose that he must have experimented with dried and shrunken ones. The doubled-up, immensely elongated proboscis, projecting beyond the abdomen, would answer as well as a pointed extremity to the abdomen, which in the case of Acropera I imagine is the instrument for removing the pollen-masses; but I presume that with Gongora it is not the viscid disc, but the broad and free ends of the pollen-masses which are inserted into the stigmatic cavity. As in the case of Acropera, I found it scarcely possible to insert the pollen-masses of Gongora into the stigma; but some which were removed from the anther and left exposed to the sun for nearly five hours, became much shrunk and formed thin sheets; and these could be inserted without much difficulty into the cleft-like entrance of the stigma. The pollinia attached to an insect flying about in the torrid zone would shrink after a time; and the delay thus caused would ensure the flowers being fertilised with pollen from a distinct plant.

With respect to Stanhopea, Dr. Crüger says[6] that in the West Indies a bee (Euglossa) often visits the flowers for the sake of gnawing the labellum, and he caught one with a pollinium attached to its back; but he adds that he cannot understand how the pollen-masses are inserted into the narrow mouth of the stigma. With Stanhopea oculata I found that the pollinia could almost always be attached to my naked or gloved finger, by gently sliding it down the concave surface of the arched column; but this occurred only within a short time after the expansion of the flowers, whilst they are highly odoriferous. By again sliding my finger down the column, the pollinia were almost always rubbed off by the sharp edge of the stigmatic chamber, and were left adhering close to its entrance. Flowers thus treated occasionally, though rarely, yielded capsules. The removal of the pollinia from my finger seemed to depend on the existence of a point projecting beyond the viscid disc, and which I suspect is specially adapted for this purpose. If this be so, the pollen-masses must emit their tubes without being inserted into the stigmatic chamber. I may add that the pollen-masses shrink very little by being thoroughly dried, and could not in this state be easily inserted.

The entrance into the stigma is in like manner, as I hear from Fritz Müller,[7] so much contracted in Cirrhæa and Notylia, which belong to another subdivision of the Vandeæ, that the pollinia can be inserted into it only with extreme difficulty. In the case of Cirrhæa, he found that this could be effected more easily, after they had shrunk a little from being left to dry for half an hour or an hour. He observed two flowers with pollen-masses naturally inserted by some means into their stigmas. On several occasions after forcing the end of a pollen-mass into the mouth of the stigma, he witnessed a most curious process of deglutition. The extremity of the pollen-mass swells from imbibing moisture, and as the chamber gradually widens downwards, the swelling part is forced downwards; so that the whole is at last drawn inwards and disappears. In the case of Notylia, Fritz Müller observed that the entrance into the stigma became a little larger after the flower had remained expanded for about a week. In whatever manner this latter plant is fertilised, it is certain that it must be impregnated with pollen from a distinct plant; as it offers one of those extraordinary cases in which its own pollen acts like poison on the stigma.

In the last edition of this work it was shown that the ovaria of mature flowers of Acropera do not contain any ovules. But I erred greatly in the interpretation of this fact, for I concluded that the sexes were separate. I was however soon convinced of my error by Mr. Scott, who succeeded in artificially fertilising the flowers with their own pollen. A remarkable discovery by Hildebrand,[8] namely, that in many Orchids the ovules are not developed unless the stigma is penetrated by the pollen-tubes, and that their development occurs only after an interval of several weeks or even months, explains the state of the ovarium in Acropera, as observed by me. According also to Fritz Müller,[9] the ovules of many endemic Epidendreæ and Vandeæ in Brazil remain in a very imperfect state of development for some months, and even in one case for half a year, after the flowers had been fertilised. He suggests that a plant which produces hundreds of thousands of ovules, would waste much power if these were formed and did not happen to be fertilised, and we know that fertilisation is a doubtful and difficult operation with many Orchids. It would therefore be an advantage to such plants, if the ovules were not at all developed until their fertilisation was assured by the pollen-tubes having already penetrated the stigma.

Coryanthes.—I will conclude this chapter by giving an account of the fertilisation of the flowers of Coryanthes, which is effected in a manner that might perhaps have been inferred from their structure, but would have appeared utterly incredible had it not been repeatedly witnessed by a careful observer, namely, the late Dr. Crüger, Director of the Botanical Gardens at Trinidad. The flowers are very large and hang downwards. The distal portion of the labellum (L) in the following woodcut, fig. 27, is converted into a large bucket (B). Two appendages (H), arising from the narrowed base of the labellum, stand directly over the bucket and secrete so much fluid that drops may be seen falling into it. This fluid is limpid and so slightly sweet that it does not deserve to be called nectar, though evidently of the same nature; nor does it serve to attract insects. M. Ménière estimates that the total quantity secreted by a single flower is about an English ounce.[10] When the bucket is full the fluid overflows by the spout (P).


Fig. 27.

Darwin - The various contrivances by which orchids are fertilized by insects (1877) - Fig 27.png

Coryanthes speciosa. (Copied from Lindley's 'Vegetable Kingdom.')

L. labellum.
B. bucket of the labellum.
H. fluid-secreting appendages.
P. spout of bucket, over-arched by the end of the column, bearing the anther and stigma.

This spout is closely over-arched by the end of the column, which bears the stigma and pollen-masses in such a position, that an insect forcing its way out of the bucket through this passage would first brush with its back against the stigma and afterwards against the viscid discs of the pollinia, and thus remove them. We are now prepared to hear what Dr. Crüger says about the fertilisation of an allied species, the C. macrantha, the labellum of which is provided with crests.[11] I may premise that he sent me specimens of the bees which he saw gnawing these crests, and they belong, as I am informed by Mr. F. Smith, to the genus Euglossa. Dr. Crüger states that these bees may be "seen in great numbers disputing with each other for a place on the edge of the hypochil (i. e. the basal part of the labellum). Partly by this contest, partly perhaps intoxicated by the matter they are indulging in, they tumble down into the 'bucket,' half-full of a fluid secreted by organs situated at the base of the column. They then crawl along in the water towards the anterior side of the bucket, where there is a passage for them between the opening of this and the column. If one is early on the look-out, as these Hymenopteræ are early risers, one can see in every flower how fecundation is performed. The humble-bee, in forcing its way out of its involuntary bath, has to exert itself considerably, as the mouth of the epichil (i. e. the distal part of the labellum) and the face of the column fit together exactly, and are very stiff and elastic. The first bee, then, which is immersed will have the gland of the pollen-mass glued to its back. The insect then generally gets through the passage, and comes out with this peculiar appendage, to return nearly immediately to its feast, when it is generally precipitated a second time into the bucket, passing out through the same opening, and so inserting the pollen-masses into the stigma while it forces its way out, and thereby impregnating either the same or some other flower. I have often seen this; and sometimes there are so many of these humble-bees assembled that there is a continual procession of them through the passage specified."

There cannot be the least doubt that the fertilisation of the flower absolutely depends on insects crawling out through the passage formed by the extremity of the labellum and the over-arching column. If the large distal portion of the labellum or bucket had been dry, the bees could easily have escaped by flying away. Therefore we must believe that the fluid is secreted by the appendages in such extraordinary quantity and is collected in the bucket, not as a palatable attraction for the bees, as these are known to gnaw the labellum, but for the sake of wetting their wings, and thus compelling them to crawl out through the passage.

I have now described, perhaps in too much detail, a few of the many contrivances by which the Vandeæ are fertilised. The relative position and shape of the parts—friction, viscidity, elastic and hygrometric movements, all nicely related to one another—come into play. But all these appliances are subordinate to the aid of insects. Without their aid, not a plant belonging to this tribe, in the species of the twenty-nine genera examined by me, would set a seed. It is also certain in a majority of the cases, that insects withdraw the pollinia only when retreating from the flower, and by carrying them away, effect a union between two flowers, generally on distinct plants. This can hardly fail to occur in all the many cases in which the pollinia slowly change their position, when removed from the rostellum, in order to assume a proper direction for striking the stigma; for the insects during this interval will have had time to fly from the flowers on one plant which will serve as the male, to those on another plant which will serve as the female.

  1. I may here remark that Delpino ('Fecondazione nelle Piante,' Firenze, 1867, p. 19) says he has examined flowers of Vanda, Oncidium, Epidendrum, Phaius, and Dendrobium, and is able to confirm in general my statements.
  2. See letter with a drawing by Hermann Müller, 'Nature,' 1878, p. 223.
  3. Mr. Belt suggests ('The Naturalist in Nicaragua,' 1874, p. 133) that the great length of the nectary of this plant serves to prevent other moths which are not well-adapted for the fertilisation of the flowers from sucking the nectar, and that its development can thus be accounted for. I have no doubt of the truth of this principle, but it is hardly applicable here, as the moth has to be compelled to drive its proboscis as deeply down as possible into the flower.
  4. Mr. Scott has observed that after the flowers of Acropera and of two species in the allied genus of Gongora have been fertilised, an abundance of nectar exudes from the front of the column; but at no other time could he find a trace of nectar. This exudation can, therefore, be of no use to the plant with respect to its fertilisation, and must be viewed as an excretion.
  5. 'Journ. Linn. Soc. Bot.' vol. viii. 1864, p. 131.
  6. 'Journ. Linn. Soc. Bot.' vol. viii. 1864, p. 130. Bronn has described the structure of Stanhopea devoniensis, in his German translation of the first edition of this work.
  7. 'Bot. Zeitung,' 1868, p. 630.
  8. 'Bot. Zeitung,' 1863, Oct. 30, et seq., and Aug. 4, 1865.
  9. 'Bot. Zeitung,' 1868, p. 164.
  10. 'Bulletin de la Soc. Bot. de France,' tom. ii. 1855, p. 351.
  11. ' Journal of Linn. Soc. Bot.' vol. viii. 1864, p. 130. There is a drawing of this species in Paxton's 'Mag. of Botany,' vol. v. p. 31, but it is too complicated to be reproduced. There is also a drawing of C. feildingii in 'Journal of Hort. Soc.' vol. iii. p. 16. I am indebted to Mr. Thiselton Dyer for informing me of these figures.