The Various Contrivances by which Orchids are Fertilised by Insects/Chapter 8

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The Various Contrivances by which Orchids are Fertilised by Insects
Charles Darwin
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CHAPTER VIII.

CYPRIPEDEÆ—HOMOLOGIES OF THE FLOWERS OF ORCHIDS.

Cypripedium, differs much from all other Orchids—Labellum in the form of a slipper with two small orifices by which insects can escape—Manner of fertilisation by small bees of the genus Andrena—Homological nature of the several parts of the flowers of the Orchideæ—Wonderful amount of modification which they have undergone.


We have now armed at Lindley's last and seventh tribe, including, according to most botanists, only a single genus, Cypripedium, which differs from all other Orchids far more than any other two of these do from one another. An enormous amount of extinction must have swept away a multitude of intermediate forms, and has left this single genus, now widely distributed, as a record of a former and more simple state of the great Orchidean Order. Cypripedium possesses no rostellum; for all three stigmas are fully developed, though confluent. The single anther, which is present in all other Orchids, is here rudimentary, and is represented by a singular shield-like projecting body, deeply notched or hollowed out on its lower margin. There are two fertile anthers which belong to an inner whorl, represented in ordinary Orchids by various rudiments. The grains of pollen are not united together by threes or fours, as in so many other genera, nor are they tied together by elastic threads, nor furnished with a caudicle, nor cemented into waxy masses. The labellum is of ​large size, and is a compound organ as in all other Orchids.

The following remarks apply only to the six species which I have examined, namely, C. barbatum, purpuratum, insigne, venustum, pubescens and acaule; though I have casually looked at some other kinds. The basal part of the labellum is folded round the short


Fig. 35.

Cypripedium.

a'.a. anther.
a'. rudimentary, shield-like anther.
a'.s. stigma.
a'.l. labellum.
A. Flower viewed from above, with the sepals and petals, excepting the labellum, partly cut off. The labellum has been slightly depressed, so that the dorsal surface of the stigma is exposed; the edges of the labellum have thus become a little separated and the toe or extremity stands lower than is natural.
B. Side view of column, with all the sepals and petals removed.


column, so that its edges nearly meet along the dorsal surface; and the broad extremity is folded over in a peculiar manner, forming a sort of shoe, which closes up the end of the flower. Hence arises the English name of Ladies'-slipper. The overarching edges of the labellum are inflected or sometimes only smooth and ​polished internally; and this is of much importance, as it prevents insects which have once entered the labellum from escaping through the great opening in the upper surface. In the position in which the flower grows, as here represented, the dorsal surface of the column is uppermost. The stigmatic surface is slightly protuberant, and is not adhesive; it stands nearly parallel to the lower surface of the labellum. With a flower in its natural state, the margin of the dorsal surface of the stigma can be barely distinguished between the edges of the labellum and through the notch in the rudimentary, shield-like anther (a'); but in the drawing (s, fig. A) the margin of the stigma has been brought outside the edges of the depressed labellum, and the toe is a little bent downwards, so that the flower is represented as rather more open than it really is. The edges of the pollen-masses of the two lateral anthers (a) can be seen through the two small orifices or open spaces in the labellum (fig. A) on each side, close to the column. These two orifices are essential for the fertilisation of the flower.

The grains of pollen are coated by and immersed in viscid fluid, which is so glutinous that it can be drawn out into short threads. As the two anthers stand behind and above the lower convex surface (see fig. B) of the stigma, it is impossible that the glutinous pollen can without some mechanical aid get on to this, the efficient surface of the stigma. The economy here shown by Nature in her manner of gaining the same end is surprising. In all the other Orchids seen by me, the stigma is viscid and more or less concave, by which means the dry pollen, transported by means of the viscid matter secreted by the rostellum or modified stigma, is retained. In Cypripedium the pollen is glutinous, and assumes the function of viscidity, which ​in all other Orchids except Vanilla belongs exclusively to the rostellum and the two confluent stigmas. These latter organs, on the other hand, in Cypripedium entirely lose their viscidity, and at the same time become slightly convex, so as more effectually to rub off the glutinous pollen adhering to the body of an insect. Moreover in several of the North American species, as in C. acaule and pubescens, the surface of the stigma is beset, as Professor Asa Gray remarks,[1] "with minute, rigid, sharp-pointed papillæ, all directed forwards, which are excellently adapted to brush off the pollen from an insect's head or back." There is one partial exception to the above rule of the pollen of Cypripedium being viscid while the stigma is not viscid and is not convex; for in C. acaule the pollen is more granular and less viscid, according to Asa Gray, than in the other American species, and in C. acaule alone the stigma is slightly concave and viscid. So that here the exception almost proves the truth of the general rule.

I have never been able to detect nectar within the labellum, and Kurr[2] makes the same remark with respect to C. calceolus. The inner surface of the labellum, however, in those species which I examined, is clothed with hairs, the tips of which secrete little drops of slightly viscid fluid. And these if sweet or nutritious would suffice to attract insects. The fluid when dried forms a brittle crust on the summits of the hairs. Whatever the attraction may be, it is certain that small bees frequently enter the labellum.

Formerly I supposed that insects alighted on the labellum and inserted their proboscides through either ​of the orifices close to the anthers; for I found that when a bristle was thus inserted the glutinous pollen adhered to it, and could afterwards be left on the stigma; but this latter part of the operation was not well effected. After the publication of my book Professor Asa Gray wrote to me[3] that he was convinced from an examination of several American species that the flowers were fertilised by small insects entering the labellum through the large opening on the upper surface, and crawling out by one of the two small orifices close to the anthers and stigma. Accordingly I first introduced some flies into the labellum of C. pubescens, through the large upper opening, but they were either too large or too stupid, and did not crawl out properly. I then caught and placed within the labellum a very small bee which seemed of about the right size, namely, Andrena parvula, and this by a strange chance proved, as we shall presently see, to belong to the genus on which in a state of nature the fertilisation of C. calceolus depends. The bee vainly endeavoured to crawl out again the same way by which it had entered, but always fell backwards, owing to the margins being inflected. The labellum thus acts like one of those conical traps with the edges turned inwards, which are sold to catch beetles and cockroaches in the London kitchens. It could not creep out through the slit between the folded edges of the basal part of the labellum, as the elongated, triangular, rudimentary stamen here closes the passage. Ultimately it forced its way out through one of the small orifices close to one of the anthers, and was found when caught to be smeared with the glutinous pollen. I then put the same bee back into the labellum; and again it crawled out through one of the small ​orifices, always covered with pollen. I repeated the operation five times, always with the same result. I afterwards cut away the labellum, so as to examine the stigma, and found its whole surface covered with pollen. It should be noticed that an insect in making its escape must first brush past the stigma and afterwards one of the anthers, so that it cannot leave pollen on the stigma, until being already smeared with pollen from one flower it enters another; and thus there will be a good chance of cross-fertilisation between two distinct plants. Delpino[4] with much sagacity foresaw that some insect would be discovered to act in this manner; for he argued that if an insect were to insert its proboscis, as I had supposed, from the outside through one of the small orifices close to one of the anthers, the stigma would be liable to be fertilised by the plant's own pollen: and in this he did not believe, from having confidence in what I have often insisted on—namely, that all the contrivances for fertilisation are arranged so that the stigma shall receive pollen from a distinct flower or plant. But these speculations are now all superfluous; for, owing to the admirable observations of Dr. H. Müller,[5] we know that Cypripedium calceolus in a state of nature is fertilised in the manner just described by bees belonging to five species of Andrena.

Thus the use of all the parts of the flower,—namely, the inflected edges, or the polished inner sides of the labellum,—the two orifices and their position close to the anthers and stigma,—the large size of the medial rudimentary stamen,—are rendered intelligible. An ​insect which enters the labellum is thus compelled to crawl out by one of the two narrow passages, on the sides of which the pollen-masses and stigma are placed. We hare seen that exactly the same end is gained in the case of Coryanthes by the labellum being half-filled with secreted fluid; and in the case of Pterostylis and some other Australian Orchids by the labellum being irritable, so that when touched by an entering insect it shuts up the flower, with the exception of a single narrow passage.[6]


Homological Nature of the several Parts of the Flowers of the Orchideæ.


The theoretical structure of few flowers has been so largely discussed as that of the Orchideæ; nor is this surprising, seeing how unlike they are to common flowers; and here will be a convenient place for considering this subject. No group of organic beings can be well understood until their homologies are made out; that is, until the general pattern, or, as it has often been called, the ideal type, of the several members of the group is intelligible. No one member may now exist exhibiting the full pattern; but this does not make the subject less important to the naturalist,—probably makes it more important for the full understanding of the group.

The homologies of any being, or group of beings, ​can be most surely made out by tracing their embryological development when that is possible; or by the discovery of organs in a rudimentary condition; or by tracing, through a long series of beings, a close gradation from one part to another, until the two parts or organs, though employed for widely different functions and most unlike each other, can be joined by a succession of short links. No instance is known of a close gradation between two organs, unless they are homologically one and the same organ.

The importance of the science of Homology rests on its giving us the key-note of the possible amount of difference in plan within any group; it allows us to class under proper heads the most diversified organs; it shows us gradations which would otherwise have been overlooked, and thus aids us in classification; it explains many monstrosities; it leads to the detection of obscure and hidden parts, or mere vestiges of parts, and shows us the meaning of rudiments. Besides these uses, Homology clears away the mist from such terms as the scheme of nature, ideal types, archetypal patterns or ideas, &c.; for these terms come to express real facts. The naturalist, thus guided, sees that all homologous parts or organs, however much they may be diversified, are modifications of one and the same ancestral organ; in tracing existing gradations he gains a clue in tracing, as far as that is possible, the probable course of modification through which beings have passed during a long line of generations. He may feel assured that, whether he follows embryological development, or searches for the merest rudiment, or traces gradations between the most different beings, he is pursuing the same object by different routes, and is tending towards the knowledge of the actual progenitor of the group, as it once grew and ​lived. Thus the subject of Homology gains largely in interest.

Although this subject, under whatever aspect it be viewed, will always be most interesting to the student of nature, it is very doubtful whether the following details on the homological nature of the flowers of Orchids will possess any interest for the general reader. If, indeed, he cares to see how much light an acquaintance with homology, though far from perfect, throws on a subject, this will, perhaps, be nearly as good an instance as could be given. He will see how curiously a flower may be moulded out of many separate organs,—how perfect the cohesion of primordially distinct parts may become,—how organs may be used for purposes widely different from their proper uses,—how other organs may be entirely suppressed, or leave mere useless emblems of their former existence. Finally, he will see how enormous has been the amount of change which these flowers have undergone from their parental or typical form.

Robert Brown first clearly discussed the homologies of Orchids,[7] and left, as might be expected, little to be done. Guided by the general structure of monocotyledonous plants and by various considerations, he propounded the doctrine that the flower properly consists of three sepals, three petals, six anthers in two whorls or circles (of which only one anther belonging to the outer whorl is perfect in all the common forms), and of three pistils, with one of them modified into the rostellum. These fifteen organs are arranged as usual, alternately, three within three, in five whorls. Of the existence of three of the anthers in two of ​the whorls, R. Brown offers no sufficient evidence, but believes that they are combined with the labellum, whenever that organ presents crests or ridges. In these views Brown is followed by Lindley.[8]

Brown traced the spiral vessels in the flower by making transverse sections,[9] and only occasionally, as far as it appears, by longitudinal sections. As spiral vessels are developed at a very early period of growth, and this circumstance always gives much value to a part in making out homologies; and as they are apparently of high functional importance, though their function is not well known, it appeared to me, guided also by the advice of Dr. Hooker, to be worth while to trace upwards all the spiral vessels from the six groups surrounding the ovarium. Of the six ovarian groups of vessels, I will call (though not correctly) that under the labellum the anterior group; that under the upper sepal the posterior group; and the two groups on the two sides of the ovarium the antero-lateral and postero-lateral groups.

The result of my dissections is given in the following diagram (fig. 36). The fifteen little circles represent ​so many groups of spiral vessels, in every case traced down to one of the six large ovarian groups. They alternate in five whorls, as represented; but I have not attempted to give the actual distances at which they stand apart. In order to guide the eye, the three central groups running to the three pistils are connected by a triangle.


Fig. 36.

Section of the Flower of an Orchid.

The little circles show the position of the spiral vessels.


a1 a2.S S. Stigmas; Sr, stigma modified into the rostellum.
a1 a2.A1. Fertile anther of the outer whorl; A2 A3, anthers of the same whorl combined with the lower petal, forming the labellum.
a1 a2. Rudimentary anthers of the inner whorl (fertile in Cypripedium), generally forming the clinandrum; a3, third anther of the same whorl, when present, forming the front of the column.


Five groups of vessels run into the three sepals together with the two upper petals; three enter the ​labellum; and seven run up the great central column. These vessels are arranged, as may be seen, in rays proceeding from the axis of the flower; and all on the same ray invariably run into the same ovarian group; thus the vessels supplying the upper sepal, the fertile anther (A1), and the upper pistil or stigma (i. e. the rostellum Sr), all unite and form the posterior ovarian group. Again, the vessels supplying, for instance, the left lower sepals, the corner of the labellum and one of the two stigmas (S) on the same side, unite and form the antero-lateral group; and so with all the other vessels.

Hence, if the existence of groups of spiral vessels can be trusted, the flower of an Orchid certainly consists of fifteen organs, in a much modified and confluent condition. We see three stigmas, with the two lower ones generally confluent, and with the upper one modified into the rostellum. We see six stamens, arranged in two whorls, with generally one alone (A1) fertile. In Cypripedium, however, two stamens of the inner whorl (a1 and a2) are fertile, and in other Orchids these two are represented more plainly in various ways than the remaining stamens. The third stamen of the inner whorl (a3), when its vessels can be traced, forms the front of the column: Brown thought that it often formed a medial excrescence, or ridge, cohering to the labellum; or, in the case of Glossodia,[10] a filamentous organ, freely projecting in front of the labellum. The former conclusion does not agree with my dissections; about Glossodia I know nothing. The two infertile stamens of the outer whorl (A2, A3) were believed by Brown to be only occasionally represented, and then by lateral excre​scences on the labellum; but I find the corresponding vessels invariably present in the labellum of every Orchid examined,—even when the labellum is very narrow or quite simple, as in Malaxis, Herminium, or Habenaria.

We thus see that an Orchid-flower consists of five simple parts, namely, three sepals and two petals; and of two compounded parts, mamely, the column and labellum. The column is formed of three pistils, and generally of four stamens, all completely confluent. The labellum is formed of one petal with two petaloid stamens of the outer whorl, likewise completely confluent. I may remark, as making this fact more probable, that in the allied Marantaceæ the stamens, even the fertile stamens, are often petaloid, and partially cohere. This view of the nature of the labellum explains its large size, its frequently tripartite form, and especially the manner of its coherence to the column, unlike that of the other petals.[11] As rudimentary organs vary much, we can thus perhaps understand the variability, which as Dr. Hooker informs me is characteristic of the excrescences on the labellum. In some Orchids which have a spur-like nectary, the two sides are apparently formed by the two modified stamens; thus in Gymnadenia conopsea (but not in Orchis pyramidalis), the vessels, proceeding from the two antero-lateral ovarian groups, run down the sides of the nectary; those from the single anterior group run down the exact middle of the nectary, then returning up the opposite side form the mid-rib of the labellum. The sides of the nectary being thus formed of two distinct organs, apparently explains the tendency, ​as in Calanthe, Orchis mono, &c., to the bifurcation of its extremity.

The number, position, and course of all the spiral vessels exhibited in the diagram (fig. 36) were observed in some Vandeæ and Epidendreæ.[12] In the Malaxeæ ​all were observed excepting a3, which is the most difficult one to trace, and apparently is oftenest absent. In the Cypripedeæ, again, all were traced except a3,[13] which, I feel pretty sure, was here really absent: in this tribe the stamen (A1) is represented by a conspicuous shield-like rudiment, and a1 and a2 are developed into two fertile anthers. In the Ophreæ and Neotteæ all were traced, with the important exception of the vessels belonging to the three stamens (a1, a2, and a3) of the inner whorl. In Cephalanthera grandiflora, I clearly saw a3 proceeding from the anterior ovarian group, and running up the front of the column. This anomalous Orchid has no rostellum, and the vessel marked Sr in the diagram was entirely absent, though seen in every other species.

Although the two anthers (a1 and a2) of the inner whorl are not fully and normally developed in any Orchid, excepting Cypripedium, their rudiments are generally present and are often utilised; for they often form the membranous sides of the cup-like clinandrum on the summit of the column, which includes and protects the pollen-masses. These rudiments thus aid their fertile brother-anther. In the young flower-bud ​of Malaxis paludosa, the close resemblance between the two membranes of the clinandrum and the fertile anther, in shape, texture, and in the height to which the spiral vessels extended, was most striking: it was impossible to doubt that in these two membranes we had two rudimentary anthers. In Evelyna, one of the Epidendreæ, the clinandrum was similarly formed, as were the horns of the clinandrum in Masdevallia, which serve in addition to keep the labellum at the proper distance from the column. In Liparis pendula and some other species, these two rudimentary anthers form not only the clinandrum, but likewise wings, which project on each side of the entrance into the stigmatic cavity, and serve as guides for the insertion of the pollen-masses. In Acropera and Stanhopea, as far as I could make out, the membranous borders of the column, down to its base, were also thus formed; but in other cases, as in Cattleya, the wing-like borders of the column seem to be simple developments of the two pistils. In this latter genus, as well as in Catasetum, these same two rudimentary stamens, judging from the position of the vessels, serve chiefly to strengthen the back of the column; and the strengthening of the front of the column is the sole function of the third stamen of the inner whorl (a3), in those cases in which it was observed. This third stamen runs up the middle of the column to the lower edge, or lip, of the stigmatic cavity.

I have said that in the Ophreæ and Neotteæ the spiral vessels of the inner whorl, marked a1, a2, a3 in the diagram, are entirely absent, and I looked carefully for them; but in nearly all the members of these two tribes, two small papillæ, or auricles as they have been often called, stand in exactly the position which the two first of these three anthers would have occupied, ​had they been developed. Not only do they stand in this position, but the column in some cases, as in Cephalanthera, has on each side a prominent ridge, running from them to the bases or mid-ribs of the two upper petals; that is, in the proper position of the filaments of these two stamens. It is, again, impossible to doubt that the two membranes of the clinandrum in Malaxis are formed by these two anthers in a rudimentary and modified condition. Now, from the perfect clinandrum of Malaxis, through that of Spiranthes, Goodyera, Epipactis latifolia, and E. palustris (see fig. 16, p. 101, and fig. 15, p. 94), to the minute and slightly flattened auricles in the genus Orchis, a perfect gradation can be traced. Hence I conclude that these auricles are doubly rudimentary; that is they are rudiments of the membranous sides of the clinandrum, these membranes themselves being rudiments of the two anthers so often referred to. The absence of spiral vessels running to the auricles is by no means sufficient to overthrow the views here advocated as to the much disputed nature of these structures; that such vessels may quite disappear, we have proof in Cephalanthera grandiflora, in which the rostellum and its vessels are completely aborted.

Finally, then, with respect to the six stamens which ought to be represented in every Orchid: the three belonging to the outer whorl are always present, the upper one being fertile (except in Cypripedium), and the two lower ones invariably petaloid and forming part of the labellum. The three stamens of the inner whorl are less plainly developed, especially the lower one, a3, which, when it can be detected, serves only to strengthen the column, and, in some rare cases, according to Brown, forms a separate projection or filament; the two upper anthers of this inner whorl are fertile ​in Cypripedium, and in other cases are generally represented either by membranous expansions, or by minute auricles without spiral vessels. These auricles, however, are sometimes quite absent, as in some species of Ophrys.

On this view of the homologies of Orchid-flowers, we can understand the existence of the conspicuous central column,—the large size, generally tripartite form, and peculiar manner of attachment of the labellum,—the origin of the clinandrum,—the relative position of the single fertile anther in most of the genera, and of the two fertile anthers in Cypripedium,—the position of the rostellum, as well as of all the other organs,—and lastly, the frequent occurrence of a bilobed stigma, and the occasional occurrence of two distinct stigmas. I have encountered only one case of difficulty, namely in Habenaria and the allied genus, Bonatea. These flowers have undergone such an extraordinary amount of distortion, owing to the wide separation of their anther-cells and of the two viscid discs of the rostellum, that any anomaly in them is the less surprising. The anomaly relates only to the vessels supplying the sides of the upper sepal and of the two upper petals; for the vessels running into their midribs and into all the other more important organs pursue the same identical course as in the other Ophreæ. The vessels which supply the sides of the upper sepal, instead of uniting with the midrib and entering the posterior ovarian group, diverge and enter the postero-lateral groups. Again, the vessels on the anterior side of the two upper petals, instead of uniting with those of the midrib and entering the postero-lateral ovarian groups, diverge, or wander from their proper course, and enter the antero-lateral groups.

This anomaly is so far of importance, as it throws ​some doubt on the view that the labellum is always an organ compounded of one petal and two petaloid stamens; for if any one were to assume that from some unknown cause the lateral vessels of the lower petals had diverged in an early progenitor of the Orchidean order from their proper course into the antero-lateral ovarian groups, and that this structure had been inherited by all existing Orchids, even by those with the smallest and simplest labellums, I could answer only as follows; but the answer is, I think, satisfactory. From the analogy of other monocotyledonous plants, we might expect the hidden presence of fifteen organs in the flowers of the Orchideæ, arranged alternately in five whorls; and in these flowers we find fifteen groups of vessels exactly thus arranged. Hence there is a strong probability that the vessels, A2 and A3, which enter the sides of the labellum, not in one or two cases, but in all the Orchids seen by me, and which occupy the precise position which they would have occupied had they supplied two normal stamens, do really represent modified and petaloid stamens, and are not lateral vessels of the labellum which have wandered from their proper course. In Habenaria and Bonatea,[14] on the other hand, the vessels proceeding ​from the sides of the upper sepal and of the two upper petals, which enter the wrong ovarian groups, cannot possibly represent any lost but once distinct organs.

We have now finished with the general homologies of the flowers of Orchids. It is interesting to look at one of the magnificent exotic species, or, indeed, at one of our humblest forms, and observe how profoundly it has been modified, as compared with all ordinary flowers,—with its great labellum, formed of one petal and two petaloid stamens,—with its singular pollen-masses, hereafter to be referred to,—with its column formed of seven cohering organs, of which three alone perform their proper function, namely, one anther and two generally confluent stigmas,—with the third stigma modified into the rostellum and incapable of being fertilised,—and with three of the anthers no longer functionally active, but serving either to protect the pollen of the fertile anther, or to strengthen the column, or existing as mere rudiments, or entirely suppressed. What an amount of modification, cohesion, abortion, and change of function do we here see! Yet hidden in that column, with its surrounding petals and sepals, we know that there are fifteen groups of vessels, arranged three within three, in alternate order, which probably have been preserved to the present time from being developed at a very early period of growth, before the shape or existence of any part of the flower is of importance for the well-being of the plant.

Can we feel satisfied by saying that each Orchid was created, exactly as we now see it, on a certain "ideal type;" that the omnipotent Creator, having fixed on one plan for the whole Order, did not depart from this ​plan; that he, therefore, made the same organ to perform diverse functions—often of trifling importance compared with their proper function—converted other organs into mere purposeless rudiments, and arranged all as if they had to stand separate, and then made them cohere? Is it not a more simple and intelligible view that all the Orchideæ owe what they have in common, to descent from some monocotyledonous plant, which, like so many other plants of the same class, possessed fifteen organs, arranged alternately three within three in five whorls; and that the now wonderfully changed structure of the flower is due to a long course of slow modification,—each modification having been preserved which was useful to the plant, during the incessant changes to which the organic and inorganic world has been exposed?

  1. ↑ 'American Journal of Science,' vol. xxxiv, 1862, p. 428
  2. ↑ 'Bedeutung der Nektarien,' 1833, p. 29.
  3. ↑ See also 'American Journal of Science,' vol. xxxiv. 1862, p, 427.
  4. ↑ 'Fecondazione nelle Piante Antocarpee,' 1867, p. 20.
  5. ↑ 'Verh. d. Nat. Ver. für Pr. Rheinland und Westfal.' Jahrg. xxv. III. Folge, v. Bd. p. 1: see also 'Befruchtung der Blumen,' 1873, p. 76.
  6. ↑ Selenipedium palmifolium is one of the Cypripedeæ, and according to Dr. Crüger ('Journ. Linn. Soc. Bot.' vol. viii. 1864, p. 134) bears very fragrant flowers, which "in all probability are always impregnated by insects. The labellum is, like some Aristolochia-flowers, constructed after the fish-pot system, i. e. a funnel-shaped opening conducts into it, and insects find it difficult to escape through the same. The only other opening near the base of the labellum is partly closed by the sexual apparatus, and the insect has to force its way out there."
  7. ↑ I believe his latest views are given in his celebrated paper, read Nov. 1–15, 1831, and published in the 'Linnean Transactions,' vol. xvi. p. 685.
  8. ↑ Professor Asa Gray has described in the 'American Journal of Science,' July 1866, a monstrous flower of Cypripedium candidum, and remarks on it, "here we have (and perhaps the first direct) demonstration that the orchideous type of flower has two staminal verticils, as Brown always insisted." Dr. Crüger also advances evidence ('Journ. Linn. Soc. Bot.' vol. viii. 1864:, p. 132) in favour of the presence of five whorls of organs; but he denies that the homologies of the parts can be deduced from the course of the vessels, and he does not admit that the labellum is formed by the union of one petal with two petaloid stamens.
  9. ↑ 'Linn. Transact.' vol. xvi. p. 696–701. Link in his 'Bemerkungen über der Bau der Orchideen' ('Botanische Zeitung,' 1849, p. 745) seems to have also trusted to transverse sections. Had he traced the vessels upwards I cannot believe that he would have disputed Brown's view of the nature of the two anthers in Cypripedium. Brongniart in his admirable paper ('Annales des Sciences Nat.' tom. xxiv. 1831) incidentally shows the course of some of the spiral vessels.
  10. ↑ See Brown's observations under Apostasia in Wallich's 'Plantæ Asiaticæ rariores,' 1830, p. 74.
  11. ↑ Link remarks on the manner of coherence of the labellum to the column in his "Bemerkungen" in 'Bot. Zeitung,' 1849, p. 745.
  12. ↑ It may be advisable to give a few details on the flowers which I dissected; but I looked to special points, such as the course of the vessels in the labellum, in many cases not worth here giving. In the Vandeæ I traced all the vessels in Catasetum tridentatum and saccatum; the great group of vessels going to the rostellum separate (as likewise in Mormodes) from the posterior ovarian group, beneath the bifurcation supplying the upper sepal and fertile anther; the anterior ovarian group runs a little way along the labellum before it bifurcates and sends a group (a3) up the front of the column; the vessels proceeding from the postero-lateral group run up the back of the column, on each side of those running to the fertile anther, and do not go to the edges of the clinandrun. In Acropera luteola the base of the column, where the labellum is attached, is much produced, and the vessels of the whole anterior ovarian group are similarly produced; those (a3) going up the front of the column are abruptly reflected back; the vessels at the point of reflexion are curiously hardened, flattened, and produced into odd crests and points. In an Oncidium I traced the vessels Sr to the viscid gland of the pollinium. Among the Epidendreæ I traced all the vessels in a Cattleya; and all in Evelyna carivata except a3, which I did not search for. In the Malaxeæ I traced all in Liparis pendula except a3, which I do not believe is present. In Malaxis paludosa I traced nearly all the vessels. In Cypripedium barbatum and purpuratum I traced all except a3, which I am nearly sure does not exist. In the Neotteæ I traced in Cephalanthera grandiflora all the vessels, excepting that to the aborted rostellum and those to the two auricles a1 and a2, which were certainly absent. In Epipactis I traced all excepting a1, a2, and a3, which are certainly absent. In Spiranthes autumnalis the vessel Sr runs to the bottom of the fork of the rostellum: there are no vessels to the membranes of the clinandrum in this Orchid nor in Goodyera. In none of the Ophreæ do the vessels a1, a2, and a3 occur. In Orchis pyramidalis I traced all the others, including two to the two separate stigmas: in this species the contrast between the vessels of the labellum and of the other sepals and petals is striking, as in the latter the vessels do not branch, whilst the labellum has three vessels the lateral ones running of course into the antero-lateral ovarian group. In Gymnadenia conopsea I traced all the vessels; but I am not sure whether the vessels supplying the sides of the upper sepal do not, as in the allied Habenaria, wander from their proper course and enter the postero-lateral ovarian group: the vessel Sr, going to the rostellum, enters the little folded crest of membrane, which projects between the bases of the anther-cells. Lastly, in Habenaria chlorantha I traced all the vessels, excepting as in the other Ophreæ the three of the inner staminal whorl, and I looked carefully for a3: the vessel supplying the fertile anther runs up the connective membrane between the two anther-cells, but does not bifurcate: the vessel to the rostellum runs up to the top of the shoulder or ledge beneath the connective membrane of the anther, but does not bifurcate and extend to the two widely-separated viscid discs.
  13. ↑ From Irmisch's ('Beiträge zur Biologie der Orchideen,' 1853, pp. 78 and 42) description of the development of the flower-bud of Cypripedium, it would appear that there is a tendency to the formation of a free filament in front of the labellum, as in the case of Glossodia before mentioned; and this will perhaps account for the absence of spiral vessels, proceeding from the anterior ovarian group and coalescing with the column. In Uropedium, a genus which A. Brongniart ('Annal. des. Sc. Nat.,' 3rd series, Bot. tom. xiii. p. 114) considers closely allied to, and even perhaps a monstrosity of, Cypripedium, a third fertile anther occupies this same position.
  14. ↑ In Bonatea speciosa, of which I have examined only dry specimens sent me by Dr. Hooker, the vessels from the sides of the upper sepal enter the postero-lateral ovarian group, exactly as in Habenaria. The two upper petals are divided down to their bases, and the vessels supplying the anterior segment and those supplying the anterior portion of the posterior segment unite and then run, as in Habenaria, into the antero-lateral (and therefore wrong) group. The anterior segments of the two upper petals cohere with the labellum, causing it to have five segments, which is a most unusual fact. The two wonderfully protuberant stigmas also cohere to the upper surface of the labellum; and the lower sepals apparently also cohere to its under side. Consequently a section of the base of the labellum divides one lower petal, two petaloid anthers, portions of the two upper petals, and apparently of the two lower sepals and the two stigmas: altogether the section passes through the whole of or through portions of either seven or nine organs. The base of the labellum is here as complex an organ as the column of other Orchids.
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