The Various Contrivances by which Orchids are Fertilised by Insects/Chapter 1
Throughout the following volume I have followed, as far as I conveniently could, the arrangement of the Orchideæ given by Lindley. The British species belong to five of his tribes, the Ophreæ, Neotteæ, Arethuseæ, Malaxeæ and Cypripedeæ, but the two latter tribes contain each only a single genus. Various British and foreign species belonging to the several tribes are described in the first eight chapters. The eighth also contains a discussion on the homologies of the flowers of the Orchideæ. The ninth chapter is devoted to miscellaneous and general considerations.
The Ophreæ include most of our common British species, and we will begin with the genus Orchis. The reader may find the following details rather difficult to understand; but I can assure him, if he will have patience to make out the first case, the succeeding ones will be easily intelligible. The accompanying diagrams (fig. 1, p. 8) show the relative position of the more important organs in the flower of the Early Orchis (O. mascula). The sepals and the petals have been removed, excepting the labellum with its nectary. The nectary is shown only in the side view (n, fig. A); for its enlarged orifice is almost hidden in shade in the front view (B). The stigma (s) is bilobed, and consists of two almost confluent stigmas; it lies under the pouch-formed rostellum (r). The anther (a, in B and A) consists of two rather widely separated cells, which are longitudinally open in front: each cell includes a pollen-mass or pollinium.
A pollinium removed out of one of the two anther-cells is represented by fig. C; it consists of a number of wedge-formed packets of pollen-grains (see fig. F, in which the packets are forcibly separated), united together by excessively elastic, thin threads. These threads become confluent at the lower end of each pollen-mass, and compose the straight elastic caudicle (c, C). The end of the caudicle is firmly attached to the viscid disc (d, C), which consists (as may be seen in the section of the pouch-formed rostellum, fig. E) of a minute oval piece of membrane, with a ball of viscid matter on its under side. Each pollinium has its separate disc; and the two balls of viscid matter lie enclosed together (fig. D) within the rostellum.The rostellum is a nearly spherical, somewhat pointed projection (r, figs. A and B) overhanging the two almost confluent stigmas, and must be fully described, as every detail of its structure is full of significance. A section through one of the discs and balls of viscid matter is given (fig. E); and a front view of both viscid within the rostellum (fig. D) is likewise given. This latter figure (D) probably best serves to explain the structure of the rostellum; but it must be understood that the front lip is here considerably depressed. The lowest part of the anther is united to the back of the rostellum, as may be seen in fig. B. At an early period of growth the rostellum
Description of Fig. 1.
|a. anther, consisting of two cells.||n. nectary.|
|r. rostellum.||p. pollen-mass.|
|s. stigma.||c. caudicle of pollinium.|
|l. labellum.||d. viscid disc of pollinium.|
B. Front view of flower, with all sepals and petals removed, except the labellum.
C. One pollinium, showing the packets of pollen-grains, the caudicle, and viscid disc.
D. Front view of the caudicles of both pollinia with the discs lying within the rostellum, its lip being depressed.
E. Section through one side of the rostellum, with the included disc and caudicle of one pollinium, lip not depressed.
F. Packets of pollen-grains, tied together by elastic threads, here extended. (Copied from Bauer.)
consists of a mass of polygonal cells, full of brownish matter, which cells soon resolve themselves into two balls of extremely viscid semi-fluid matter, void of structure. These viscid masses are slightly elongated, almost flat on the top, and convex below. They lie quite free within the rostellum (being surrounded by fluid), except at the back, where each viscid ball adheres to a small portion or disc of the exterior membrane of the rostellum. The ends of the two caudicles are strongly attached externally to these two little discs of membrane.
The membrane forming the whole exterior surface of the rostellum is at first continuous; but as soon as the flower opens the slightest touch causes it to rupture transversely in a sinuous line, in front of the anther-cells and of the little crest or fold of membrane (see fig. D) between them. This act of rupturing makes no difference in the shape of the rostellum, but converts the front part into a lip, which can be depressed easily. This lip is represented considerably depressed in fig. D, and its edge is seen, fig. B, in the front view. When the lip is thoroughly depressed, the two balls of viscid matter are exposed. Owing to the elasticity of the hinder part, the lip or pouch, after being pressed down, springs up again and encloses the two viscid balls.
I will not affirm that the rupturing of the exterior membrane of the rostellum never takes place spontaneously; and no doubt the membrane is prepared for rupture by having become very weak along defined lines; but several times I saw the act ensue from an excessively slight touch—so slight that I conclude that the action is not simply mechanical, but, for the want of a better term, may be called vital. We shall hereafter meet with other cases, in which the slightest touch or the vapour of chloroform causes the exterior membrane of the rostellum to rupture along certain defined lines.
At the same time that the rostellum becomes transversely ruptured in front, it probably (for it was impossible to ascertain this fact from the position of the parts) ruptures behind in two oval lines, thus separating and freeing from the rest of the exterior surface of the rostellum the two little discs of membrane, to which the two caudicles are attached externally, and to which the two balls of viscid matter adhere internally. The line of rupture is thus very complex, but strictly defined.
As the two anther-cells are open longitudinally in front from top to bottom, even before the flower expands, it follows that as soon as the rostellum is properly ruptured from the effects of a slight touch, its lip can be depressed easily, and, the two little discs of membrane being already separate, the two pollinia now lie absolutely free, but are still embedded in their proper places. So that the packets of pollen and the caudicles still lie within the anther-cells; the discs still form part of the rostellum, but are separate; and the balls of viscid matter still lie concealed within the rostellum.
Now let us see in the case of Orchis mascula (fig. 1) how this complex mechanism acts. Suppose an insect to alight on the labellum, which forms a good landing-place, and to push its head into the chamber (see side view, A, or front view, B), at the back of which lies the stigma (s), in order to reach with its proboscis the end of the nectary; or, which does equally well to show the action, push very gently a sharply-pointed common pencil into the nectary. Owing to the pouch-formed rostellum projecting into the gangway of the nectary, it is scarcely possible that any object can be pushed into it without the rostellum being touched. The exterior membrane of the rostellum then ruptures in the proper lines, and the lip or pouch is easily depressed. When this is effected, one or both of the viscid balls will almost infallibly touch the intruding body. So viscid are these balls that whatever they touch they firmly stick to. Moreover the viscid matter has the peculiar chemical quality of setting, like a cement, hard and dry in a few minutes' time. As the anther-cells are open in front, when the insect withdraws its head, or when the pencil is withdrawn, one pollinium, or both, will be withdrawn, firmly cemented to the object, projecting up like horns, as shown (fig. 2)
by the upper figure, A. The firmness of the attachment of the cement is very necessary, for if the pollinia were to fall sideways or backwards they could never fertilise the flower. From the position in which the two pollinia lie in their cells, they diverge a little when attached to any object. Now suppose that the insect flies to another flower, or let us insert the pencil (A, fig. 2), with the attached pollinium, into the same or into another nectary: by looking at the diagram (fig. 1, A) it will be evident that the firmly attached pollinium will be simply pushed against or into its old position, namely, into the anther-cell. How then can the flower be fertilised? This is effected by a beautiful contrivance: though the viscid surface remains immovably affixed, the apparently insignificant and minute disc of membrane to which the caudicle adheres is endowed with a remarkable power of contraction (as will hereafter be more minutely described), which causes the pollinium to sweep through an angle of about ninety degrees, always in one direction, viz., towards the apex of the proboscis or pencil, in the course of thirty seconds on an average. The position of the pollinium after the movement is shown at B in fig. 2. After this movement, completed in an interval of time which would allow an insect to fly to another plant, it will be seen, by turning to the diagram (fig. 1, A), that, if the pencil be inserted into the nectary, the thick end of the pollinium now exactly strikes the stigmatic surface.
Here again comes into play another pretty adaptation, long ago noticed by Robert Brown. The stigma is very viscid, but not so viscid as when touched by a pollinium to pull the whole off an insect's head or off a pencil, yet sufficiently viscid to break the elastic threads (fig. 1, F) by which the packets of pollen-grains are tied together, and leave some of them on the stigma. Hence a pollinium attached to an insect or to a pencil can be applied to many stigmas, and will fertilise all. I have often seen the pollinia of Orchis pyramidalis adhering to the proboscis of a moth, with the stump-like caudicles alone left, all the packets of pollen having been left glued to the stigmas of the successively visited flowers.
One or two other little points must be noticed. The balls of viscid matter within the pouch-formed rostellum are surrounded with fluid; and this is very important, for, as already mentioned, the viscid matter sets hard when exposed to the air for a very short time. I have pulled the balls out of their pouches, and found that they had entirely lost the power of adhesion after a few minutes. Again, the little discs of membrane, the movement of which, as causing the movement of the pollinia, is so absolutely indispensable for the fertilisation of the flower, lie at the upper and back surface of the rostellum, and are closely enfolded and thus kept damp within the bases of the anther-cells; and this is very necessary, as an exposure of about thirty seconds causes the movement of depression to take place; but as long as the disc is kept damp, the pollinia remain ready for action whenever removed by an insect.
Lastly, as I have shown, the pouch, after being depressed, springs up to its former position; and this is likewise of great service; for if this action did not take place, and an insect after depressing the lip failed to remove the two viscid balls, or if it removed one alone, then in the first case both, and in the second case one would be left exposed to the air; consequently one or both would quickly lose all adhesiveness, and the pollinium would be rendered absolutely useless. That with many kinds of Orchids insects often remove only one of the two pollinia at a time is certain; it is even probable that they generally remove only one, for the lower and older flowers almost always have both pollinia removed, whilst the younger flowers close beneath the buds, which will have been seldomer visited, have frequently only one pollinium removed. In a spike of Orchis maculata, I found as many as ten flowers, chiefly the upper ones, which had only one pollinium removed; the other pollinium being still in its proper place with the lip of the rostellum well closed up; so that all the mechanism was perfect for its subsequent removal by some other insect.
When the first edition of this book was published, I had not seen any insects visiting the flowers of the present species; but a friend watched some plants, and saw them visited by several humble-bees, apparently Bombus muscorum; and Dr. H. Müller has seen four other species of Bombus at work. He caught ninety-seven specimens, and of these thirty-two had pollinia attached to their heads.
The description now given of the action of the organs in Orchis mascula applies to O. morio, fusca, maculata, and latifolia. These species present slight and apparently co-ordinated differences in the length of their caudicles, in the direction of the nectary, in the shape and position of the stigma, but they are not worth detailing. In all, the pollinia when removed from the anther-cells undergo the curious movement of depression, which is so necessary to place them in a right position on an insect's head for striking the stigmatic surface of another flower. Six species of humble-bees, the hive-bee and two other kinds have been seen by H. Müller and myself visiting the flowers of Orchis morio. On some of the hive-bees from ten to sixteen pollen-masses adhered; to the head of Eucera longicornis eleven, to the head of Osmia rufa several, and several to the bare surface close above the mandibles of Bombus muscorum. H. Müller has seen twelve different kinds of bees visiting the flowers of O. latifolia, which are also visited by Diptera. My son George observed for some time plants of O. maculata, and saw many specimens of a fly (Empis livida) inserting their proboscides into the nectary; and subsequently the same fact was observed by me. He brought home six specimens of this Empis, with pollinia attached to their spherical eyes, on a level with the bases of the antennæ. The pollinia had undergone the movement of depression, and stood a little above and parallel to the proboscis: hence they were in a position excellently adapted to strike the stigma. Six pollinia were thus attached to one specimen, and three to another. My son also saw another and smaller species (Empis pennipes) inserting its proboscis into the nectary; but this species did not act so well or so regularly as the other in fertilising the flowers. One specimen of this latter Empis had five pollinia, and a second had three pollinia, attached to the dorsal surface of its convex thorax. H. Müller has seen two other genera of Diptera at work on this orchis, with pollinia attached to the front part of their bodies; and on one occasion he saw a humble-bee visiting the flowers.
We now come to Orchis (sub-genus, Anacamptis) pyramidalis, one of the most highly organised species which I have examined, and which is ranked by several botanists as a distinct genus. The relative position of the parts (fig. 3) is here considerably different from what it is in O. mascula and its allies. There are two quite distinct rounded stigmatic surfaces (s, s, A) placed on each side of the pouch-formed rostellum. This latter organ, instead of standing some height above the nectary, is brought down (see side view B) so as to overhang and partially to close its orifice. The ante-chamber to the nectary, formed by the union of the edges of the labellum to the column, which is large in O. mascula and its allies, is here small. The pouch-formed rostellum is hollowed out on the under side in the middle: it is filled with fluid. The viscid disc is single and of the shape of a saddle (figs. C and E); it carries on its nearly flat top or seat the two caudicles of the pollinia, the ends of which firmly adhere to its upper surface. Before the membrane of the rostellum ruptures, the saddle-formed disc can be clearly seen to be continuous with the rest of the surface. The disc is partially hidden and kept damp (which is of great importance) by the over-folding bases of the two anther-cells. It consists of several layers of minute cells, and is therefore rather thick; it is lined beneath with a layer of highly adhesive matter, which is formed within the rostellum. It corresponds strictly to the two minute, oval, separate discs to which the two caudicles of O. mascula and its allies are attached.
When the flower opens and the rostellum has become symmetrically ruptured, either from a touch or spontaneously (I know not which), the slightest pressure depresses the lip, that is, the lower and bilobed portion of the exterior membrane of the rostellum, which projects into the mouth of the nectary.
Description of Fig. 3.
|s, s.||stigma.||l′.||guiding plate on the labellum.|
B. Side view, with all the sepals and petals removed, with the labellum longitudinally bisected, and with the near side of the upper part of the nectary cut away.
C. The two pollinia attached to the saddle-shaped viscid disc.
D. The disc after the first act of contraction, with no object seized.
E. The disc seen from above, and flattened by force, with one pollinium removed; showing a depression in its surface, by which the second movement of the pollinium is effected.
F. The pollinia removed by the insertion of a needle into the nectary, after the saddle has clasped the needle by the first act of contraction.
G. The same pollinia after the second movement and their consequent depression.
When the lip is depressed, the under and viscid surface of the disc, still remaining in its proper place, is uncovered, and is almost certain to adhere to the touching object. Even a human hair, when pushed into the nectary, is stiff enough to depress the lip or pouch; and the viscid surface of the saddle adheres to it. If, however, the lip be pushed only slightly, it springs back and recovers the under side of the saddle.
The perfect adaptation of the parts is well shown by cutting off the end of the nectary and inserting a bristle at that end; consequently in a reversed direction to that in which moths insert their proboscides; and it will be found that the rostellum may easily be torn or penetrated, but that the saddle is rarely or never caught. When the saddle together with the pollinia is removed on a bristle, the under lip instantly curls closely inwards, and leaves the orifice of the nectary more open than it was before; but whether this is of much service to the moths which frequent the flowers, and consequently to the plant, I will not pretend to decide.
Lastly, the labellum is furnished with two prominent ridges (l′, figs. A, B), sloping down to the middle and expanding outwards like the mouth of a decoy; these ridges serve to guide any flexible body, like a fine bristle or hair, into the minute and rounded orifice of the nectary, which, small as it already is, is partly choked up by the rostellum. This contrivance of the guiding ridges may be compared to the little instrument sometimes used for guiding a thread into the fine eye of a needle.
Now let us see how these parts act. Let a moth insert its proboscis (and we shall presently see how frequently the flowers are visited by Lepidoptera) between the guiding ridges of the labellum, or insert a fine bristle, and it is conducted safely to the minute orifice of the nectary, and can hardly fail to depress the lip of the rostellum; this being effected, the bristle comes into contact with the now naked and sticky under surface of the suspended saddle-formed disc. When the bristle is removed, the saddle with the attached pollinia is removed. Almost instantly, as soon as the saddle is exposed to the air, a rapid movement takes place, and the two flaps curl inwards, and embrace the bristle. When the pollinia are pulled out by their caudicles, by a pair of pincers, so that the saddle has nothing to clasp, I observed that the flaps curled inwards so as to touch each other in nine seconds (see fig. D), and in nine more seconds the saddle was converted by the flaps curling still more inwards into an apparently solid ball. The proboscides of the many moths which I have examined, with the pollinia of this Orchis attached to them, were so thin that the tips of the flaps just met on the under side. Hence a naturalist, who sent me a moth with several saddles attached to its proboscis, and who did not know of this movement, very naturally came to the extraordinary conclusion that the moth had cleverly bored through the exact centres of the so-called sticky glands of some Orchid.
Of course this rapid clasping movement helps to fix the saddle upright on the proboscis, which is very important; but the viscid matter setting hard rapidly would probably suffice for this end, and the real object gained by the clasping or curling movement is the divergence of the pollinia. The pollinia, being attached to the flat top or seat of the saddle, project at first straight up and nearly parallel to each other; but as the flat top curls round the cylindrical and thin proboscis, or round a bristle, the pollinia necessarily diverge. As soon as the saddle has clasped the bristle and the pollinia have diverged, a second movement commences, which action, like the last, is exclusively due to the contraction of the saddle-shaped disc of membrane, as will be more fully described in the ninth chapter. This second movement is the same as that in O. mascula and its allies, and causes the divergent pollinia, which at first projected at right angles to the needle or bristle (see fig. F), to sweep through an angle of nearly ninety degrees towards the tip of the needle (see fig. G), so as to become depressed and finally to lie in the same plane with the needle. In three specimens, this second movement was effected in from thirty to thirty-four seconds after the removal of the pollinia from the anther-cells, and therefore in about fifteen seconds after the saddle had clasped the bristle.
The use of this double movement becomes evident if a bristle with pollinia attached to it, which have diverged and become depressed, be pushed between the guiding ridges of the labellum into the nectary of the same or another flower (compare figs. A and G); for the two ends of the pollen-masses will be found now to have acquired such a position that the end of the one strikes against the stigma on the one side, and the end of the other at the same moment strikes against the stigma on the opposite side. The secretion on the stigmas is so viscid that when the pollinia are withdrawn, the elastic threads by which the packets of pollen are bound together are ruptured; and some dark-green grains may be seen, even by the naked eye, remaining on the two white stigmatic surfaces. I have shown this little experiment to several persons, and all have expressed the liveliest admiration at the perfection of the contrivance by which this Orchid is fertilised.
As in no other plant, or indeed in hardly any animal, can adaptations of one part to another, and of the whole to other organisms widely remote in the scale of nature, be named more perfect than those presented by this Orchis, it may be worth while briefly to sum them up. As the flowers are visited both by day and night-flying Lepidoptera, it is not fanciful to believe that the bright-purple tint (whether or not specially developed for this purpose) attracts the day-fliers, and the strong foxy odour the night-fliers. The upper sepal and two upper petals form a hood protecting the anther and stigmatic surfaces from the weather. The labellum is developed into a long nectary in order to attract Lepidoptera, and we shall presently give reasons for suspecting that the nectar is purposely so lodged that it can be sucked only slowly (very differently from what occurs in most other plants), in order to give time for the viscid matter on the under side of the saddle to set hard and dry. He who will insert a fine and flexible bristle into the expanded mouth of the flower between the sloping ridges on the labellum, will not doubt that they serve as guides and effectually prevent the bristle or proboscis from being inserted obliquely into the nectary. This latter circumstance is of manifest importance, for, if the proboscis were inserted obliquely, the saddle-formed disc would become attached obliquely, and after the compounded movement of the pollinia they would not strike the two lateral stigmatic surfaces.
Then we have the rostellum partially closing the mouth of the nectary, like a trap placed in a run for game; and the trap so complex and perfect, with its symmetrical lines of rupture forming the saddle-shaped disc above, and the lip of the pouch below; and, lastly, this lip so easily depressed that the proboscis of a moth can hardly fail to uncover the viscid disc and adhere to it. But if this fails to occur, the elastic lip rises and covers again the viscid surface, so as to keep it damp. The viscid matter within the rostellum is attached to the saddle-shaped disc alone, and is surrounded by fluid, so that it does not set hard till the disc is withdrawn. The upper surface of the saddle, with the attached caudicles, is also kept damp by the bases of the anther-cells, until it is withdrawn, and then the curious clasping movement instantly commences, causing the pollinia to diverge, followed by the movement of depression, which movements together are exactly fitted to cause the ends of the two pollen-masses to strike the two stigmatic surfaces. These stigmatic surfaces are not so sticky as to tear off the whole pollinium from the proboscis of the moth, but by rupturing the elastic threads to secure a few packets of pollen, leaving plenty for other flowers.
But let it be observed that, although the moth probably takes a considerable time to suck the nectar of a flower, yet the movement of depression in the pollinia does not commence (as I know by trial) until they are fully withdrawn; nor will the movement be completed, and the pollinia properly placed for striking the stigmatic surfaces, until about half a minute has elapsed, which will give ample time for the moth to fly to another plant, and thus effect a union between two distinct individuals.
Orchis ustulata resembles O. pyramidalis in some important respects, and differs from it in others. The labellum is deeply channelled, and the channel which replaces the guiding ridges of O. pyramidalis leads to the small triangular orifice of the short nectary. The upper angle of the triangle is overhung by the rostellum, the pouch of which is rather pointed below. In accordance with this position of the rostellum, close to the mouth of the nectary, the stigma is double and lateral. This species shows in an interesting manner how easily two distinct stigmas, like those of O. pyramidalis, might be converted into a single one, by becoming at first slightly lobed like that of O. mascula, and then acquiring its present structure. For directly beneath the rostellum there is a narrow transverse rim, formed of true stigmatic tissue, which connects together the two lateral stigmas; so that if this rim were widened, the two stigmas would be converted into a single transverse one. Conversely a single stigma might thus easily be converted into a double one. The pollinia undergo the usual movement of depression, and in acquiring this position the two diverge slightly, so as to be ready to strike the two lateral stigmas.
Orchis (sub-genus Himantoglossum) hircina.—A fine specimen of this extremely rare British plant, the Lizard Orchis, with its curious elongated labellum, was sent me by Mr. Oxenden. The two pollinia arise from a single almost square disc; and when they are removed from their cells, they do not diverge, but become depressed, sweeping through an angle of ninety degrees, in about thirty seconds. They are then in a proper position for striking the single large stigma which lies beneath the rostellum. In the case of O. pyramidalis we have seen that the depression of the two pollinia is effected by the contraction of the disc in front of each, two furrows or valleys being there formed; whilst with the present species, the whole front of the disc contracts or sinks down, the front part being thus separated from the hinder part by an abrupt step.
Aceras (Orchis) anthropophora.—The caudicles of the pollinia are unusually short; the nectary consists of two minute rounded depressions in the labellum; the stigma is transversely elongated; and lastly the two viscid discs lie so close together within the rostellum that they affect each other's outline. This latter fact is worth notice, as a step towards the two becoming absolutely confluent, as in the following species of Aceras, in O. pyramidalis and hircina. Nevertheless, in Aceras a single pollinium is sometimes removed by insects, though more rarely than with the other species of Orchis.
Aceras (Orchis) longibracteata.—Mr. Moggridge has given an interesting account, together with a figure, of this plant which grows in the South of France. The pollinia are attached to a single viscid disc. When they are removed they do not diverge as in O. pyramidalis, but converge and then undergo the movement of depression. The most remarkable point about this species is that insects seem to suck nectar out of minute open cells in the honeycombed surface of the labellum. The flowers are visited by various hymenopterous and dipterous insects; and the author saw the pollinia attached to the forehead of a large bee, the Xylocopa violacea.
Neotinea (Orchis) intacta.—Mr. Moggridge sent me from North Italy living specimens of this very rare British plant, which, as he informed me, is remarkable from producing seeds without the aid of insects. When insects were carefully excluded by me, almost all the flowers produced capsules. Their fertilisation follows from the pollen being extremely incoherent, so as to fall spontaneously on the stigma. Nevertheless a short nectary is present, the pollinia possess small viscid discs, and all the parts are so arranged that, if insects were to visit the flowers, the pollen-masses would almost certainly be removed and carried to another flower, but not so effectually as with most other orchids.
Serapias cordigera, an inhabitant of the South of France, has been described by Mr. Moggridge in the paper just referred to. The pollinia are attached to a single viscid disc; when first withdrawn, they are bent backwards, but soon afterwards move forwards and downwards in the usual manner. As the stigmatic cavity is narrow, the pollinia are guided into it by two guiding plates.
Nigritella angustifolia.—This Alpine species is said by Dr. H. Müller to differ from all ordinary orchids in the ovarium not being twisted; so that the labellum stands on the upper side of the flower, and insects alight on the opposite sepals and petals. As a consequence of this, when a butterfly inserts its proboscis into the narrow entrance of the nectary, the viscid discs become attached to the lower surface of the proboscis, and the pollinia afterwards move upwards, instead of as in all other orchids downwards. They are then in the proper position for striking the stigma of the next flower which is visited. Dr. Müller remarks that the flowers are frequented by an extraordinary number of butterflies.
I have now described the structure of most of the British and of a few foreign species in the genus Orchis and its close allies. All these species, with the exception of the Neotinea, require the aid of insects for their fertilisation. This is obvious from the fact that the pollinia are so closely embedded in the anther-cells, and the ball of viscid matter in the pouch-formed rostellum, that they cannot be shaken out by violence. We have also seen that the pollinia do not assume the proper position, for striking the stigmatic surface until some time has elapsed; and this indicates that they are adapted to fertilise, not their own flowers, but those on a distinct plant. To prove that insects are necessary for the fertilisation of the flowers, I covered up a plant of Orchis morio under a bell-glass, before any of its pollinia had been removed, leaving three adjoining plants uncovered; I looked at the latter every morning, and daily found some of the pollinia removed, till all were gone with the exception of those in a single flower low down on one spike, and of those in one or two flowers on the summits of all the spikes, which were never removed. But it should be observed that when only a very few flowers remain open on the summits of the spikes, these are no longer conspicuous, and would consequently be rarely visited by insects. I then looked at the perfectly healthy plant under the bell-glass, and it had, of course, all its pollinia in the anther-cells. I tried an analogous experiment with specimens of O. mascula with the same result. It deserves notice that the spikes which had been covered up, when subsequently left uncovered, never had their pollinia carried away by insects, and did not, of course, set any seed, whereas the adjoining plants produced plenty of seed. From this fact it may be inferred that there is a proper season for each kind of Orchis, and that insects cease their visits after the proper season has passed.
With many of the hitherto mentioned species, and with several other European kinds, the sterility of the flowers, when protected from the access of insects, depends solely on the pollen-masses not coming into contact with the stigma. This has been proved to be the case by Dr. Hermann Müller, who, as he informs me, applied the pollen-masses of Orchis pyramidalis (44), fusca (6), militaris (14), variegata (3), coriophora (6), morio (4), maculata (18), mascula (6), latifolia (8), incarnata (3), Ophrys muscifera (8), Gymnadenia conopsea (14), albida (8), Herminium monorchis (6), Epipogon aphyllus (2), Epipactis latifolia (14), palustris (4), Listera ovata (5), and Cypripedium calceolus (2), to their own stigmas, and full-sized capsules, containing seeds in appearance good, were formed. The numbers placed after the names of the species show how many flowers were tried in each case. These facts are remarkable, because Mr. Scott and Fritz Müller have proved that various exotic species, both in this country and in their native homes, invariably fail to yield seed-capsules, when the flowers are fertilised with their own pollen.
From the observations already given, and from what will hereafter be shown with respect to Gymnadenia, Habenaria, and some other species, it is a safe generalisation that species with a short and not very narrow nectary are fertilised by bees and flies; whilst those with a much elongated nectary, or one having a very narrow entrance, are fertilised by butterflies or moths, these being provided with long and thin proboscides. We thus see that the structure of the flowers of Orchids and that of the insects which habitually visit them, are correlated in an interesting manner,—a fact which has been amply proved by Dr. H. Müller to hold good with many of the Orchideæ and other kinds of plants.
With respect to Orchis pyramidalis, which possesses, as we have seen, an elongated nectary, Mr. Bond was so kind as to send me a large number of Lepidoptera, out of which I selected twenty-three species, enumerated in the following list, with the pollinia of this Orchid, which can easily be recognised, attached to their proboscides.
Leucania lithargyria (two specimens).
Eubolia mensuraria (two specimens).
Heliothis marginata (two specimens).
Xylophasia sublustris (two specimens).
A large majority of these moths and butterflies had two or three pairs of pollinia attached to them, and invariably to the proboscis. The Acontia had seven pair (fig. 4), and theFig. 4.Head and proboscis of Acontia luctuosa with seven pair of pollinia of Orchis pyramidalis attached to the proboscis. Caradrina no less than eleven pair! The proboscis of this latter moth presented an extraordinary arborescent appearance. The saddle-formed discs, each bearing a pair of pollinia, adhered to the proboscis, one before the other, with perfect symmetry; and this follows from the moth having always inserted its proboscis into the nectary in exactly the same manner, owing to the presence of the guiding plates on the labellum. The unfortunate Caradrina, with its proboscis thus encumbered, could hardly have reached the extremity of the nectary, and would soon have been starved to death. Both these moths must have sucked many more than the seven and eleven flowers, of which they bore the trophies, for the earlier attached pollinia had lost much of their pollen, showing that they had touched many viscid stigmas.
The above list proves that many different species of Lepidoptera visit the same kind of Orchis. The Hadena dentina also frequents Habenaria. Probably all the Orchids provided with elongated nectaries are visited indifferently by many kinds of moths. Whether any of the British Orchids are fertilised exclusively by special insects confined to certain localities is very doubtful; but we shall hereafter see that Epipactis latifolia seems to be fertilised by wasps alone. I have twice observed plants of Gymnadenia conopsea, which had been transplanted into a garden many miles from its native home, with nearly all their pollinia removed. Mr. Marshall of Ely has made the same observation on similarly transplanted specimens of O. maculata. On the other hand fifteen plants of Ophrys muscifera had not one pollen-mass there removed. Malaxis paludosa was placed in a bog about two miles from that in which it naturally grew; and it had most of its pollinia immediately removed.
The list which follows serves to show that insects in most cases perform the work of fertilisation effectually. But the list by no means gives a fair idea how effectually it is done; for I have often found nearly all the pollinia removed, but kept an exact record only in exceptional cases, as may be seen by the appended remarks. Moreover, in most cases, the pollinia which had not been removed were in the upper flowers beneath the buds, and many of these would probably have been subsequently carried away. I have often found an abundance of pollen on the stigmas of flowers which had not their own pollinia removed, showing that they had been visited by insects. In many other cases the pollinia had been removed, but no pollen had been as yet left on the stigmas.
|Number of flowers with both or one pollinium removed. Flowers lately open excluded.||Number of flowers with only one pollinium removed. These flowers are included in the column to the left.||Number of flowers with neither pollinium removed.|
|Orchis morio. Three small plants. N. Kent||22||2||6|
|Orchis morio. Thirty-eight plants. N. Kent. These plants were examined after nearly four weeks of extraordinarily cold and wet weather in 1860; and therefore under the most unfavourable circumstances||110||23||193|
|Orchis pyramidalis. Two plants. N. Kent and Devonshire||39||. .||8|
|Orchis pyramidalis. Six plants from two protected valleys. Devonshire||102||. .||66|
|Orchis pyramidalis. Six plants from a much exposed bank. Devonshire||57||. .||166|
|Orchis maculata. One plant. Staffordshire. Of the twelve flowers which had not their pollinia removed, the greater number were young, flowers under the buds||32||6||12|
|Orchis maculata. One plant. Surrey||21||5||7|
|Orchis maculata. Two plants. N. and S. Kent||28||17||50|
|Orchis latifolia. Nine plants from S. Kent, sent me by the Rev. B. S. Malden. The flowers were all mature||50||27||119|
|Orchis fusca. Two plants. S. Kent. Flowers quite mature, and even withered||8||5||54|
|Aceras anthropophora. Four plants. S. Kent||63||6||34|
In the second lot of O. morio, in the preceding list, we see the injurious effects of the extraordinary cold and wet season of 1860 on the visits of insects, and, consequently, on the fertilisation of this Orchid, very few seed-capsules having been produced.
I have examined spikes of O. pyramidalis in which every single expanded flower had its pollinia removed. The forty-nine lower flowers of a spike from Folkestone (sent me by Sir Charles Lyell) actually produced forty-eight fine seed-capsules; and of the sixty-nine lower flowers in three other spikes, seven alone had failed to produce capsules. These facts show how well moths and butterflies perform their office of marriage-priests.
The third lot of O. pyramidalis in the above list, grew on a steep grassy bank, overhanging the sea near Torquay, and where there were no bushes or other shelter for Lepidoptera; being surprised how few pollinia had been removed, though the spikes were old and very many of the lower flowers withered, I gathered, for comparison, six other spikes from two bushy and sheltered valleys, half a mile on each side of the exposed bank; these spikes were certainly younger, and would probably have had several more of their pollinia removed; but in their present condition we see how much more frequently they had been visited by moths, and consequently fertilised, than those growing on the much exposed bank. The Bee Ophrys and O. pyramidalis grow mingled together in many parts of England; and they did so here, but the Bee Ophrys, instead of being, as usual, the rarer species, was here much more abundant than O. pyramidalis. No one would readily have suspected that one chief reason of this difference probably was, that the exposed situation was unfavourable to Lepidoptera, and therefore to the seeding of O. pyramidalis; whereas, as we shall hereafter see, the Bee Ophrys is independent of insects.
Many spikes of O. latifolia were examined, because, being familiar with the usual state of the closely-allied O. maculata, I was surprised to find in nine nearly withered spikes (as maybe seen in the list) how few pollinia had been removed. In one instance, however, O. maculata had been even worse fertilised; for seven spikes with 315 flowers, produced only forty-nine seed-capsules—that is, on an average only seven capsules on each spike. In this case the plants formed larger beds than I had ever before seen; and I imagine that there were too many flowers for the insects to visit and fertilise all of them. On some other plants of O. maculata growing at no great distance, above thirty capsules had been produced by each spike.
Orchis fusca offers a still more curious case of imperfect fertilisation. I examined ten fine spikes from two localities in South Kent, sent to me by Mr. Oxenden and Mr. Malden: most of the flowers on these spikes were partly withered, with the pollen mouldy even in the uppermost flowers; we may therefore infer that no more pollinia would have been removed. I examined all the flowers only in two spikes, on account of the trouble from their withered condition, and the result may be seen in the list, namely, fifty-four flowers with both pollinia in place, and only eight with one or both removed. In this Orchid, and in O. latifolia, neither of which had been sufficiently visited by insects, there were more flowers with one pollinium than with both removed. I casually examined many flowers in the other spikes of O. fusca, and the proportion of pollinia removed was evidently not greater than in the two in the list. The ten spikes bore altogether 358 flowers, and, in accordance with the few pollinia removed, only eleven capsules had been formed: five of the ten spikes produced not a single capsule; two spikes had only one, and one had as many as four capsules. As corroborating what I have before said with respect to pollen being often found on the stigmas of flowers which retain their own pollinia, I may add that, of the eleven flowers which had produced capsules, five had both pollinia still within their now withered anther-cells.
From these facts the suspicion naturally arises that O. fusca is so rare a species in Britain from not being sufficiently attractive to insects, and to its not producing a sufficiency of seed. C. K. Sprengel noticed, that in Germany O. militaris (ranked by Bentham as the same species with O. fusca) is likewise imperfectly fertilised, but more perfectly than our O. fusca; for he found five old spikes bearing 138 flowers which had set thirty-one capsules; and he contrasts the state of these flowers with those of Gymnadenia conopsea, in which almost every flower produces a capsule.
An allied and curious subject remains to be discussed. The existence of a well-developed spur-like nectary seems to imply the secretion of nectar. But Sprengel, a most careful observer, thoroughly searched many flowers of O. latifolia and morio, and could never find a drop of nectar; nor could Krünitz find nectar either in the nectary or on the labellum of O. morio, fusca, militaris, maculata or latifolia. I have looked to all our common British species and could find no trace of nectar; I examined, for instance, eleven flowers of O. maculata, taken from different plants growing in different districts, and taken from the most favourable position on each spike, and could not find under the microscope the smallest bead of nectar. Sprengel calls these flowers "Scheinsaftblumen," or sham-nectar-producers;—he believes that these plants exist by an organized system of deception, for he well knew that the visits of insects were indispensable for their fertilisation. But when we reflect on the incalculable number of plants which have lived during a great length of time, all requiring that insects should carry the pollen-masses from flower to flower in each generation; and as we further know from the number of the pollen-masses attached to their proboscides, that the same insects visit a large number of flowers, we can hardly believe in so gigantic an imposture. He who believes in Sprengel's doctrine must rank the sense or instinctive knowledge of many kinds of insects, even bees, very low in the scale. To test the intellect of moths and butterflies I tried the following little experiment, which ought to have been tried on a larger scale. I removed a few already-opened flowers on a spike of O. pyramidalis, and then cut off about half the length of the nectaries of the six next non-expanded flowers. When all the flowers were nearly withered, I found that thirteen of the fifteen upper flowers with perfect nectaries had their pollinia removed, and two alone had their pollinia still in the anther-cells; of the six flowers with their nectaries cut off, three had their pollinia removed, and three were still in place; and this indicates that moths do not go to work in a quite senseless manner.
Nature may be said to have tried this same experiment, but not quite fairly; for Orchis pyramidalis, as shown by Mr. Bentham, often produces monstrous flowers without a nectary, or with a short and imperfect one. Sir C. Lyell sent me several spikes from Folkestone with many flowers in this condition: I found six without a vestige of a nectary, and their pollinia had not been removed. In about a dozen other flowers, having either short nectaries, or with the labellum imperfect, the guiding ridges being either absent or developed in excess and rendered foliaceous, the pollinia in one alone had been removed, and the ovarium of another flower was swelling. Yet I found that the saddle-formed discs in these eighteen flowers were perfect, and that they readily clasped a needle when inserted in the proper place. Moths had removed the pollinia, and had thoroughly fertilised the perfect flowers on the same spikes; so that they must have neglected the monstrous flowers, or, if visiting them, the derangement in the complex mechanism of the parts had hindered the removement of the pollinia, and prevented their fertilisation.
Notwithstanding these several facts I still suspected that nectar must be secreted by our common Orchids, and I determined to examine O. morio rigorously. As soon as many flowers were open, I began to examine them for twenty-three consecutive days: I looked at them after hot sunshine, after rain, and at all hours: I kept the spikes in water, and examined them at midnight, and early the next morning: I irritated the nectaries with a bristle, and exposed them to irritating vapours: I took flowers which had lately had their pollinia removed by insects, of which fact I had independent proof on one occasion by finding grains of some foreign pollen within the nectary; and I took other flowers, which judging from their position on the spike, would soon have had their pollinia removed; but the nectary was invariably quite dry. After the publication of the first edition of this work, I one day saw various kinds of bees visiting repeatedly the flowers of this same Orchid, so that this was evidently the proper time to examine their nectaries; but I failed to detect under the microscope even the minutest drop of nectar. So it was with the nectaries of O. maculata at a time when I repeatedly saw flies of the genus Empis keeping their proboscides inserted into them for a considerable length of time. Orchis pyramidalis was examined with equal care with the same result, for the glittering points within the nectary were absolutely dry. We may therefore safely conclude that the nectaries of the above-named Orchids neither in this country nor in Germany ever contain nectar.
Whilst examining the nectaries of O. morio and maculata, and especially of O. pyramidalis and hircina, I was surprised at the degree to which the inner and outer membranes forming the tube or spur were separated from each other,—also at the delicate nature of the inner membrane, which could be penetrated very easily,—and, lastly, at the quantity of fluid contained between the two membranes. So copious is this fluid, that, after cutting off the extremities of the nectaries of O. pyramidalis, and gently squeezing them on glass under the microscope, such large drops of fluid exuded from the cut ends, that I concluded that at last I had found nectaries which contained nectar; but when I carefully made, without any pressure, a slit along the upper surface of other nectaries from the same plants, and looked into them, their inner surfaces were quite dry.
I then examined the nectaries of Gymnadenia conopsea (a plant ranked by some botanists as a true Orchis) and of Habenaria bifolia, which are always full of nectar up to one-third or two-thirds of their length. The inner membrane presented the same structure and was covered with papillæ as in the foregoing species; but there was a plain difference in the inner and outer membranes being closely united, instead of being in some degree separated from each other and charged with fluid. I was therefore led to conclude that insects penetrate the lax inner membrane of the nectaries of the above-named Orchids, and suck the copious fluid between the two membranes. This was a bold hypothesis; for at the time no case was known, of insects penetrating with their delicate proboscides even the laxest membrane. But I have now heard from Mr. Trimen, that at the Cape of Good Hope moths and butterflies do much injury to peaches and plums by puncturing their unbroken skins. In Queensland, Australia, a moth, the Ophideres fullonica, bores through the thick rind of the orange with its wonderful proboscis, provided with formidable teeth. There is therefore not the least difficulty in believing that Lepidoptera with their delicate proboscides, and bees with their much stronger ones, could penetrate with ease the soft inner membrane of the nectaries of the above-named Orchids. Dr. H. Müller is also convinced that insects puncture the thickened bases of the standard petals of the Laburnum, and perhaps the petals of some other flowers, so as to obtain the included fluid.
The various kinds of bees which I saw visiting the flowers of Orchis morio remained for some time with their proboscides inserted into the dry nectaries, and I distinctly saw this organ in constant movement. I observed the same fact with Empis in the case of O. maculata; and on afterwards opening several of the nectaries, I occasionally detected minute brown specks, due as I believe to the punctures made some time before by these flies. Dr. H. Müller, who has often watched bees at work on several species of Orchis, the nectaries of which do not contain any free nectar, fully accepts my view. On the other hand, Delpino still maintains that Sprengel is right, and that insects are continually deceived by the presence of a nectary, though this contains no nectar. His belief is founded chiefly on a statement by Sprengel that insects soon find out that it is of no use to visit the nectaries of these orchids, as shown by their fertilising only the lower and first opened flowers. But this statement is completely contradicted by my observations previously given, from which it follows that very many of the upper flowers are fertilised; for instance, on a spike of O. pyramidalis with between fifty and sixty flowers, no less than forty-eight had their pollinia removed. Nevertheless, as soon as I learnt that Delpino still believed in Sprengel's view, I selected during the unfavourable season of 1875 six old spikes of O. maculata, and divided each into halves, so as to observe whether many more capsules were produced by the lower than by the upper half. This certainly was not always the case; for in some of the spikes no difference could be detected between them; in others there were more capsules in the lower, while in others there were more in the upper half. A spike of O. pyramidalis examined in the same manner produced twice as many capsules in the upper as in the lower half. Bearing in mind these facts and others before given, it appears to me incredible that the same insect should go on visiting flower after flower of these Orchids, although it never obtains any nectar. Insects, or at least bees, are by no means destitute of intelligence. They recognise from a distance the flowers of the same species, and keep to them as long as they can. When humble-bees have bitten holes through the corolla, as they often do, so as to reach the nectar more easily, hive-bees immediately perceive what has been done and take advantage of the perforations. When flowers having more than a single nectary are visited by many bees, so that the nectar is exhausted in most of them, the bees which afterwards visit such flowers insert, their proboscides only into one of the nectaries, and if they find this exhausted, they instantly pass on to another flower. Can it be believed that bees which show this much intelligence, should persevere in visiting flower after flower of the above-named Orchids, and in keeping their proboscides in constant movement for some time within the nectaries, in the hope of obtaining nectar which is never present? This, as I have said, seems to me utterly incredible.
It has been shown how numerous and beautiful are the contrivances for the fertilisation of Orchids. We know that it is of the highest importance that the pollinia, when attached to the head or proboscis of an insect, should be fixed symmetrically, so as not to fall either sideways or backwards. We know that in the species as yet described the viscid matter of the disc sets hard in a few minutes when exposed to the air, so that it would be a great advantage to the plant if insects were delayed in sucking the nectar, time being thus allowed for the disc to become immovably affixed. It is manifest that insects must be delayed by having to bore through several points of the inner membrane of the nectary, and to suck the nectar from the intercellular spaces; and we can thus understand why the nectaries of the above-named species of Orchis do not contain free nectar, but secrete it internally between the two membranes.
The following singular relation supports this view in a striking manner. I have found free nectar within the nectaries of only five British species of Ophreæ, namely, in Gymnadenia conopsea and albida, in Habenaria bifolia and chlorantha, and in Peristylus (or Habenaria) viridis. The first four of these species have the viscid surfaces of the discs of their pollinia naked or not enclosed within pouches, and the viscid matter does not rapidly set hard when exposed to the air, as if it did, it would immediately have been rendered useless; and this shows that it must differ in chemical nature from that in the foregoing species of Orchis. But to make sure of this fact I removed the pollinia from their anther-cells, so that the upper as well as the under surfaces of the viscid discs were freely exposed to the air; in Gymnadenia conopsea the disc remained sticky for two hours, and in Habenaria chlorantha for more than twenty-four hours. In Peristylus viridis the viscid disc is covered by a pouch-formed membrane, but this is so minute that botanists have overlooked it. I did not, when examining this species, see the importance of ascertaining exactly how soon the viscid matter set hard; but I copy from my notes the words written at the time: "disc remains sticky for some time when removed from its little pouch."
Now the meaning of these facts is clear: as the viscid matter of the discs of these five latter species is so adhesive that it serves to attach the pollinia firmly to the insects which visit the flowers, without setting hard, there would be no use in the insects being delayed by having to bore holes at several points through the inner membrane of the nectaries; and in these five species, and in these alone, we find copious nectar ready stored for rapid suction in open nectaries. On the other hand, whenever the viscid matter sets hard by exposure for a short time to the air, it would manifestly be advantageous to the plant, if insects were delayed in obtaining the nectar; and in all such species the nectar is lodged within intercellular spaces, so that it can be obtained only by the inner membrane being penetrated at several points, and this will require time. If this double relation is accidental, it is a fortunate accident for the plants; but I cannot believe it to be so, and it appears to me one of the most wonderful cases of adaptation which has ever been recorded.
- Dr. H. Müller ('Die Befruchtung der Blumen durch Insekten,' 1873, p. 84) has timed humble-bees at work on the spikes flowers of Orchis mascula, and finds that this statement is correct.
- 'Transactions of the Linnean Society,' vol. xvi. p. 731.
- 'Die Befruchtung,' &c., p. 84.
- M. M. Girard caught a longicorn beetle, Strangalia atra, with a tuft of the pollen-masses of this orchis attached to the front of its mouth: 'Annales de la Soc. Entomolog. de France,' tom. ix. 1869, p. xxxi.
- The late Prof. Treviranus has confirmed ('Botanische Zeitung,' 1863, p. 241) all my observations, but points out two unimportant inaccuracies in the drawing which I have given.
- I am greatly indebted to Mr. G. Chichester Oxenden of Broome Park for fresh specimens of this Orchis, and for his never-tiring kindness in supplying me with living plants, and information regarding many of the rarer British Orchids.
- The separation of this genus is evidently artificial. It is a true Orchis, but with a very short nectary. Dr. Weddell has described ('Annales des Sc. Nat.,' 3 ser. Bot. tom. xviii. p. 6) the occurrence of numerous hybrids, naturally produced, between this Aceras and Orchis galeata.
- 'Journ. Linn. Soc. Bot.' vol. viii. 1865, p. 256. He gives also a figure of Orchis hircina.
- 'Nature,' Dec. 31, 1874, p. 169.
- An abstract of their observations in given in my 'Variation of Animals and Plants under Domestication,' chap. xvii. 2nd edit, vol. ii. p. 114.
- Some remarks to this effect were given in my "Notes on the Fertilisation of Orchids," in 'Annals and Mag. of Nat. Hist.' Sept. 1869, p. 2.
- M. Ménière (in 'Bull. Bot. Soc. de France,' tom. i. 1854, p. 370) says he saw in Dr. Guépin's collection, bees collected at Saumur with the pollinia of Orchids attached to their heads; and he states that a person who kept bees near the Jardin de la Faculté (at Toulouse?) complained that his in bees returned from the garden with their heads charged with yellow bodies, of which they could not free themselves. This is good evidence how firmly the pollinia are attached. There is, however nothing to show whether the pollinia in these cases belonged to the genus Orchis or to some other genus of the family.
- I am indebted to Mr. Parfitt for an examination of this moth, which is mentioned in the 'Entomologist's Weekly Intelligencer,' vol. ii. p. 182, and vol. iii. p. 3, Oct. 3, 1857. The pollinia were erroneously thought to belong to Ophrys apifera. The pollen had changed from its natural green colour to yellow; on washing it, however, and drying it, the green tint returned.
- 'Gardener's Chronicle,' 1861, p. 73. Mr. Marshall's communication was in answer to some remarks of mine on this subject previously published in the 'Gardener's Chronicle,' 1860, p. 528.
- In the summer of 1875, which was a very wet one, I gathered six unusually fine spikes of O. pyramidalis. These bore 302 flowers, excluding fourteen which were still fully expanded and capable of being fertilised; and on this occasion only 119 flowers produced capsules, 183 having failed to do so. Six spikes of O. maculata bore 187 flowers, of which eighty-two produced capsules, 105 having failed.
- 'Das entdeckte Geheimniss,' etc. s. 404.
- Quoted by J. G. Kurr in his 'Untersuchungen über die Bedeutung der Nektarien,' 1833, s. 28. See also 'Das entdeckte Geheimniss,' s. 403.
- Kurr ('Bedeutung der Nekarien,' 1833, p. 123) cut off the nectaries of fifteen flowers of Gymnadenia conopsea, and they did not produce a single capsule: he also treated in the same manner fifteen flowers of Platanthera or Habenaria bifolia, and these set only five capsules; but then it should be observed that the nectaries of both these orchids contain free nectar. He also cut off the corolla, leaving the nectary, of forty flowers of Orchis morio, and these set no capsules; and this case shows that insects are guided to the flowers by the corolla, Sixteen flowers of Platanthera treated in the same manner bore only one capsule. Similar experiments made by him on Gymnadenia seem to me open to doubt.
- 'Handbook of the British Flora,' 1858, p. 501.
- My son Francis has described and figured this organ in the 'Q. Journal of Microscopical Science,' vol. xv. 1875, p. 385.
- 'Die Befruchtung,' &c. p. 235.
- Treviranus confirms ('Bot. Zeitung,' 1863, p. 10) a statement made by Salisbury, that when the filaments in the flowers of another leguminous plant, Edwardsia, fall off, or when they are cautiously separated, a large quantity sweet fluid flows from the points of separation; and as beforehand there was no trace of any such fluid, it must have been contained, as Treviranus remarks, within the cellular tissue. I may add an apparently similar, but really distinct case, namely, the presence of nectar in several monocotyledonous plants (as described by Ad. Brongniart in 'Bull. Soc. Bot. de France,' tom. i. 1854, p. 75) between the two walls (feuillets) which form the divisions of the ovarium. But the nectar in this case is conducted to the outside by a channel; and the secreting surface is homologically an exterior surface.
- 'Die Befruchtung,' &c. p. 84.
- 'Ult. Osservazioni sulla Dicogamia,' 1875, p. 121'.