The Various Contrivances by which Orchids are Fertilised by Insects/Chapter 3
ARETHUSEÆ.
Cephalanthera grandiflora.—This Orchid is remarkable from not possessing a rostellum, which is so eminently characteristic of the order. The stigma is large, and the anther stands above it. The pollen is extremely friable and readily adheres to any object. The grains are tied together by a few weak elastic threads; but they are not cemented together, so as to form compound pollen-grains, as in almost all other Orchids.[1] In this latter character and in the complete abortion of the rostellum we have evidence of degradation; and Cephalanthera appears to me like a degraded Epipactis, a member of the Neotteæ, to be described in the next chapter.
The anther opens whilst the flower is in bud and partly expels the pollen, which stands in two nearly free upright pillars, each nearly divided longitudinally into halves. These subdivided pillars rest against or even overhang the upper square edge of the stigma, which rises to about one-third of their height (see front view B, and side view C, in fig. 13). Whilst the flower is still in bud, the pollen-grains which rest against the upper sharp edge of the stigma (but not those in the upper or lower parts of the mass) emit a multitude of
Fig. 13.
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Cephalanthera grandiflora.
tubes; and these deeply penetrate the stigmatic tissue. After this period the stigma bends a little forward, and the result is that the two friable pillars of pollen are drawn a little forward and stand almost completely free from the anther-cells, being tied to the edge of the
stigma and supported by the penetration of the pollen-tubes. Without this support the pillars would soon fall
down.
The flower stands upright, with the lower part of the labellum turned up parallel to the column (fig. A). The tips of the lateral petals never become separated;[2] so that the pillars of pollen are protected from the wind, and as the flower stands upright they do not fall down from their own weight. These are points of much importance to the plant, as otherwise the pollen would have been blown or fallen down and been wasted. The labellum is formed of two portions; when the flower is mature, the small triangular distal portion turns down at right angles to the basal portion; and thus offers a small landing-place for insects in front of the triangular entrance, situated half-way up the almost tubular flower. After a short time, as soon as the flower is fully fertilised, the small distal portion of the labellum rises up, shuts the triangular door, and again perfectly encloses the organs of fructification.
Although I have often searched for nectar within the cup of the labellum, I have never found even a trace. The terminal portion of the labellum is frosted with globular papillæ of an orange colour, and within the cup there are several transversely wrinkled, longitudinal ridges of a darker orange tint. These ridges are often gnawed by some animal, and I have found minute, bitten-off fragments lying within the base of the cup. In the summer of 1862 the flowers were visited less frequently by insects than is usual, as shown by the unbroken state of the pollen-masses; nevertheless, out of seventeen flowers which were examined one day, five had their ridges gnawed, and on the next day, seven out of nine other flowers were in this state. As there was no appearance of slime, I do not believe that they had been attacked by slugs; but whether they had been gnawed by winged insects, which alone would be effectual for cross-fertilisation, I know not. The ridges had a taste like that of the labellum of certain Vandeæ, in which tribe (as we shall hereafter see) this part of the flower is often gnawed by insects. Cephalanthera is the only British Orchid, as far as I have observed, which attracts insects, by thus offering to them solid food.
The early penetration of the stigma by a multitude of pollen-tubes, which were traced far down the stigmatic tissue, apparently gives us another case, like that of the Bee Ophrys, of perpetual self-fertilisation. I was much surprised at this fact, and asked myself: Why does the distal portion of the labellum open for a short period? what is the use of the great mass of pollen above and below that layer of grains, the tubes of which alone penetrate the upper edge of the stigma? The stigma has a large flat viscid surface; and during several years I have almost invariably found masses of pollen adhering to its surface, and the friable pillars by some means broken down. It occurred to me that, although the flowers stand upright, and the pillars are well protected from the wind, yet that the pollen-masses might ultimately topple over from their own weight, and so fall on the stigma, thus completing the act of self-fertilisation. Accordingly, I covered with a net a plant having four buds, and examined the flowers as soon as they had withered; the broad stigmas of three of them were perfectly free from pollen, but a little had fallen on one corner of the fourth. With the exception of the summit of one pillar in this latter flower, all the other pillars still stood upright and unbroken. I looked at the flowers of some surrounding plants, and everywhere found, as I had so often done before, broken-down pillars and masses of pollen on the stigmas.
From the usual state of the pillars of pollen, as well as from the gnawed condition of the ridges on the labellum, it may be safely inferred that insects of some kind visit the flowers, disturb the pollen, and leave masses of it on the stigmas. We thus see that the turning down of the distal portion of the labellum, by which a temporary landing-place and an open door are afforded,—the upturned labellum, by which the flower is made tubular so that insects are compelled to crawl close by the stigmatic surface,—the pollen readily cohering to any object, and standing in friable pillars protected from the wind,—and, lastly, the large masses of pollen above and below that layer of grains, the tubes of which alone penetrate the edge of the stigma,—are all co-ordinated structures, far from useless; and they would be quite useless if these flowers were always self-fertilised.
To ascertain how far the early penetration of the upper edge of the stigma by the tubes of those grains which rest on it, is effectual for fertilisation, I covered up a plant, just before the flowers opened, and removed the thin net as soon as they had begun to wither. From long experience I am sure that this temporary covering could not have injured their fertility. The four covered flowers produced seed-capsules as fine in appearance as those on any of the surrounding plants. When ripe, I gathered them, and likewise capsules from several of the surrounding plants, growing under similar conditions, and weighed the seed in a chemical balance. The seeds from the four capsules on the uncovered plants weighed 1.5 grain; whilst those from an equal number of capsules on the covered plant weighed under 1 grain; but this does not give a fair idea of the relative difference of their fertility, for I observed that a great number of the seeds from the covered plant consisted of minute and shrivelled husks. Accordingly I mixed the seeds well together, and took four little lots from one heap and four little lots from the other heap, and, having soaked them in water, compared them under the microscope: out of forty seeds from the uncovered plants there were only four bad ones, whereas out of forty seeds from the covered-up plants there were at least twenty-seven bad; so that there were nearly seven times as many bad seeds from the covered plants, as from those left free to the access of insects.
We may therefore conclude that this orchid is constantly self-fertilised, although in a very imperfect manner; but this would be highly useful to the plant, if insects failed to visit the flowers. The penetration of the pollen-tubes, however, is apparently even more serviceable by retaining the pillars of pollen in their proper places, so that insects, in crawling into the flowers, may get dusted with pollen. Self-fertilisation also may, perhaps, be aided by insects, carrying pollen from the same flower on to the stigma; but an insect thus smeared with pollen could hardly fail likewise to cross the flowers on other plants. From the relative position of the parts, it seems indeed probable (but I omitted to prove this by the early removal of the anthers, so as to observe whether pollen was brought to the stigma from other flowers) that an insect would more frequently get dusted by crawling out of a flower than by crawling into one; and this would of course facilitate a cross between distinct individuals. Hence Cephalanthera offers only a partial exception to the rule that the flowers of Orchids are generally fertilised by pollen from another plant.
Cephalanthera ensifolia.—According to Delpino,[3] the flowers of this species are visited by insects, as shown by the removal of the pollen-masses. He believes that this is effected by their bodies being first rendered sticky by means of the stigmatic secretion. It is not clear whether the flowers also fertilise themselves. Each pollen-mass is divided into two, instead of being merely sub-divided, so that there are four distinct pollen-masses.
Pogonia ophioglossoides.—The flowers of this plant, an inhabitant of the United States, resemble, as described by Mr. Scudder,[4] those of Cephalanthera in not having a rostellum, and in the pollen-masses not being furnished with caudicles. The pollen consists of powdery grains not united by threads. Self-fertilisation seems to be effectually prevented; and the flowers on distinct plants must intercross, for each plant generally bears only a single flower.
Pterostylis trullifolia and longifolia.—I may here briefly mention some Orchids, inhabitants of Australia and New Zealand, which are included by Lindley in the same family of the Arethuseæ with Cephalanthera and Pogonia, and are remarkable from their labella being extremely sensitive or irritable. Two of the petals and one of the sepals form a hood which encloses the column, as may be seen at A in the accompanying figure of Pterostylis longifolia.
The distal portion of the labellum affords a landing-place for insects, in nearly the same manner as with Cephalanthera; but when this organ is touched it rapidly springs up, carrying with it the touching insect, which is thus temporarily imprisoned within the otherwise almost completely closed flower. The labellum
Fig. 14.
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Pterostylis longifolia. (Copied from Mr. R. D. Fitzgerald's 'Australian Orchids.')
remains shut from half an hour to one hour and a half, and on reopening is again sensitive to a touch. Two membranous shields project on each side of the upper part of the column, with their edges meeting in front, as may be seen in fig. B. In this drawing the petal on the near side has been cut away, and the labellum is represented in the position which it assumes after having been touched. As soon as the labellum has thus risen, an imprisoned insect cannot escape except by crawling through the narrow passage formed by the two projecting shields. In thus escaping it can hardly fail to remove the pollinia, as, before coming into contact with them, its body will have been smeared with the viscid matter of the rostellum. On being imprisoned in another flower, and on again escaping by the same passage, it will almost certainly leave at least one of the four pollen-masses on the adhesive stigma, and thus fertilise the flower.
All that I have here said is taken from the admirable description given by Mr. Cheeseman[5] of Pterostylis trullifolia; but I have copied the figure of P. longifolia from Mr. Fitzgerald's great work on the Australian Orchids, as it shows plainly the relation of all the parts.
Mr. Cheeseman placed insects within several flowers of P. trullifolia, and saw them afterwards crawl out, generally with pollinia attached to their backs. He also proved the importance of the irritable labellum by removing it from twelve flowers whilst young, and in this case insects which entered the flowers would not have been compelled to crawl out through the passage; and not one of these flowers produced a capsule. The flowers seem to be frequented exclusively by Diptera; but what attraction they present is not known, as they do not secrete nectar. Mr. Cheeseman believes that hardly a quarter of the flowers produce capsules; notwithstanding that on one occasion he examined 110 flowers in a withered condition, and seventy-one of these had pollen on their stigmas, and only twenty-eight had all four pollinia still within their anthers. All the New Zealand species bear solitary flowers, so that distinct plants cannot fail to be intercrossed. I may add that Mr. Fitzgerald also placed a small beetle on the labellum of P. longifolia, which was instantly carried into the flower and imprisoned; afterwards he saw it crawl out with two pollinia attached to its back. Nevertheless he doubts, from reasons which seem to me quite insufficient, whether the sensitiveness of the labellum is not as great a disadvantage as an advantage to the plant.
Mr. Fitzgerald has described another Orchid belonging to the same sub-tribe, Caladenia dimorpha, which has an irritable labellum. He kept a plant in his room, and says: "A house-fly lighting on the lip was carried by its spring against the column, and becoming entangled in the gluten of the stigma, and struggling to escape, removed the pollen from the anther and smeared it on the stigma." He adds, "Without some such aid the species of this genus never produce seed." But from the analogy of other Orchids we may feel sure that insects usually behave very differently from the fly which he saw caught on the stigma, and no doubt they carry the pollen-masses from plant to plant. The labellum of another Australian genus, Calæna, one of the Arethuseæ, is said by Dr. Hooker[6] to be irritable; so that when touched by an insect it shuts up suddenly against the column, and temporarily encloses its prey as it were within a box. The labellum is covered by curious papillæ, which, as far as Mr. Fitzgerald has seen, are not gnawed by insects.
Mr. Fitzgerald describes and figures several other genera, and states with respect to Acianthus fornicatus and exsertus that neither species produce seeds if protected from insects, but are easily fertilised by pollen placed on their stigmas. Mr. Cheeseman[7] has witnessed the fertilisation of Acianthus sinclairii in New Zealand, the flowers of which are incessantly visited by Diptera, without whose aid the pollinia are never removed. Out of eighty-seven flowers borne by fourteen plants, no less than seventy-one matured capsules. This plant according to the same observer exhibits one remarkable peculiarity, namely, that the pollen-masses are attached to the rostellum by means of the exserted pollen-tubes, which serve as a caudicle; and the pollen-masses are thus removed together with the rostellum, which is viscid, when the flowers are visited by insects. The flowers of the allied Cyrtostylis are also much frequented by insects, but the pollinia are not so regularly removed as those of the Acianthus; and with Corysanthes, only five out of 200 flowers produced capsules.
The Vanillidæ according to Lindley form a subtribe of the Arethuseæ. The large tubular flowers of Vanilla aromatica are manifestly adapted to be fertilised by insects; and it is known that when this plant is cultivated in foreign countries, for instance in Bourbon, Tahiti, and the East Indies, it fails to produce its aromatic pods unless artificially fertilised. This fact shows that some insect in its American home is specially adapted for the work; and that the insects of the above-named tropical regions, where the Vanilla flourishes, either do not visit the flowers, though they secrete an abundance of nectar, or do not visit them in the proper manner.[8] I will mention only two peculiarities in the structure of the flowers: the anterior part of the pollen-masses is semi-waxy and the posterior part somewhat friable; the grains are not cemented together into compound grains, and the single grains are not united by fine elastic threads but by viscid matter; this matter would aid in causing the pollen to adhere to an insect, but I should have thought that such aid was superfluous, as the viscid rostellum is well developed. The other peculiarity is that the labellum, in front of the stigma, and some way beneath it, is furnished with a stiff hinged brush, formed of a series of combs one over the other, which point downwards. This structure would allow an insect to crawl easily into a flower, but would compel it whilst retreating to press close against the column; and in doing so it would remove the pollen-masses, leaving them on the stigma of the next flower which was visited.
The genus Sobralia is allied to Vanilla, and Mr. Cavendish Browne informs me that he saw a large humble-bee enter a flower of S. macrantha in his hothouse, and when it crawled out it had the two large pollen-masses firmly fixed to its back, nearer to the tail than to the head. The bee then looked about, and seeing no other flower re-entered the same one of the Sobralia, but quickly retreated, leaving the pollen-masses on the stigma, with the viscid discs alone adhering to its back. The nectar of this Guatemala Orchid seemed too powerful for our British bee, for it stretched out its legs and lay for a time as if dead on the labellum, but afterwards recovered.
- ↑ This separation of the grains was observed, and is represented, by Bauer in the plate published by Lindley in his magnificent 'Illustrations of Orchidaceous Plants.'
- ↑ Bauer figures the flowers much more widely expanded than is here represented: all that I can say is that I have not seen them in this condition.
- ↑ 'Ult. Osservaz. sulla Dicogamia,' part ii. 1875, p. 149.
- ↑ 'Proc. Boston Soc. Nat. Hist.' vol. ix. 1863, p. 182.
- ↑ 'Transact. New Zealand Institute,' vol. v. 1873, p. 352; and vol. vii. p. 351.
- ↑ 'Flora of Tasmania,' vol. ii. p. 17.
- ↑ 'Transact. New Zealand Institute,' vol. vii. 1875, p, 349.
- ↑ For Bourbon see 'Bul. Soc. Bot. de France,' tom. i. 1854, p. 290. For Tahiti see H. A. Tilley, 'Japan, the Amour, &c.,' 1861, p. 375. For the East Indies see Morren in 'Annals and Mag. of Nat. Hist' 1839, vol. iii. p. 6. I may give an analogous but more striking case from Mr Fitzgerald, who says "that Sarcochilus parviflorus (one of the Vandeæ) produces capsules not unfrequently in the Blue Mountains of New South Wales; removed from thence to Sydney, a number of plants, though flowering well, have not borne any seed if left to themselves, though invariably fertile when the pollen-masses were removed and placed on the stigma." Yet the Blue Mountains are less than one hundred miles distant from Sydney.