ORCHIDS, FORMS OF FLOWERS, POLLINATION
Spiranthes, with the common name ladies tresses describing its attractive attire, is a cosmopolitan orchid genus of about forty species, distinguished by spiraled spikes of flowers fertilized by moths and bees. Darwin obtained plants of Spiranthes autumnalis (= S. spiralis) from his correspondent Alexander More on the Isle of Wight, and also found them while on vacation at Torquay. In his orchid book, Darwin described the long, flat “boat-formed” structure (viscidium) attached at the base of the pollinia (pollen packets), bearing adhesive fluid that guaranteed strong attachment to a pollinator’s proboscis (or even a faux proboscis—Darwin and More experimented with blades of grass and needles, poking them into flowers to trigger the “curious contrivance” of movement and transfer of the pollinia). He saw a bee with five such “boats” stuck to its proboscis, carrying them away to another flower for pollination.
Darwin’s delight and wonder at the elaborate pollination mechanism of Spiranthes is evident in his meticulous description, from the fit of an insect’s proboscis in the flower to how the flowers transition from female to male up the spike, termed “protandry” today. The flowers are functionally male at first, presenting pollinia but not allowing access to the stigma. They open further as they age—hence, the older, lower, flowers in a spike become pollinated first since insects tend to “alight at the bottom and crawl up the spike,” as Darwin observed, depositing pollen from the top-most flowers of the last spike they visited and, making their way up, picking up a fresh supply for the next plant.
Eager to see how general his findings were, Darwin asked botanist Asa Gray to examine the North American species Spiranthes cernua and S. gracilis: “I enclose sketch about Spiranthes; if you will observe your species, I should be infinitely obliged.” Gray later confirmed the observations: “The difference between the older flowers and those first opened is striking. The latter presents the disc—the former the stigma.”137
Spiranthes autumnalis.—This Orchid with its pretty name of Ladies’-tresses, presents some interesting peculiarities. The rostellum is a long, thin, flat projection, joined by sloping shoulders to the summit of the stigma. In the middle of the rostellum a narrow vertical brown object (fig. C) may be seen, bordered and covered by transparent membrane. This brown object I will call “the boat-formed disc.” It forms the middle portion of the posterior surface of the rostellum and consists of a narrow strip of the exterior membrane in a modified condition. When removed from its attachment, its summit (fig. E) is seen to be pointed, with the lower end rounded; it is slightly bowed, so as altogether to resemble a boat or canoe. …
The stigma lies beneath the rostellum and projects with a sloping surface, as may be seen at B in the side-view: its lower margin is rounded and fringed with hairs. On each side a membrane (cl, B) extends from the edges of the stigma to the filament of the anther, thus forming a membranous cup or clinandrum, in which the lower ends of the pollen-masses lie safely protected. …
The tubular flowers are elegantly arranged in a spire round the spike, and project from it horizontally (fig. A). The labellum is channelled down the middle, and is furnished with a reflexed and fringed lip, on which bees alight; its basal internal angles are produced into two globular processes, which secrete an abundance of nectar. The nectar is collected (n, fig. B) in a small receptacle in the lower part of the labellum. Owing to the protuberance of the inferior margin of the stigma and of the two lateral inflexed nectaries, the orifice into the nectar-receptacle is much contracted. When the flower first opens, the receptacle contains nectar, and at this period, the front of the rostellum, which is slightly furrowed, lies close to the channelled labellum; consequently a passage is left, but so narrow that only a fine bristle can be passed down it. In a day or two, the column moves a little farther from the labellum, and a wider passage is left for insects to deposit pollen on the stigmatic surface. On this slight movement of the column the fertilisation of the flower absolutely depends. …
Spiranthes autumnalis. A. Side view of flower in its natural position, with the two lower sepals alone removed. The labellum can be recognised by its fringed and reflexed lip. B. Side view of a mature flower, with all the sepals and petals removed. The position of the labellum (which has moved from the rostellum) and the upper sepal is shown by the dotted lines. C. Front view of the stigma, and of the rostellum with its embedded central disc. D. Front view of the stigma and of the rostellum after the viscid disc has been removed. E. Viscid disc, removed from the rostellum, greatly magnified, viewed posteriorly, and with the attached elastic threads of the pollen-masses; the pollen-grains have been removed from the threads.
a. anther. p. pollen-masses. t. threads of the pollen-masses. cl. margin of clinandrum. r. rostellum. s. stigma. n. nectar-receptacle.
We thus see how beautifully everything is contrived that the pollinia should be withdrawn by insects visiting the flowers. They are already attached to the disc by their threads, and, from the early withering of the anther-cells, they hang loosely suspended but protected within the clinandrum. The touch of the proboscis causes the rostellum to split in front and behind, and frees the long, narrow, boat-formed disc, which is filled with extremely viscid matter, and is sure to adhere longitudinally to the proboscis. When the bee flies away, so surely will it carry away the pollinia. As the pollinia are attached parallel to the disc, they adhere parallel to the proboscis. When the flower first opens and is best adapted for the removal of the pollinia, the labellum lies so close to the rostellum that the pollinia attached to the proboscis of an insect cannot possibly be forced into the passage so as to reach the stigma; they would be either upturned or broken off: but we have seen that after two or three days, the column becomes more reflexed and moves from the labellum,—a wider passage being thus left. When I inserted the pollinia attached to a fine bristle into the nectar-receptacle of a flower in this condition (n, fig. B), it was pretty to see how surely the sheets of pollen were left adhering to the viscid stigma. …
Hence in Spiranthes, recently expanded flower, which has its pollinia in the best state for removal, cannot be fertilised; and mature flowers will be fertilised by pollen from younger flowers, borne, as we shall presently see, on a separate plant. In conformity with this fact, the stigmatic surfaces of the older flowers are far more viscid than those of the younger flowers. Nevertheless, a flower which in its early state had not been visited by insects would not necessarily, in its later and more expanded condition, have its pollen wasted: for insects, in inserting and withdrawing their proboscides, bow them forwards or upwards, and would thus often strike the furrow in the rostellum. I imitated this action with a bristle, and often succeeded in withdrawing the pollinia from old flowers. I was led to make this trial from having at first chosen old flowers for examination; and on passing a bristle, or fine culm of grass, straight down into the nectary, the pollinia were never withdrawn; but when it was bowed forward, I succeeded. Flowers which have not had their pollinia removed can be fertilised as easily as those which have lost them; and I have seen not a few cases of flowers with their pollinia still in place, with sheets of pollen on their stigmas.
At Torquay I watched for about half an hour a number of these flowers growing together, and saw three humble-bees of two kinds visit them. … The next day I watched the same flowers for a quarter of an hour and caught another humble-bee at work; one perfect pollinium and four boat-formed discs adhered to its proboscis, one on the top of the other, showing how exactly the same part of the rostellum had each time been touched.
The bees always alighted at the bottom of the spike, and, crawling spirally up it, sucked one flower after the other. I believe humble-bees generally act in this manner when visiting a dense spike of flowers, as it is the most convenient method; on the same principle that a woodpecker always climbs up a tree in search of insects. This seems an insignificant observation; but see the result. In the early morning, when the bee starts on her rounds, let us suppose that she alighted on the summit of a spike; she would certainly extract the pollinia from the uppermost and last opened flowers; but when visiting the next succeeding flower, of which the column in all probability would not as yet have moved from the labellum (for this is slowly and very gradually effected), the pollen-masses would be brushed off her proboscis and wasted. But nature suffers no such waste. The bee goes first to the lowest flower, and, crawling spirally up the spike, effects nothing on the first spike which she visits till she reaches the upper flowers, and then she withdraws the pollinia. She soon flies to another plant, and, alighting on the lowest and oldest flower, into which a wide passage will have been formed from the greater reflexion of the column, the pollinia strike the protuberant stigma. If the stigma of the lowest flower has already been fully fertilised, little or no pollen will be left on its dried surface; but on the next succeeding flower, of which the stigma is adhesive, large sheets of pollen will be left. Then as soon as the bee arrives near the summit of the spike she will withdraw fresh pollinia, will fly to the lower flowers on another plant, and fertilise them; and thus, as she goes her rounds and adds to her store of honey, she continually fertilises fresh flowers and perpetuates the race of our autumnal Spiranthes, which will yield honey to future generations of bees.
Trifolium pratense. Watercolor by Elizabeth Wharton, British Flowers.