ORCHIDS, FORMS OF FLOWERS, POLLINATION
Epipactis is a large orchid genus of some seventy species, placed in the tribe Neotteae, a group with free-standing anthers. Darwin was curious about Epipactis palustris (the marsh helleborine), and in 1860, he requested fresh flowers from the botanist Alexander More from the Isle of Wight, so he could examine and manipulate the various parts of the flowers to determine their pollination mechanism. He also asked More to observe pollinators and do experiments on flowers in their native habitat to confirm his observations on the movement of the labellum and pollinia in promoting pollination. It was a poor time for pollination studies in the field, however, as the summer of 1860 was one of the coldest and wettest on record in the British Isles. “The only chance of seeing insects at work,” he wrote More hopefully, “would be the first bright day after this miserable weather or a bright gleam of few hours in middle of one of our gloomy days.”67 Darwin later noted that “we see the injurious effects of the extraordinary cold and wet season of 1860 in the infrequency of the visits of insects.”68 His son William made observations on the Isle of Wight several years later, describing mainly honeybees pollinating the plants.
A related species, Epipactis latifolia (= E. helleborine) appeared in Darwin’s yard unexpectedly, prompting him to report it to the Gardeners’ Chronicle.69 Although not rare then or now (it has become widely naturalized even in North America, where it is considered a weed by some), what was surprising, as he described in his report, was where it appeared—right in the middle of his gravel walking path, now known as the sand-walk. Given the history of disturbance, first as a carriage road and then as his gravel footpath, had this orchid been lying dormant for years? Or maybe a seed was blown there from some far-flung locale?
Not one to waste an opportunity, he carefully observed the guest orchid over several seasons, and found wasps to be its main pollinators, sucking nectar out of the big cup-shaped labellum while pollen masses became attached to their foreheads to be carried to other flowers.70 He would have surely marveled at more modern studies showing that, like many flowering plants, E. helleborine orchids have additional tricks up their sleeves (er, flowers) to facilitate pollination. They produce a chemical cocktail in their nectar that includes both attractants and compounds with narcotic or soporific qualities that could function to keep the wasps in the vicinity, increasing the likelihood of visitation to additional flowers.71
Epipactis palustris. The flowers stand out (fig. A) almost horizontally from the stem. The labellum is curiously shaped, as may be seen in the drawings: the distal half, which projects beyond the other petals and forms an excellent landing-place for insects, is joined to the basal half by a narrow hinge, and naturally is turned a little upwards, so that its edges pass within the edges of the basal portion. So flexible and elastic is the hinge that the weight of even a fly, as Mr. More informs me, depresses the distal portion; it is represented in fig. B in this state; but when the weight is removed it instantly springs up to its former position (fig. A), and with its curious medial ridges partly closes the entrance into the flower. The basal portion of the labellum forms a cup, which at the proper time is filled with nectar.
Epipactis palustris. A. Side view of flower (with the lower sepals alone removed) in its natural position. B. Side view of flower, with the distal portion of the labellum depressed, as if by the weight of an insect. C. Side view of flower, with all the sepals and petals removed, excepting the labellum, of which the near side has been cut away; the massive anther is seen to be of large size. D. Front view of column, with all the sepals and petals removed
a. anther, with the two open cells seen in the front view. a’. rudimentary anther, or auricle, referred to in a future chapter. r. rostellum. s. stigma. l. labellum.
Now let us see how all the parts, which I have been obliged to describe in detail, act. When I first examined these flowers, I was much perplexed: trying in the same manner as I should have done with a true Orchis, I slightly pushed the protuberant rostellum downwards, and it was easily ruptured; some of the viscid matter was withdrawn, but the pollinia remained in their cells. Reflecting on the structure of the flower, it occurred to me that an insect in entering one in order to suck the nectar would depress the distal portion of the labellum, and consequently would not touch the rostellum; but that, when within the flower, it would be almost compelled, from the springing up of this distal half of the labellum, to rise a little upwards and back out parallel to the stigma. I then brushed the rostellum lightly upwards and backwards with the end of a feather and other such objects; and it was pretty to see how easily the membranous cap of the rostellum came off, and how well from its elasticity it fitted any object, whatever its shape might be, and how firmly it clung to the object owing to the viscidity of its under surface. Large masses of pollen, adhering by the elastic threads to the cap of the rostellum were at the same time withdrawn.
Nevertheless, the pollen-masses were not removed nearly so cleanly as those which had been naturally removed by insects. I tried dozens of flowers, always with the same imperfect result. It then occurred to me that an insect in backing out of the flower would naturally push with some part of its body against the blunt and projecting upper end of the anther, which overhangs the stigmatic surface. Accordingly, I so held a brush that, whilst brushing upwards against the rostellum, I pushed against the blunt solid end of the anther (see fig. C); this at once eased the pollinia, and they were withdrawn in an entire state. At last I understood the mechanism of the flower. …
Epipactis latifolia.—This species agrees with the last in most respects. The rostellum, however, projects considerably further beyond the face of the stigma, and the blunt upper end of the anther less so. The viscid matter lining the elastic cap of the rostellum takes a longer time to get dry. The upper petals and sepals are more widely expanded than in E. palustris: the distal portion of the labellum is smaller and is firmly united to the basal portion so that it is not flexible and elastic; it apparently serves only as a landing-place for insects. The fertilisation of this species depends simply on an insect striking in an upward and backward direction the highly-protuberant rostellum, which it would be apt to do when retreating from the flower after having sucked the copious nectar in the cup of the labellum. Apparently it is not at all necessary that the insect should push upwards the blunt upper end of the anther; at least I found that the pollinia could be removed easily by simply dragging off the cap of the rostellum in an upward or backward direction.
As some plants grew close to my house, I have been able to observe here and elsewhere their manner of fertilisation during several years. Although hive-bees and humble-bees of many kinds were constantly flying over the plants, I never saw a bee or any Dipterous insect visit the flowers; but in Germany, Sprengel caught a fly with the pollinia of this plant attached to its back. On the other hand, I have repeatedly observed the common wasp (Vespa sylvestris) sucking the nectar out of the open cup-shaped labellum. I thus saw the act of fertilisation effected by the pollen-masses being removed by the wasps and afterwards carried attached to their foreheads to other flowers. … It is very remarkable that the sweet nectar of this Epipactis should not be attractive to any kind of bee. If wasps were to become extinct in any district, so probably would the Epipactis latifolia.
Fragaria vesca. Watercolor by Elizabeth Blackwell, A Curious Herbal.