Fig. 81. Pollinia of Orchis pyramidalis with the disc contracted.
Fig. 82. Pollinia of Orchis pyramidalis:—a, as withdrawn by the insertion of a needle; b, as after the second contraction.
When the flower opens, the saddle-shaped disc is set free by the rupture of the membrane of the rostellum. Then the rostellum, which projects into the minute round orifice of the nectary, is easily depressed by the proboscis of a moth, and as the now naked and sticky under-surface of the saddle-shaped disc is uncovered, it adheres to the proboscis, and is withdrawn with it into the air, carrying the two pollinia ([fig. 80]) on its exterior surface. Almost instantly the saddle is exposed to the air, a rapid movement takes place. The two flaps curl inwards ([fig. 81]), and embrace the proboscis, and the pollinia, at first parallel, become divergent. A second movement now takes place, which causes the divergent pollinia ([fig. 81]), which are at right angles to the proboscis, to sweep through ninety degrees towards the tip of the proboscis, so as to become depressed, and lie on each side of it. Hence, when a moth pushes its proboscis between the guiding ridges of the labellum into the nectary of another flower, the two thick ends of the pollinia will exactly strike against the two stigmas. These stigmas are so viscid that they hold and rupture the elastic threads which bind the packets of pollen grains together, and some dark green grains are seen even with the naked eye remaining on the two white stigmatic surfaces. In the Orchids, as in all other flowering plants, tubes sent out by the pollen grains penetrate through the stigmas, and fertilize the ovules in the twisted ovary. The double movement of the pollinia, while on the moth’s proboscis, is owing to the rapidity with which the viscid matter contracts and dries. Both butterflies and moths frequent this Orchis. Mr. Darwin enumerates twenty-three species of these insects which he had seen with the pollinia of the Orchis pyramidalis attached to their proboscides. Many had two or three pairs; the proboscis of the Acherontia had seven, and that of the Caradium had no less than eleven of these saddles attached at regular distances from top to bottom of its proboscis. Few of the species of Orchis are visited by bees.
On account of some resemblance in form, the Ophreæ are named after insects. The Fly Ophrys differs in no material respect from the other Orchids. The stem or caudicle of the pollinium, instead of being straight, as in the Orchis mascula, is doubly and almost rectangularly bent. The upper membrane of the disc to which the stalk of the pollinium is fixed, being the summit of the rostellum, is exposed to the air, and becomes dry when the flower opens, consequently the disc, though viscid enough on its under-side to stick to an insect’s head, is incapable of shrinking, and causing that depression of the pollinium, characteristic of all the species of Orchis. The labellum has no spur, but at its base, just below the stigma, there is a deep depression representing the nectary; and as the pollinia, which cannot be shaken out of their cells, or pouches, are certainly, though rarely, extracted, Mr. Darwin conceives that small insects crawl along the labellum to its base, strike against one of the pouches, extract a pollinium, and fly with it sticking on their head to another blossom, and that while bending their heads into the hollow at the base of the labellum, the pollinium, owing to its doubly bent stalk, strikes the sticky stigmatic surface, and leaves pollen grains on it. There can be no doubt that this plant is visited occasionally by insects, as it cannot be fructified without them; but it is scentless, and as no nectar has as yet been found in it, their motive for visiting it is unknown.
Fig. 83. Epipactis palustris:—Side views of flower, with lower sepals cut away: A, with lip in natural position; B, with lip depressed as by an insect.
The fructification of the Ophrys apifera, or Bee Ophrys, is independent of insects, for the stalks of the pollinia are extremely long, thin, and flexible; and although their viscid discs still remain in their pouches, as soon as the flower expands, and the anther cells or pouches open, the heavy thick ends of the pollinia fall out of their cells, and hang freely down in the air exactly opposite to the stigmatic surface. A breath of air is sufficient to make them vibrate, strike the stigma with their pollen mass, and leave pollen grains on its sticky surface.
Fig. 84. Epipactis palustris:—C, side view of flower, with sepals and petals and half the labellum removed: D, front view of column: a, anther; r, rostellum; s, stigma; l, labellum.