8 NATIVE BRITISH ORCHIDACE^ 



In Listera and Neottia the rostellum is a tongue-shaped organ immediately below 

 the anther, which deposits the poUinia on its upper surface. The rostellum encloses 

 a series of longitudinal tubes charged with adhesive fluid at high pressure. Its under- 

 surface is sensitive and, when touched by the head of an insect, a sort of mmiature 

 explosion occurs. Two drops of liquid glue are violently expelled, coalesce, and 

 attach themselves simultaneously to the head of the insect and the thm ends ot the 

 poUinia solidifying almost instantly. This mechanism is peculiar to these two genera. 

 In Spiranthes the rostellum is a green projection from the top of the stigrna with 

 a detachable brownish linear gland, very viscid below, to the upper surface of which 

 the pollinia become attached. Its linear shape enables it to adhere lengthwise to the 

 proboscis of insects. When removed, the green supports are left behmd like a two- 

 pronged fork. In Goodyera the rosteUum is similar, except that the viscidium is 

 circutar and truncate above, and the prongs of the fork curved. 



We now come to the Ophrydese, the most higlily orgamsed and specialised tribe 

 of the Monandry. In Hermimim the rostellum, hitherto always single, divides mto 

 two separate viscidia with a pollinium attached to each. These are shaped so as to 

 fit like a cap on a joint or elbow of the leg of very small insects. They are hollow 

 beneath and filled with adhesive matter, and rest on a strip of membrane, easdy 

 pushed aside, which keeps the viscid matter from drying up, and is the forerunner 

 of the pouch of Orchis, etc. In Cwloglosstm there is a further development. Darwm 

 says the oval ball of viscid matter is enclosed in a small pouch.^ In Vlatanthera there 

 is an extraordinary development. The viscidia or discs are large, attached to the ends 

 of the caudicles sideways, so that they look inwards, facing each other, and become 

 attached to the eyes or sides of the base of the proboscis of moths. The viscid matter 

 is naked. In Gjmnadenia the viscidia are also uncovered, and form part of the roof 

 of the mouth of the spur. They are linear and designed to adhere longitudmaUy 

 to the proboscis of Lepidoptera. In Neotinea the tiny viscid discs are enclosed m a 

 little pouch fuU of liquid, in which they lie free. It has the stigmas of Gymnadma, 

 and seems to be a half-way house to Orchis, of which it has the anther and rosteUum. 

 In Anacamptis, wliich seems to be the climax of the Gymnadenia Ime of descent, the 

 linear viscidia of the latter appear, by a half-turn outwards, to have become jomed 

 by their broader ends, with the pollinia rather close together. When withdrawn their 

 free ends curl round, grasp, and become cemented by their viscous under-surface 

 to the proboscis of Lepidoptera. The stigmas are on each side of the co umn at a 

 litde distance from each other. The curling action of the viscidium makes the pollmia 

 diverge a little, so that their tips become slightly wider apart, and are m the right 



I Darwin Vert Onh. p. 65. This is not a pouch left behind when the poUinia are removed as in 

 Or^£Tc!"but the sS of the viscidium, removed with it, and no doubt a forerunner of the pouch 

 of the rostellum. 



