133° 



A TEXTBOOK OF THEORETICAL BOTANY 



downwards and is deeply cleft. The whole of the perianth is greenish- 

 yellow and the labellum has a bright green line down the centre in the posi- 

 tion of the nectar groove. The rostellum is relatively large and leaf-like 



Fig. 1242. — Listera ovata. A, Flowering shoot. B, Front view of flower. C, Vertical section 

 of flower with pollinia exposed above the downwardly curved rostellum. D, The same 

 at later stage, the pollinia ha\ing been removed and the rostellum erected. 



and at the slightest touch it expels mucilage, which appears as two white 

 tenacious drops. The anther is situated behind the rostellum and dehisces 

 while the flower is still in the bud. When it opens the pollinia are freely 

 exposed. They lie on their backs with their apices converging towards the 

 upper part of the rostellum, which at this stage arches over the stigmatic 

 surface. The rostellum then moves slightly forward and draws the pollinia 

 clear of the anther. 



Ichneumon flies and other small insects use the labellum to alight upon 

 and creep up it while they lick up the nectar secreted in the groove. When 

 they reach the top of the groove they strike their heads against the rostellum, 

 which immediately expels two drops of mucilage. These run together and 

 adhere, both to the head of the insect and also to the tips of the pollinia, 

 so that, when the insect flies away, it carries the pollinia with it. During this 

 process the rostellum remains curved over the stigmatic surface so that self- 

 pollination is prevented. Later, as the flower matures, the rostellum be- 

 comes more erect and the stigmatic surface is freely exposed. The surface 

 becomes sticky and the nectar groove again becomes filled with nectar. ' If 



