2102 A TEXTBOOK OF THEORETICAL BOTANY 



B. Acrotonae 



Caudicle arising from the apex of the polHnium. The anthers erect 

 or incumbent, filaments short, slender and generally closely joined to the 

 column, usually deciduous, but if persisting they soon wither. 



I. Polychondreae. Pollen granular and soft; anthers mostly persistent; 

 inflorescence always terminal. 



In this tribe are a large number of genera including the following: 

 Epipactis, Cephalanthera, Spiranthes, Listera, Neottia, Goodyera and Vanilla. 



With the exception of Vanilla all these genera are represented in the 

 British Flora. Vanilla is a small tropical genus with about thirty species. 

 They are climbers with fleshy leaves. The only important species is V. 

 planifolia, which is widely cultivated for its capsules, from which the spice 

 Vanilla is prepared. 



The genus Epipactis contains ten north temperate species, two of which, 

 E. helleborine (Broad Helleborine) and E. palustris (Marsh Helleborine), are 

 common in Britain. Epipactis helleborine is pollinated by wasps. The flowers 

 are borne horizontally and the outer part of the labellum forms a platform for 

 the insect visitor, the inner part forming a cup containing nectar. The lower 

 part of the stigma is bilobed, with a small, almost globular rostellum pro- 

 jecting above it. It is covered with a viscid cap which can be easily lifted 2 

 off by pressure from inside and below. The anthers dehisce longitudinally 

 before the flower opens so that the two sessile pollinia are exposed. Their 

 pollen grains are united into small packets by elastic threads, which are 

 joined to cords fastened to the posterior lobes of the cap covering the rostel- 

 lum. When an insect visits the flower it alights on the front part of the 

 labellum and crawls into the flower. It is prevented from touching the 

 pollinia by the rostellum. As the insect creeps out it lifts the rostellum 

 cap and comes into contact with and removes the pollinia on its head or 

 back. When it visits another flower these pollinia will already be in a 

 position suitable to touch the stigmatic surfaces as the insect enters the 

 flower. 



The genus Cephalanthera contains ten species, three of which are found 

 in Britain. The flowers are characterized by the complete absence of a 

 rostellum, the pollen germinating in situ and fertilizing its own stigma. 

 This is well seen in C. darnasoniiim (Fig. 2040) (White Helleborine) in 

 which the pollen is loose and friable, the grains being almost entirely 

 separated from one another with only a very few threads uniting them here 

 and there (Fig. 2041). The anthers dehisce before the flowers open and the 

 pollinia fall on to the upper margin of the stigma which lies below them. 

 In this way automatic self-pollination inevitably takes place. Cross-pollina- 

 tion by means of insects is, however, possible. The labellum is vertical and 

 closes the flower but the anterior part folds down and serves as a platform 

 and the visiting insect scatters the pollen broadcast into the upright flowers. 

 In so doing, it will itself receive a liberal share and when it subsequently 

 visits another flower it may shake off some of this pollen on to the stigma. 



