2104 



A TEXTBOOK OF THEORETICAL BOTANY 



After pollination has taken place the terminal lobe of the labellum becomes 

 erect and once again closes up the flower. The British species of 

 Cephalanthera are found chiefly in woods, especially beech woods. 



Turning to the next genus in this group, Listera, we have two British 

 species. L. ovata (Fig. 2042) (Tway-blade) is one of the commoner British 



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Fig. 2042. — Listera ovata. Flowering plant. 



orchids found not infrequently in woods. It is not very noticeable, for the 

 flowers are greenish-yellow. The stalk bears only a single pair of rather 

 large ovate leaves. The flowers (Fig. 2043) are rather small; the labellum 

 is forked and the rostellum contains a viscid fluid which is expelled at the 

 slightest touch. This fluid comes into contact with the ends of the pollinia, 

 which when set free from the anthers lie on the concave back of the rostel- 

 lum. The effect of an insect visit is to bring away one or both pollinia in the 

 viscid drop, which sets hard in a few seconds. Nectar is secreted in a narrow 

 furrow along the upper half of the lip and this is sought after by small 

 crawling injects. As they crawl further in they find themselves under the 

 overhanging crest of the rostellum and should they raise their heads or in 

 any way touch this crest, the explosion of the liquid follows and thereafter 

 the pollinia are cemented firmly on their heads. As the insect flies away the 

 pollinia are withdrawn and when another flower is visited the pollen will 

 be liberally scattered on the stigma. Some twenty species of the genus are 

 recognized, which are widely distributed in north temperate regions. 

 Listera cordata is similar but smaller and more northerly in distribution. 



