2io6 



A TEXTBOOK OF THEORETICAL BOTANY 



back of the mucilage receptacle. The 

 oblique surfaces of the stigma project be- 

 low the rostellum. 



Darwin observed that the flowers were 

 visited by humble bees. The mucilage 

 receptacles with their adherent pollinia 

 cling to their probosces so that eventually 

 only the lateral parts of the rostellum are 

 left. After the flower has been open for a 

 day or two the labellum moves away from 

 the rostellum thus widening the approach 

 to the stigmas. When this has happened 

 a bee with pollinia on it entering the flower 

 will cause pollination to take place. Since 

 this time-interval must elapse before 

 pollination can occur it follows that the 

 pollinia concerned are more likely to be 

 derived from a diflFerent plant than from 

 the flowers of the same inflorescence. 



The genus Goodyera contains about 

 forty species distributed mainly in North 

 America. Species also occur in tropical 

 Asia, in New Caledonia and the Masca- 

 renes. One species, G. repens, is found in 

 Britain. Darwin studied the pollination 

 (Fig. 2046) mechanism and found that it 

 was worked by humble bees. The flowers 

 are small and white but only feebly 

 fragrant. The rostellum is square and 

 shieldlike and projects beyond the stigma. 

 On being lightly pressed mucilage is exuded 

 from the projecting surface which is easily 

 displaced upwards. In this process it 

 carries with it a membranous strip to the 

 posterior end of which the pollinia adhere. 

 The anther lobes dehisce while the flower 

 is still in the bud and the pollinia cling 

 with their anterior sides to the back of the 

 rostellum. In this way they are completely 

 exposed. The posterior part of the 

 labellum contains nectar while the anterior 

 part serves as a platform, but owing to 

 the narrowness of the entrance only the proboscis of the insect can be 

 inserted and in search for the nectar it is almost certain to strike against 

 and remove the pollinia. In older flowers the labellum moves away from 



i 



Fig. 2044. — Spiranthes spiralis. 

 Lady's Tresses. Inflorescence 

 enlarged X 2. Plant growing in 

 the lawn outside University 

 College, Cardiff. 



