1342 A TEXTBOOK OF THEORETICAL BOTANY 



into the flower. In doing so it comes into contact with the rostellum and on 

 withdrawing its head brings away attached to it two polhnia from the anther. 

 For the details of the process see Chapter XXX, p. 2098. 



The astonishing variety of mechanisms among Orchids may be learnt 

 from Darwin's fascinating book, " On the Fertilization of Orchids by Insects". 

 Some, as we have seen already (p. 1339), depart far from the Orchis type 

 described under Orchidaceae in Chapter XXX. Some are definitely pinch- 

 trap mechanisms, for example the Australasian Pterostylis longifolia, which 

 has a sensitive labellum (Fig. 1251). The flower has the form of an upright 

 hood with the petals and one sepal closely overlapping except for a 



Fig. 125 1. — Pterostylis Ivngifolio. A, Side view of flower with 

 the sehsitive labellum protruding. B, Vertical section of 

 flower show'ing labellum retracted and one of the side- 

 pieces of the column, between which is the insect's only 

 channel of escape. {After Darivin.) 



narrow opening in front, through which the labellum protrudes. This has a 

 distal portion like a tongue covered with long papillae and a proximal por- 

 tion Hke a piece of watch spring. It responds to the lightest touch and when 

 an insect alights on it the spring-like part folds up, lifting the distal part 

 like a drawbridge and imprisoning the insect in the flower. Two large ear- 

 pieces project from the sides of the column, forming a short tube which is 

 the only avenue of escape. Any insect passing out that way is certain to 

 remove the pollinia. There is no nectar and the flowers seem to be visited 

 exclusively by Diptera. 



The orchid genus Catasetiim presents an extraordinary and probably 

 unique case. The species are all dioecious and, while the male plants bear 

 flowers which are easily distinguishable, as to species, the flowers on the 

 female plants are not only quite difterent from the males, but may be quite 

 similar to one another in distinct species, with the result that the taxonomy 

 of the genus was for long in confusion. The male flowers possess an explo- 

 sive mechanism of great interest, a feature which they share with some 



