254 SOUTH AFRICAN FLOWERING PLANTS. 



but the third is modified to form a projection (?•.), 

 called the rostdliun. As this stands between the anther 

 and the stigmas, by far the greater number of orchids 

 are prevented from self-pollination. 



In a few the rostellum is scarcely developed when 

 the " pollen masses " — for the pollen grains are all 

 coherent into a so-called poUinium (IV.) of a different 

 orchid — can fall over it on to the stigma, and some few 

 orchids are cleistogamous ; all such do not require 

 insects to move the pollen masses. (III., sp.) is one of 

 the side petals. 



When an insect alights in search for honey in 

 orchids, the pollen masses become withdrawn from the 

 anther cells by a sticky secretion which fixes them 

 upon the head of the insect. On entering another 

 flower, the pollen mass then hits the stigma, the 

 glutinous surface of which retains it, when the insect 

 retires. Numerous differences exist in the methods of 

 pollination in orchids, many of which are described 

 by Darwin, in his book " The Fertilization of Orchids." 



(II.) is a diagram suitable for by far the greater 

 number of orchids, showing the position of the three 

 sepals and three petals (i.e. six " leaves " of the superior 

 " perianth ") ; the labellum is the petal (5) next to the 

 stem or axis (x.), and therefore posterior. There should 

 be six stamens, but only one bears pollen ; the others 

 are only detected as ridges on the perianth leaves. 

 Their positions are indicated by rings and a dot. 



