748 



BOTANY 



PART II 



/r 



FIG. 833. Orchis militaris ( nat. size). 



pollen of each of the two pollen 

 sacs is joined together by an 

 interstitial substance which 

 continues below to form a 

 stalk ; the whole structure, 

 which has a waxy consistence, 

 is called a pollinium, and the 

 stalk goes by the name of the 

 caudicle. The caudicles ter- 

 minate below in contact with 

 the rostellum which forms tough 

 adhesive discs. This relation 

 to the rostellum serves to keep 

 the pollinia, which lie free in 

 the pollen sacs, in position, and 

 the adhesive discs attach the 

 pollinia to any body that comes 

 in contact with them. If an 

 insect alights on the lower lip 

 and attempts to reach the nectar 

 secreted in the spur, its head or 

 tongue must touch the rostel- 

 lum and the pollinia will become 

 attached to it. As the adhesive 

 discs dry they cause the pollinia 

 to bend forward, so that Avhen 

 the insect visits a second flower 

 they will be brought in contact 

 with the stigmatic surfaces. 



All Orchids are similarly 

 adapted to insect visitors, 

 though in many the contri vances 

 are far more complicated ; pol- 

 lination does not take place in 

 the absence of the insects ( 47 ). 

 In many cases the adaptations 

 are so specialised to particular 

 insects that no other insect will 

 do instead. Thus Vanilla (Fig. 

 832) brought from its American 

 habitats to other tropical coun- 

 tries remains sterile on account 

 of the absence of the pollinating 

 insect. On this fact being dis- 

 covered artificial pollination 

 was resorted to and the plants 

 can thus be induced to bear 

 fruits regularly. It should be 

 mentioned that in some forms, 

 e.g. Vanilla, the pollen remains 

 powdery. Many tropical 



