"U 



BOTANY 



PAKT II 



Fio. 782.— Orchis ■iiiUitarii'. (J iiat. size.) 



The caudicles terminate below 

 ill contact with the rostelluni 

 which forms touf^h adhesive 

 discs. Tliis relation to the 

 rostellum serves to keep the 

 pollinia, which lie free in the 

 pollen-sacs, in position, and the 

 adhesive discs attach the pol- 

 linia to any body that comes in 

 contact with them. If an in- 

 sect 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 be- 

 come attached to it. As the 

 adhesive discs dry they cause 

 the pollinia to bend forward, 

 so that when 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 contriv- 

 ances are far more complicated ; 

 pollination does not take place 

 in the absence of the insects. 

 In many cases the adaptations 

 are so specialised to particular 

 insects that no other insect will 

 do instead. Thus Vanilla 

 brought from its American habi- 

 tats to other tropical countries 

 remains sterile on account of the 

 absence of the iioUinating insect. 

 On this fact being discovered 

 artificial pollination was re- 

 sorted to and the plants can thus 

 be induced to bear fruits regu- 

 larly. It should be mentioned 

 that in some forms, e.g. Vanilla, 

 the pollen remains powdery. 

 Many tropical Orchids are cul- 

 tivated in greenhouses on ac- 

 count of the beauty of their 

 flowers. 



Fossil Angiosperms (*"') 



The first undoulited Angio- 

 sperms appear in the Upper 



