Classification of Plants 275 



The rostellum is an interesting example of an organ which 

 has lost its original use and has then acquired a new way of 

 serving the plant. (Miss Duthie has found that the stigmatic 

 surface in some species of Pterygodium extends some distance 

 up on to the arms of the rostellum, which have not entirely 

 lost their original function.) 



The three carpels unite to form a single-chambered ovary, 

 which is filled with an immense number of ovules. The fruit 

 is an inferior capsule. 



Dr. Bolus mentions that, with all the wonderful provisions for insect 

 pollination, some orchids produce new plants entirely by tubers. This is 

 possibly owing to the dying out of the insects which could effect pollina- 

 tion, owing to bush fires or other causes. The Western orchids are 

 usually tuberous-rooted. Many in the East send up new shoots from 

 creeping rhizomes, the food being sometimes stored in the lower swollen 

 parts of stems. 



FIG. 248. I. Floral Diagram of Disa. (From Edmonds and Marloth's 

 " Elementary Botany for South Africa ".) II. Pollen masses of an orchid joined 

 with caudicles to a single gland. (From Thome" and Bennett's " Structural and 

 Physiological Botany".) 



KEY TO SOME OF THE LARGER GENERA OF ORCHIDS. 



A. Anthers lid-like. 

 B. Pollen masses 2-8, waxy. Not joined to a 



gland of the rostellum. 

 Petals linear ; lip entire ; terrestrial or 



epiphytic. Pollinia 4 . . . . LIPARIS. 

 BB. Pollen masses waxy, stalk united to a gland 



of the rostellum. 

 C. Lip not spurred ; 3-lobed ; epiphytic . POLYSTACHYA. 



18 * 



