DEVELOPMENT OF PLANTS 



407 



districts of the tropics, where they more commonly appear as 

 epiphytes upon the trunks of trees and in the crevices of rocks. 

 Such conditions are met by the development of a thick mantle 

 of cells, the velamen, about the aerial roots which absorbs the 

 moisture from the air and doubtless the enlargement of the leaf 

 base into a bulbous storage organ enables these plants to antici- 



FiG. 291. An epiphytic orchid growing upon the branch of a tree. The 

 coarse roots, r, are surrounded by a mantle of cells which takes up the mois- 

 ture from the atmosphere, b, storage organs formed from the base of the 

 leaves, enabling the plant to produce flowers and fruit. The smaller stalks, 

 c, are the shriveled remains of these organs after flowering and fruiting. 



pate in this way the heavy demands that will be made upon them 

 in the flowering season (Fig. 291). The terrestrial forms are 

 largely parasitic or saprophytic and associated with mycorrhiza, 

 and this has resulted in some of the forms in the suppression of 

 various organs of the plant as the primary root or even the entire 

 root system, as is illustrated in the coral root orchid, where the 



