390 



THE ORCHIDALES 



ledons, their variations have adapted them as a rule to peculiar 

 conditions. They are especially abundant in the mountainous 

 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- 

 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 

 leaves have also become reduced to mere scales and the chloro- 



FIG. 292. A simple type of the Orchidales: A, the moccasin flower, 

 Cypripedium /, labellum; p, the two unmodified petals; s, sepals, two 

 being united below the labellum; b, bract, partially concealing the inferior 

 ovary. B, section of the flower /, labellum; s, stigma; an, anther; st, 

 shield-like sterile stamen covering the two anthers and stigma; o, ovary; 

 b, bract. 



phyll has disappeared. The organs of the flowers are subject to 

 such remarkable variations that the various parts may appear at 

 first somewhat difficult to recognize. It is evident that the same 

 line of variation noted in the higher families of the Liliales has 



