422 THE ORCHIDALES 



highest group of the monocotyledons and their flowers are a 

 source of wonder and admiration, owing to the singular beauty 

 and delicacy of their mechanical construction. Variation in this 

 order has occurred on a gigantic scale, resulting in a larger num- 

 ber of species (over 7,000) than is found in any of the preceding 

 orders. Nevertheless these elaborate variations have not been 

 very successful in enabling them to compete with other plants, 

 and as a result the orchids are rather rare and not at all com- 

 parable in number of individuals with the lilies and grasses. 

 Though more widely distributed than any of the other monocoty- 

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

 conditions. They are especially abundant in the mountainous 

 districts of the tropics, \vhere 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 in some of these plants into a bulbous storage organ enables 

 them to anticipate in this way the heavy demands that will be 

 made upon them in the flowering season (Fig. 291). The ter- 

 restrial 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, \vhere the leaves have also become reduced to mere scales 

 and the chlorophyll has disappeared. The dependence upon 

 fungal life has reached such a stage in some forms that the seeds 

 do not germinate unless hyphae come in contact with them 

 (p. 69). The organs of the flow r ers are subject to such remark- 

 able variations that the various parts may appear at first some- 

 what difficult to recognize. It is evident that the same line 

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

 continued, for the perianth is arranged upon a compound ovary, 

 but its parts are often sharply differentiated into calyx and 

 corolla. The sepals and petals comprising these two whorls 

 differ greatly in form, but especially to be noted is the oddly con- 

 structed petal known as the labellum (Figs. 292, 293, /). This 



