1 68 Plant Life and Evolution 



which is more or less completely fused with the 

 upper part of the carpel, forming a peculiar struc- 

 ture known as the column, which distinguishes the 

 flowers of the orchids (Fig. 22, B). The flowers 

 are strongly zygomorphic, and like most highly spe- 

 cialized flowers they are, as a rule, quite depend- 

 ent upon insect aid for pollination. The orchids, 

 which constitute the second largest family of flow- 

 ering plants, show the highest degree of specializa- 

 tion found among the monocotyledons, and also 

 exhibit remarkable plasticity, as they grow under a 

 great variety of conditions. There are many epi- 

 phytic orchids, or " air plants," and a good many 

 which have lost their chlorophyll and become sapro- 

 phytic, living upon the decayed organic matter in 

 leaf mold. The orchids, however, do not as a rule 

 seem to be capable of adapting themselves readily to 

 new conditions, and never seem able to compete suc- 

 cessfully with the more aggressive plant types when 

 they are removed from their usual environment. 

 They give the impression of a group of plants in 

 the process of evolution, making experiments in 

 various directions, and developing a great variety 

 of types, most of which, however, are more or less 

 pronounced failures. 



DICOTYLEDONS 



Dicotyledons the Highest of All Plants. The 



angiosperms, in the dicotyledons, reach their most 



