ANGIOSPERMiE 195 



the trees with great masses of their spiky leaves, with 

 here and there clusters of showy crimson bracts, or in 

 some cases gayly colored flowers. The broad, overlap- 

 ping leaf -bases, and the scales upon them, form efficient 

 reservoirs both for water and the accumulation of vege- 

 table mould, which these "air-plants" need for their 

 subsistence, as they are in no sense parasites upon the 

 trees to which they are attached. 



Probably to be i-egarded as the most specialized of all 

 the Monocotyledons are the Orchids. In these the 

 flower is strongly zygomorphic (Fig. 47, C, E), and 

 usually one petal is decidedly different from the others 

 and forms the "lip " (Z). In much the greater number 

 of them the stamens are reduced to a single one, which 

 is coherent with the upper part of the pistil and forms 

 with it the "column" (Fig. 47, D, F), but sometimes 

 two stamens are present. Usually the pollen-spores 

 are held together in masses (pollinia) by a viscid sub- 

 stance, and the position of the pollinia is such that 

 insect aid is necessary to dislodge them and transfer 

 the pollen to the stigma. We find, consequently, among 

 the orchids a wonderful variety of ingenious devices by 

 which cross-fertilization is effected. Sometimes the 

 flower is adapted to pollination by a single species of 

 insect upon which it is absolutely dependent. 



In spite of these perfect adaptations for cross-fertiliza- 

 tion, the orchids seem for some reason to be less pei- 

 fectly suited to their environment than many other 

 plants. They seldom occur in such great numbers 

 together as to make much of an impression upon the 

 aspect of the vegetation as a whole, although individually 

 they are often among the showiest of flowers. Compared 



