ANGIOSPERMZ 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 regarded 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” (2). 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 per- 
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 
‘hey are often among the showiest of flowers. Compared 
