ANGIOSPERMAE 183 
other plants, until they reach the light, while myriads 
of parasites and air-plants cover the trunks and branches 
of the trees, all striving to hold their own in the fierce 
competition. The variety shown in the flowers and 
fruits of these plants, as well as in the leaves and stems, 
is almost infinite, and in these respects the Angiosperms 
stand far above all other plants. In spite of this 
extraordinary variety, the essential structure of the 
flowers and seeds of the Angiosperms is remarkably 
uniform, and with little question they constitute a 
perfectly homogeneous class. 
The Angiosperms fall naturally into two subclasses, — 
Monocotyledons and Dicotyledons. These show many 
points of similarity in their structure, but the differ- 
ences are sufficient to make somewhat doubtful the 
exact relationship of the two. 
THE MOoONOCOTYLEDONS 
The Monocotyledons are usually simpler than the 
Dicotyledons, both as regards their tissues and their 
flowers, although among them are certain groups, like 
the orchids, which are among the most specialized of 
all Angiosperms. As a rule they have narrow leaves 
with unbranched veins, and the vascular bundles never 
exhibit secondary thickening. The roots therefore 
never become very thick, and a tap-root is never devel- 
oped. While it is true that the parts of the flower are 
usually arranged in whorls of three, there are so many 
exceptions to the rule that it cannot be used as a satis- 
factory diagnostic character of the group as a whole. 
The embryo of the Monocotyledons is characterized 
