. ANGIOSPERMiE 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 aud 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 Monocotyledons 



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 



