30 THE BOOT, OR DESCRIPTIVE AXIS. 



plants firmly upon other plants or trees, while they derive their nour- 

 ishment wholly from the air. The long-moss (Tillandsia) and Conop- 

 seum arc examples. 



144. Parasites — three classes. Very different in nature arc the 

 roots of those plants called parasites, which feed upon the juices of 

 other plants or trees. Such roots penetrate the bark of the nurse-plant 

 to the cambium layer beneath, and appropriate the stolen juices to their 

 own growth, as the dodder and misseltoc. Other parasite?, although 

 standing in the soil, arc fixed upon foreign roots, and thence derive 

 either their entire sustenance, as the beach-drops and other leafless, col- 

 orless plants; or apart of their sustenance, as the cow-wheat (Melam- 

 pyrum), Gcrardia. 



145. Subterranean stems. As there aro aerial roots, so thero arc subterranean 

 Btems. These are frequently mistaken for roots, but may be known by their habit- 

 ually and regularly producing buds. Of this nature are the tubers of the Irish po- 

 tato, the root-stock of the sweet flag, the bulb of the tulip. But even the true 

 root may sometimes develop buds — accidentally as it were, in consequence of soma 

 injury to the upper axis, or some other unnatural condition. 



CHAP TEE V. 



THE STEM, OR ASCENDING AXIS. 



140. Definition. That part of the plant which originates with tho 

 plumule, tends upward in its growth and expands itself to the influence 

 of the air and the light, is called the stem or ascending axis. 



147. The general idea of the axis is the central substantial portion of tho 

 plant, bearing the appendages, viz., tho root below and the leaf-organs above. 

 Although not marked by gay coloring or fantastic forms, yet we regard the stem 

 with a lively interest for its substantial value, its gracefulness and lofty propor- 

 tions, its infinite gradation of form and texturo from the tender speedwell crushed 

 beneath tho foot to the strong forest oak. 



SS. Procumbent stem — Cbiogencs hispidula. 



148. Direction of its growth. Although the first direction of 

 the stem's growth is vertical in all plants, there arc many in which thi» 

 direction docs not continue, but changes into the oblique or horizontal, 

 either just above the surface of the ground, or just beneath it. If the 



