THE STEM. 



667 



Fig. 754. Transverse section 



are termed (< aerial " stems to distinguish them from the earthy and subter- 

 -anean. The aerial roots of ivy are only used for support, and are not its 

 proper roots, though some parasitic plants strike 

 into the trees and are nourished by them. 



THE STEM. 



The stem is that portion of the plant-axis 

 which grows upwards or above ground, and may 

 be, as we have just read, subterranean. As the 

 great function of the root is to procure sustenance 

 rf for the plant, the stem assists in carrying the nourish- 

 \wood,' Showing the ment through the branches and leaves. We shall 



growth of nine years. _ t 



find two forms of stem the underground, or root- 

 stock, and the stem proper. There are in these two former several varieties 

 as under : 



1. The BULB, which is a short globular stem surrounded by thick 

 leaves, and producing 



buds as, for example, 

 the onion. 



2. The TUBER, 

 similar to the foregoing 

 in shape, having no 

 leaves, however ; the po- 

 tato is an instance. 



3. The RHIZOME 

 (root-stock), like a root, 

 only producing buds, 

 which roots do not. The 



iris will serve as an example. 



The varieties of the stem-proper are : 



(i) Filiform, or thread-like, simple, or branched, as in mosses. 



(2) The culno, a thin, hollow, and frequently- 



jointed stem. 



(3) The palm or simple stem, seen in tree- 



ferns and palms. It is marked by the 

 scars of dropped leaves. 



(4) The stalk, very common, of a green hue, 



and its life is limited to a twelve- 

 month as a rule. The so-called 

 " stem " of the hyacinth is not a stem, 

 it is a stalk, or flower-stalk, pushed 

 forth for a temporary purpose. 



(5) The ligneous stem is the perfected kind, and an example will be 

 apparent in every tree. 



Fig. 755. Section (magnified portion) of the small cut a. 



Fig. 756. Section of an endo- 

 genous stem. 



