CHAPTER XIV 



THE STALK OP THE PLANT 



THE stalk is the common support of the plant's 

 various parts. It is called annual or herba- 

 ceous when it lives only one year, as in the potato, 

 spinach, parsley, and all forms of vegetation that 

 from their soft structure belong to the class of herbs. 

 Ligneous is the name given to the stalk when, de- 

 signed to live for a greater or less number of years, 

 it is made of strong woody fibers, such as we find 

 in the trunks of trees. 



"Let us make a clean cut through any tree-trunk, 

 that of an oak for example. We shall find it divided 

 into three parts: in the center the pith or marrow, 

 very slightly developed; around the marrow the 

 wood proper; and, finally, on the outside, the bark. 

 A closer examination shows that the wood is formed 

 of concentric layers which are indicated in the cross- 

 section by a series of circles having the marrow for 

 a common center. These layers are called ligneous 

 zones or, since one is formed every year, annual 

 layers. During the summer there is a downward 

 flow, throughout the tree, of a peculiar liquid, the 

 descending sap, which constitutes the fluid nourish- 

 ment of the tree. This liquid runs between the wood 

 and the bark and becomes, little by little in its course, 



70 



