THE WORK OF STEMS 



267 



148. Diagram showing the girder- 

 like arrangement of strength- 

 ening tissues (str) in a Bulrush 

 (Hcirpus). 



will be noticed that the thick- walled cells are placed at 

 or near the periphery, where the greatest strain comes, 

 while the center is hollow or 

 occupied by pith. 



Cut off the head of a stalk 

 of Wheat, weigh the stalk, and 

 find a wire (of steel, iron, cop- 

 per, brass, or, better still, one 

 of. each) of the same length as 

 the stalk and as nearly the 

 same weight as possible ; at- 

 tach the head of Wheat to it 

 with a small bit of sealing-wax 

 ,and compare its rigidity in an upright position with 

 that of the Wheat- stalk. 



In the blade of the leaf we find that each vein is a 

 girder, or a system of girders, and usually projects 

 from the under side of the leaf (see Fig. 149), which 

 is the best theoretical construction. 



In the root we find the woody cells, not at the 



periphery, but 

 at the center 

 (Fig. 90). This 

 may seem at 

 first glance a 



149. Cross-section of a Cabbage leaf through the midrib. pOOr COnStrUC- 



tion. When we remember, however, that the strain 

 which comes upon the root is a pulling strain, we see 



