THI^J WORJv OF STKMS 



267 



148. Diagram showing the girder- 

 like arrangement of strength- 

 ening tissues {str) in a Bulrush 

 {Hcirpua). 



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 



119. Cross-section of a Cabbage leaf through the midrib. poOr COUStrUC- 



tion. When we remember, however, that the strain 

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



