290 



TYPICAL FLOWERING PLANTS 



4 V 4ij=p >J.;»Jf 



woody layer examined under a microscope 

 shows large cells on the inner side of each 

 layer, and smaller, thick-walled cells on the 

 outer side. The large cells are formed 

 when conditions are favorable to rapid 

 growth, and the smaller cells when condi- 

 tions are less favorable (Figure 294). A 

 dry season may check growth during the 

 middle of the summer so that an indistinct 

 rinsr will occur between two distinct ones. 

 This makes it impossible to tell accurately 

 the age of a tree by counting the rings. 



Every part of the woody stem has a 

 distinct use. The bark protects the tender 

 growing parts within. The xylem carries 

 water containing food materials from the 

 roots to the leaves, and the phloem carries 

 to other parts of the plant for use or for 

 storage the food which has been made from 

 the raw materials. As the stem increases 

 in thickness, only the outermost layers of 

 xylem continue to carry water, for the inner 

 layers fill up with a sub- 



stance which hardens 

 into wood. Although 

 they are dead, these 

 layers are still of use in 

 giving stiffness to the 

 tree. The work of the 

 tree goes on without 

 them, as is shown when 

 a tree decays in the cen- 

 ter. The pith in the 

 center of a tree and in 



Figure 294. — 

 Wood of Spruce. 



Greatly magni- 

 fied. 



WiL 





Figure 295. 



Photograph of Sections 

 of Wood. 



