THE NUTRITIVE ORGANS OF PLANTS 



49 



walled wood-cells are developed. Each wood-cell, when 

 separated out from the rest, and examined with the high 

 power of the compound microscope, is seen to be shaped 

 somewhat like a tiny toothpick, and the thin ends of these 

 cells fit together closely by overlapping. (See Fig. 15.) 



bast 

 cells 



sieve tube 



duct 



duct 



FIG. 15. Woody bundle of sunflower stem. 



Stems like the corn stalk and bamboo have most of their sup- 

 porting material on the outside, and these stems are in the form of 

 cylinders which are either hollow (as in grasses) or filled with pith 

 through which pass the woody bundles (as in the corn stalk). It 

 has been proved that when a given amount of material is arranged 

 in the form of a hollow tube, it will withstand a much greater strain 

 without breaking than when this material is in the form of a solid 

 rod. This mechanical principle is made use of in the construction 

 of the frame of a bicycle and of the pillars that support buildings. 



63. Stems as organs for the transmission of sap. Leaves 

 not only require an abundance of sunlight, but they must 



