STUDIES OF SEED-PLANTS 



173 



side. It is in this way that the new rings of wood and bark are 

 formed in the dicotyledonous stem. 



Longitudinal sections should be examined with the microscope 

 and compared with Fig. 50, which will serve to identify the parts seen. 



FIG. 50. Upper figure repres9nts a transverse section of a fibro-vascular 

 bundle of moonseed. Bast is at left of 2, wood is at right. Lower figure is 

 a longitudinal section of a sunflower bundle. Cambium at 2 in upper figure, 

 c, in lower one ; d and g, wood-tubes ; h, wood cells between tubes ; s, sieve- 

 tubes ; p and bp, parenchyma or pith ; 6, bast. (From Bailey, lower figure 

 after Wettstein.) 



165. Functions of Parts of Stem. Wood-cells and bast- 

 fibers are very rigid cells because of their thick walls, and 

 thus are able to give the support which is so necessary if a 

 plant is to grow very large. They also serve as a protection 

 to the tubes and more delicate cells which they surround. 



