74 STEM STRUCTURE 



the phloem. We are now interested to study the character of 

 these cells and note their adaptation to the work in hand. In 

 the xylem occur various large spaces, the vessels or ducts (Fig. 

 42, v), and smaller spaces, wood cells of different kinds. The 

 cells of the phloem are much smaller, thinner walled and less 

 numerous than those of the xylem. Therefore very thin sec- 

 tions are necessary in order to make clear all the different tissues. 



/ 



Fig. 42. One of the vascular bundles shown in Fig. 39 enlArged: x, xylem; 

 V, vessels or ducts; p, phloem; 5, sieve tube; ac, accompanying cell; c, camb- 

 ium; st, stereome. — H. O. Hanson. 



Sections from a squash stem may well be studied for this purpose 

 because the various cells of the phloem are comparatively large 

 and easily distinguishable. It should be stated that the bundle 

 of the squash is peculiar in that a phloem region is developed on 

 both sides of the xylem. The phloem, like the xylem, is also 

 characterized by large cells, the sieve tubes, so named because 

 these cells are elongated, tube like structures with the cross walls 



