NATURE OF PLANTS 



79 



lem), an outer thin-walled portion (phloem), and between these 

 two regions a delicate layer of cells, the cambium (Fig. 42). 

 The transport of all substances is largely confined to the vas- 

 cular bundles, the xylem conducting principally the crude ma- 

 terials while the bulk of the organic substances passes through 

 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 



FIG. 42. One of the vascular bundles shown in Fig. 40 enlarged: x, xylem; 

 v, vessels or ducts; p, phloem; s, sieve tube; ac, accompanying cell; c, cam- 

 bium; st, stereome. H. O. Hanson. 



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. 

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

 because the various cells of the phloem are comparatively large 



