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 c^nEned^to the vas- 

 cular bundles, the xylem conducting principally the crude rAa^ 

 terials while the bulk of the organic substances passes through 

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

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

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

 42, z>), 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 



