NATURE OF PLANTS 65 



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 

 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 



FIG. 41. One of the vascular bundles shown in Fig. 39 enlarged: x, 

 xylem; v, vessels or ducts; p, phloem; s, sieve tube; ac, accompanying 

 cell; c, cambium; st, stereome. H. O. Hanson. 



perforated like a sieve (Fig. 41, s). A small cell, the accom- 

 panying cell, is associated with the sieve tube. Usually a vary- 

 ing amount of parenchyma also occurs in the phloem and often 

 thick walled stereome fibers (Fig. 41, st). Between the xylem 

 and phloem is a region of very delicate and regularly constructed 

 cells, the cambium (Fig. 41, c). The growth of the bundle in 

 6 



