8o 



STRUCTURE OF BUNDLE 



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, which are here known as sieve tubes, 

 because these cells are tube-like structures with the cross walls 

 perforated like a sieve (Fig. 4.2, 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. 42, st). Between the xylem 

 and phloem is a region of very delicate and regularly constructed 

 cells, the cambium (Fig. 42, c). The growth of the bundle in 

 diameter and, in fact, of the entire stem is brought about very 

 largely by the formation of new cells through the division of the 



Fig. 43. Longitudinal section of the bundle shown in Fig. 42: x, xylem; 

 ph, phloem; p, pith; v-v'", annular, spiral, scalariform and pitted vessels; 

 c, cambium; s, sieve tubes; ac, accompanying cells; st, stereome. — H. O. 

 Hanson. 



cells in the cambium. The cross section of the bundle reveals 

 the arrangement and distribution of the various tissues but it 

 will be necessary to examine a section taken parallel with the 

 length of the stem, i. e., a longitudinal section, in order to arrive 

 at an understanding of the structure and character of the cells 

 themselves. Fig. 43 shows such a section of a bundle. The 



