NATURE OF PLANTS 



75 



perforated like a sieve (Fig. 42, 5). 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 

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

 the arrangement and distribution of the various tissues but it 

 wnll be necessary to examine a section taken parallel with the 



Fig. 43. Longitudinal section of the bundle shown in Fig. 41: 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. 



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 

 vessels are now seen to be tubular structures with peculiar 

 thickenings of their inner walls that assume the form of rings 

 (annular vessels), spirals (spiral vessels), or the spirals may 

 branch more or less (reticulate vessels), and often to such an 

 extent that they cover the entire surface of the wall with the 



