66 



STRUCTURE OF BUNDLE 



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 

 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 pf the structure and character of the cells 

 themselves. Fig. 42 shows such a section of a bundle. The 



FIG. 42. Longitudinal section of the bundle shown in Fig. 41 : x, xylem ; 

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

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

 Hanson. 



vessels are now seen to be tubular structures with peculiar 

 thickenings of their inner walls that assume the form of rings 

 (ajinular 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 

 exception of numerous small spaces, pores, thus forming the 

 HJttf r 1 -ypf^lg (Fig. 42, v). These peculiar sculpturings prevent 

 the crushing in and closing of these tubes and the annular and 

 spiral thickenings also permit considerable elongation of such 

 ducts which would not be possible if the thickening were more 

 uniform over the wall. It will be noticed that the later formed 



