66 



STRUCTURE < IF BUNDLE 



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

 v b) 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 t«» examine a section taken parallel with the 



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

 at an understanding of the structure ami character of the cells 



themselves. Fig. 4_' shows such a section of a bundle. The 



Fig. 4-'. Longitudinal section of the bundle shown in Fig, 41 : x, xylem; 

 fit, phloem; /\ pith; v-v'" , annular, spiral, scalariform ami pitted \< 

 c, cambium; s, sieve tubes; ac, accompanying cells; st, Btereome.— H. ( V 



1 lanson. 



els arc now seen to be tubular structures with peculiar 



thickenings of their inner walls thai assume the form of rings 



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



branch more or less (reticulate v( and often to such an 



i that the) cover tin- entire Burface of the wall with the 



ption of numerous Bmall spaces, pores, tlm^ forming the 

 pitted vessels 1 Fig. \2, v). These peculiar sculpturings prevenl 

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

 spiral thickenings also permil considerable elongation of such 

 ducts which would nol be possible if the thickening were more 

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



