290 



PLANT STRUCTURES 



through the stele (Fig. 214). This lack of regularity would 

 interfere with the organization of a cambium cylinder, and 

 we find the bundles collateral but closed — that is, with no 

 meristem left between the xylem and phloem (Fig. 271). 



Fig. 271. Cross-section of a closed collateral biindle from the stem of corn, showinp: 

 the xylem with annular (r), spiral (s), and pitted {(i) vessels; the jihloem contain- 

 ing sieve vessels {v), and separated from the xylem by no interveninn cambium; 

 both xylem and phloem surrounded by a mnss of sclerenchyma (fibers); and in- 

 vesting vessels and fibers the parenchyma (p) of the pith-like tissue through 

 which the bundles are distributed.— After Sachs. 



This lack of cambium means that stems living for sev- 

 eral years do not increase in diameter, but become columnar 



