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 bundle from the stem of corn, showing 

 the xylem with annular (r), spiral (s), and pitted (<?) vessels; the phloem contain- 

 ing sieve vessels (v), and separated from the xylem by no intervening cambium; 

 both xylem and phloem surrounded by a mass 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 



