288 



PLANT STRUCTURES 



connecting the cambium in the bundles, and thus forming 

 a camhium cylinder^ which separates the xylem and phloem 

 of the vascular cylinder. This cambium continues the for- 



FiG. 270. Cross-section of open collateral vascular bundle from stem of castor-oil 

 plant {Ricinus), showing pith cells (m), xylem containing spiral {t) and pitted ig] 

 vessels, cambium of bundle (c) and of pith rays icb), phloem containing sieve ves- 

 sels (y), three bundles of bast fibers or sclerenchyma (b), the bundle sheath con' 

 taining starch grains, and outside of it parenchyma of the cortex (r).— After Sachs 



mation of xylem tissue on the one side and phloem tissue 

 on the other in the bundles, and new parenchyma between 

 the bundles, and so the stem increases in diameter. If the 

 stem lives from year to year the addition made by the cam- 

 bium each season is marked off from that of the previous 

 season, giving rise to the so-called groivth rings or annual 

 rings, so conspicuous a feature of the cross-section of tree 



