GROWTH AND WORK OF PLANTS 



usually quite regularly arranged in rows and are rectangular in 

 form. This is the cambium portion of the bundle. The cells 

 of the cambium grow and divide, thus increasing in number. 

 The older ones on the bast side of the bundle cease to grow and 

 change into bast cells, others into sieve tubes (the cross walls 

 perforated like a sieve, see s.t. fig. 60), etc. The older ones on 

 the wood side of the bundle cease to grow and change into vessels, 

 wood fibers, etc. It will be noticed that the wood and bast at 

 no point meet around the cambium but that the cambium itself 



1 



M 



II 





i 





sp sp sc p iv c s.t. b 



Fig. 6c. 



Longitudinal section of vascular bundle of sunflower stem; sp, spiral, sc, scalariform and 

 p, pitted vessels at left; w, wood fibers with oblique cross walls; in middle are (c) cambium 

 cells with straight cross walls; next, s.t., two sieve tubes, then b, phloem or bast cells. 



extends across the medullary ray and is connected with the cam- 

 bium in the adjacent bundles. In fact, the cambium forms a 

 complete ring around in the stem, at this point separating the bast 

 and wood of all the bundles. The bundle is not therefore closed 

 but is open. This is characteristic of the bundles of the dicoty- 

 ledons as distinguished from those of the monocotyledons. The 

 stems of dicotoyledons can therefore increase in diameter indefi- 

 nitely as long as growth continues, since the cambium never com- 

 pletely passes over into permanent tissue. It extends through 

 the bundle, across the medullary ray into the adjacent bundles, 

 and keeps them open. A longitudinal section of a bundle will 



