THE CENTRAL CYLINDER. 113 



342. In the cortex, according to Olivier,^ the secondary tissues 

 are either parencliymatous or suberous (cork^-). The secondary 

 parenchyma of the integument proceeds from the periphei'al 

 layer of the central cylinder. The suberous tissue in gym- 

 nosperms and in dicotyledons with caducous primary cortex is 

 derived from the pericambial layer ; it is composed of tabular 

 cells with very short radial walls, and begins to form outside 

 of the primary liber. In the case of wood}- dicotyledons with 

 late-formed secondary vessels, and in monocotyledons, it is pro- 

 duced in the external zone of the cortical parenchyma, and is 

 composed of cubical cells. 



343. In a given species the level of the root where cork ap- 

 pears depends on the transverse diameter of the root, and also 

 on the surroundings ; in roots of the same size the cork geu- 

 erallj' appears earlier, and is more abundant in aerial than in 

 earth roots. 



The cortical parenchyma is renewed by layers of cells just out- 

 side of the sheath of the central cylinder, and its development is 

 whoUj' centrifugal. 



344. The central cylinder undergoes its most remarkable 

 changes as the root grows older, in the group of dicotyledons. 

 There is very little change, if anj', in monocotyledons, but in a 

 few of the latter some of the secondarj'^ changes now to be de- 

 scribed can be observed (e. g., Draceena). 



345. In dicotyledons, including gymnosperms, the thin-walled 

 cells of the central cylinder are in contact with the inner face of 

 the endodermis, and are known collectivelj- as the pericambium. 

 Touching this pericambium like the two ends of a bow, there 

 runs a mass of delicate cells behind each liber bundle. At the 

 point where these bows touch the inner face of the liber bundle 

 a group of cells divides tangentially, forming a cambium layer, 

 which soon gives rise within to new woodj- elements (often 

 coalescent with those of the primary wood}' bundles), and on the 

 outside to new liber elements. These new jjroductions are 

 called secondary wood and liber. 



346. In some cases — for instance Pinus — tlie cells of the 

 pericambium outside of the priraar}' woody bundles produce new 

 wood and new liber. The wood is in contact with tlie primary 

 ■wood, while the liber may serve to connect the bundles of 

 primary hber, tlius bringing about a union more or less com- 

 plete between similar elements. From these secondary pro- 



1 Annales des Sc. nat., sev 6, tome xi., 1881, p. 129. 



