CHAP, ii The Differentiation of the Tissues 209 



are relatively few in number ; when they are more numerous 

 there is left a central pith which becomes lignified. A 

 certain amount of conjunctive tissue surrounds this central 

 vascular core, forming a pericycle. The central mass corre- 

 sponds to De Bary's radial bundle, and the pericycle is 

 the pericambium of the older writers, especially Naegeli 

 and Leitgeb. This central core, delimited by the layer 

 which later on he termed the pericycle, and abutting 

 on the endodermis, he called the stele. The surrounding 

 tissues Van Tieghem recognized as a distinct cortex, 

 with an external piliferous layer which is not always a 

 morphologically distinct epidermis. 



In 1872 Van Tieghem postulated a similar plan of con- 

 struction of the stem, claiming for it a central core delimited 

 by a layer corresponding to the endodermis and clothed 

 like that of the root by a pericycle, which, however, shows 

 certain differences in the two regions. He laid great stress 

 on the point that, alike in stem and root, the fundamental 

 structure is a central cylinder and a circumferential cortex, 

 bounded on the exterior by an epidermis or a layer corre- 

 sponding to it, and delimited on the side next the stele by 

 a definite endodermis. He admitted, however, that excep- 

 tions to this typical structure occur. 



In the stem Van Tieghem recognized a differentiation 

 of the stele into conjunctive parenchyma and vascular 

 tissue, the former constituting a central pith from which 

 strands pass radially to the pericycle and separate the 

 vascular tissue into distinct bundles. The parenchyma 

 of the pith, the pericycle, and these medullary rays is of 

 a different morphological origin from that of the cortex. 



The delimitation of the tissues in the stem is not so 

 obvious as in the root, on account of the connexions with 

 the vascular system of the leaf, which cause considerable 

 interference with the continuity of the tissues at the nodes. 

 Moreover, the definition of the stele is less accurately made. 



GREEN 



