MEDICAGO SATIVA 



333 



oped, although not as much so as in the yellow-flowered, M. 

 falcata. 



The structure of the rhizome resembles that of the aerial portions 

 in the general arrangement of the tissues, but there are certain 

 definite specializations which may be related to the soil environ- 

 ment, and to the greater age that the underground portions usually 

 attain. One important difference lies in the formation of well- 

 defined Casparian thickenings 



in the endodermal layer of the 

 rhizome, followed later by a 

 pronounced lignification of the 

 radial and inner tangential walls. 

 Another development that 

 does not commonly occur in the 

 aerial stem takes place in the 

 pericyclic zone, where several 

 layers of parenchymatous cells 

 develop between the endodermis 

 and the sectors of pericyclic 

 fibers. As the rhizome increases 

 in diameter, the cells in this zone 

 adjacent to the endodermis func- 

 tion as a phellogen, producing 

 a periderm which serves as the 

 protective layer when the epi- 

 dermis and cortex are destroyed. 

 (Fig. i68.) Jones (15) has noted 

 this development in the endo- 

 dermis of the perennial portion 



— ep 



—CO 



— en 



P9 



-pel 



-ph 



— ca 



xy 



—pi 



Fig. 168. Sector of rhizome showing 

 detail of vascular ring : ca, cambium ; 

 CO, cortex ; en, endodermis ; ep, epidermis ; 

 pel, pericyclic fibers ; pg, phellogen ; ph, 

 of the stem, and attributes the phloem; pi, pith; xy,xy\em. 



origin of the phellogen to this 



layer rather than to the pericyclic cells centrad to it. He states 



that 



"When the cambium begins to produce secondary growth the endo- 

 dermis becomes a phellogen. The portions of the endodermal cell 

 walls inside the thickenings, if any have been laid down, expand con- 

 spicuously, and almost simultaneously two or three cross walls are 

 laid down interior to these thickenings. Hereupon the epidermis 

 dies, separates from the cortex, and forms the brown membranous 

 covering often seen on young stem bases early in spring. The phellem 



