MEDICAGO SATIVA 331 



is a zone of chlorenchyma; and this is limited peripherally by 

 collenchyma which is one layer in width, except at the angles of 

 the stem. A strand of collenchyma reinforces the stem at these 

 points, and may project to form a more or less pronounced longi- 

 tudinal ridge. In some instances, it extends from the epidermis 

 to the endodermis so that the continuity of the chlorenchyma is 

 interrupted. The collenchymatous cells are thickened at their 

 angles and about as long as the epidermal cells. 



The epidermal cells are elongated in the axial direction and 

 are somewhat irregular in shape. There are numerous stomata, 

 Wilson (35) reporting approximately 300 to the square millimeter. 

 Two types of epidermal hairs may develop, one being glandular 

 and two or more celled, while the other is an elongated, slender, 

 unicellular trichome similar to that found on the leaves. The 

 glandular hairs, which are not commonly found among other 

 genera of this tribe, occur in a frequency of approximately 1.^ 

 per square millimeter, and the unicellular ones are somewhat 

 more numerous. 



The Mature Stem. — As secondary thickening proceeds, an 

 interfascicular cambium develops, and a cambial cylinder is 

 formed which produces a continuous zone of lignified xylem. The 

 connective tissue cut off by the interfascicular cambium consists 

 chiefly of wood parenchyma, while the fascicular cambium con- 

 tinues to produce vessels, fibers, and xylem parenchyma. (Fig. 

 167.) As maturation continues, some of the smaller parenchyma- 

 tous cells on the inner face of the bundles may become lignified, 

 forming an uninterrupted zone of mechanical tissue in conjunction 

 with the connective tissue produced by the interfascicular cam- 

 bium. This band of lignified parenchyma frequently encloses a 

 zone of thin-walled parenchyma which surrounds the protoxylem 

 elements of each bundle. The parenchymatous cells of the pith 

 may disintegrate or become ruptured so that the mature stem 

 becomes hollow. The inner tangential, radial, and end walls of 

 the endodermis may become lignified, forming an almost con- 

 tinuous sclerenchymatous sheath. In the aerial portion of the 

 stem, the amount of secondary thickening is not usually sufficient 

 to cause any splitting of the epidermis and cortex; but, when 

 this does occur, a cortical phellogen develops. 



The Underground Stem or Rhizome. — The extent to which 

 the underground portion of the crown stem may develop is vari- 



