MEDICAGO SATIVA 32.3 



from the exarch radial to the endarch collateral arrangement is 

 completed in the veins of the cotyledons. Both van Tieghem and 

 Gerard (13), in describing the vascular anatomy of the primary 

 root and hypocotyl, state that two of the xylem groups of the 

 triarch root are continuous with the cotyledonary bundles, while 

 the third strand passes through the cotyledonary plate and con- 

 tinues as a trace to the first epicotyledonary leaf. On the other 

 hand, Compton and Winter find no direct continuity between the 

 primary vascular tissues of the root and hypocotyl and those of 

 the epicotyledonary system. They ascribe van Tieghem's interpre- 

 tation to a study of older seedlings in which the differentiation of 

 the vascular tissue is advanced to a point where the downwardly 

 diverging foliar traces are anastomosed with the hypocotyledonary 

 portions of the cotyledonary traces. 



The lower portion of the primary root is commonly triarch, 

 but a tetrarch arrangement is evident beginning approximately 

 at the region of the collar. (Fig. 164, A, B.) At about this point, 

 one of the three xylem arms appears somewhat smaller than the 

 other two and the angular divergence of the latter pair increases 

 until it approximates 180°. These two rays, which can be desig- 

 nated as the polar xylem, lie in the cotyledonary plane. The third 

 ray occupies a position at right angles to the cotyledonary plane, 

 and a fourth protoxylem group is differentiated which lies in the 

 intercotyledonary plane opposite the third ray. Thus, at the base 

 of the hypocotyl, the stele is commonly tetrarch with two large 

 xylem points in the cotyledonary plane and two smaller ones occu- 

 pying the intercotyledonary one. (Fig. 164, C, D.) During this 

 development, a fourth phloem group is differentiated, and the four 

 primary phloem strands occupy positions alternate with the xylem. 



At higher levels in the hypocotyl, the four phloem groups 

 are divided into eight groups, and these are oriented in a more 

 nearly collateral position with respect to the xylem. (Fig. 164, 

 E, F.) A few millimeters below the cotyledonary plate, the polar 

 metaxylem elements are reoriented to form two V-shaped groups 

 which, with the polar protoxylem, constitute the triad structure of 

 each cotyledonary bundle. (Fig. 164, G, H.) Coincident with 

 the reorientation of the pattern of the vascular tissues in the upper 

 hypocotyl, a central pith is differentiated; and cell enlargement 

 and division result in an increase in the diameter of the hypo- 

 cotyledonary axis. It has been suggested by Jones (15) and Rim- 



