MEDICAGO SATIVA 



32-5 



although much stretched by the rapid elongation of the hypocotyl, 

 retain their identity and position in the cotyledonary plane, 

 while those in the intercotyledonary plane eventually become dis- 

 organized and disappear. With the exception of the intercotyle- 

 donary protoxylem, all the primary xylem of the root forms a 

 continuous system with that of the cotyledonary traces. The 

 situation is somewhat complex because of an early initiation of 

 cambial activity in the hypocotyl with the consequent formation of 

 secondary xylem elements. In older seedlings, there is a further 

 complication owing to the downward differentiation of the 

 foliar traces of the epicotyledonary leaves which anastomose 

 with the primary and secondary vascular tissues of the hypocotyl. 



The Development of the Epicotyl. — The growth of the 

 epicotyl is usually retarded, so that the vascular system of the 

 cotyledons and hypocotyl is well developed and secondary thicken- 

 ing is in progress by the time the primary vascular strands of the 

 first foliage leaves have differentiated downward to the level of the 

 cotyledonary node. The median bundle of the three which supply 

 the first leaf is located in the intercotyledonary plane and con- 

 sists of collaterally oriented phloem and xylem, the latter 

 continuing as the primary xylem of the fourth ray of the hypo- 

 cotyledonary axis. The two lateral bundles are located on either 

 side of the stele, and anastomose with the secondary tissues of 

 the hypocotyl. Branches of the lateral traces supply the stipules, 

 and the main laterals continue up the petiole, anastomosing 

 with the median bundle above the point where the stipules are 

 diverged from the petiole. Nearer the lamina, the bundles sepa- 

 rate again and continue in the petiole as three parallel and distinct 

 veins. 



In the first, unifoliate leaf, the three bundles anastomose and 

 enter the blade as a single bundle. In the second, trifoliate leaf, 

 and in subsequent leaves, the terminal leaflet is supplied by the 

 median bundle and the lateral bundles enter the lateral leaflets. 

 There are cross-branches between the median and lateral bundles, 

 each of the latter receiving a branch from the median bundle, and 

 giving off a strand which joins the median bundle, after penetrating 

 for a distance into the terminal leaflet. (Fig. 170, A.") The 

 median bundle of the second leaf lies opposite that of the first 

 leaf; and, like the latter, anastomoses with the vascular system 

 of the hypocotyl at or slightly below the cotyledonary node. 



