334 THE STRUCTURE OF ECONOMIC PLANTS 



originating from the endodermis becomes the effective outer covering, 

 both in the large crown branches from axillary buds and in the small 

 stems." 



The Vascular Anatomy of the Stem. — Nageli (x}) has classi- 

 fied the genus Medicago, on the basis of its phyllotaxy and vascular 

 anatomy, in the type in which the leaves alternate in two rows 

 and the leaf trace consists of three bundles which pectinate with 

 the traces of both rows. The median bundle divides at the base 

 of the leaf into three branches of which the smallest central branch 

 enters the petiole, while the lateral ones anastomose with the 

 two lateral bundles of the trace forming the bundles which supply 

 the stipules. 



Upon entering the stem, the median bundle extends down 

 through two internodes without branching. It then curves 

 slightly to one side from its original position in an angle of the 

 stem; and, just below this point of curvature, forks into two 

 branches. These anastomose with the lateral bundles of the next 

 lower leaf trace. The two lateral bundles of each leaf trace extend 

 downward in the corners through their own internode; and, at the 

 next node, enter the vascular ring alternating with the lateral 

 bundles of the next lower trace. Each of them then unites with a 

 branch of the median bundle of the next higher trace, and extends 

 with it through two internodes. In the third internode, this 

 bundle finally anastomoses with a lateral bundle of the second 

 lower trace. Where there is an axillary bud, it is supplied by 

 two bundles which extend separately through one internode, and 

 anastomose at the next node with the lateral bundles supplying the 

 next higher leaf. (Fig. 169.) 



The Leaf. — The general structure of the stipulate, pinnately 

 compound leaf, and the course of its vascular bundles have been 

 described. Above the stipules, the three petiolar bundles extend 

 to the point of divergence of the short petiolules of the leaflets 

 where the terminal leaflet receives the median bundle, and the 

 lateral bundles become the midveins of the lateral leaflets. There 

 are branch veins between the three major bundles at this point 

 so that the three major veins of the leaflets are cross-connected. 

 (Fig. 170, y4.) In each elliptical or oval leaflet, the midvein 

 extends throughout its entire length, and the lateral branches 

 are pinnately arranged. These rebranch to form a net-veined 

 system in which the ultimate veinlets end in the mesophyll, but 



