PISUM SATIVUM 351 



At a level just above the point of divergence of the cotyledons, 

 the metaxylem bands undergo further rearrangement, and the band 

 which lies in the intercotyledonary plane on the side of the cotyle- 

 donary divergence forms an ellipse. (Fig. 177, £.) This band 

 later divides to form two bundles which anastomose with the 

 lateral groups so that two parallel metaxylem strands are formed. 

 (Fig. 177, F.) In some cases, these anastomose along their inner 

 faces, forming a central zone of metaxylem that occupies the pith 

 region a few millimeters above the cotyledonary node. (Fig. 



177, <^-) 



At this level, the vascular strand, which extends to the first leaf 

 as its median bundle, gives off a fiber strand which is diverged 

 outward into the cortex; and, a short distance above this point, 

 the second leaf trace branches in the same manner. (Fig. 177, F, 

 G.) At the middle of the first internode, as a result of these reorien- 

 tations, there are four cortical bundles, two of which consist of 

 fibers only, while the other two are fibrovascular. Within the 

 endodermis, there are two polar fibrovascular bundles, a central, 

 dumbbell-shaped zone of metaxylem, and four laterally placed 

 groups of phloem in which fibers are differentiated adjacent to the 

 pericycle. (Fig. 177, G.) 



The first trifid bract diverges at the second node, and its median 

 bundle passes out and joins the cortical fiber bundle located on that 

 side of the axis. At the same time, the laterally placed cortical 

 fibrovascular bundles branch, and a lateral bundle from each 

 becomes a lateral vein of the bract. (Fig. 177, H.) Thus, the 

 vascular system of the bract consists of a median and two lateral 

 fibrovascular bundles which later anastomose. The centrally 

 located metaxylem divides along its minor axis into two groups, 

 and the group adjacent to the bract separates longitudinally so 

 that two bundles are produced, which lie centrad to the lateral 

 phloems. (Fig. 177, H.) The two lateral bundles toward the 

 apex of the ellipse give off branches which anastomose to form the 

 median trace of the third leaf. In the third internode, the medul- 

 lary xylem remaining at the other end of the ellipse divides longi- 

 tudinally, duplicating the process which took place at the opposite 

 pole in the internode below. As a result of these vascular rear- 

 rangements, the fourth internode exhibits a typical stem structure 

 with a central pith and an endarch dictyostele which persists 

 through all higher internodes. (Fig. 178.) 



