PISUM SATIVUM 357 



a spiral course around the stem and enter the stipules at the base of 

 the petiole. 



The stipular bundles of the leaf at the fifth node anastomose with 

 bundles of the stele a short distance below the fourth node. These 

 bundles occupy a position in the cortex similar to that of the bun- 

 dles in the lower internodes, and they can be traced as strands which 

 unite with the vascular bundles of the stele that are located in a 

 plane at right angles to the leaf trace. At the fifth node, a part of 

 each stipular bundle joins with the vascular elements of the blade 

 of the leaf and with them forms its vascular supply. A repetition 

 of this vascular plan occurs at each internode above this point, in 

 this manner providing the vascular supply for each successive set 

 of stipules. 



The Upper Internodes. — Above the fourth internode, the 

 number of bundles increases until there may be twenty or more 

 constituting the vascular cylinder. The upper internodes are 

 subtriangular to quadrangular in outline and the bundles form a 

 dictyostele in which the larger ones are located at the angles of the 

 stem. 



The epidermis is well cutinized, and there are numerous stomata 

 surrounded by small guard cells that are about one-half the radial 

 dimension of the adjacent cells. The cortex consists of several 

 layers of chlorenchyma, except outside the larger bundles at the 

 angles of the stem where three or four layers of mechanical tissue 

 may be differentiated. (Fig. i8o.) The endodermis is a single 

 layer of large oval parenchymatous cells lying immediately outside 

 the groups of pericyclic fibers that cap the vascular bundles. The 

 fibers are small, thick-walled, and very compactly arranged; but 

 between the fibers and the phloem, there are two or more layers of 

 larger, thin-walled pericyclic cells. 



The bundle is collateral with a relatively active fascicular cam- 

 bium, and there are several layers of secondary phloem and xylem 

 elements formed in the larger bundles. There are few fibers in the 

 phloem as compared with the number found in the transition re- 

 gion. The radial rows of secondary xylem vessels are separated by 

 ray tissue which is thin-walled and parenchymatous. 



As the stem matures, the parenchymatous cells of the medullary 

 rays and those abutting the inner face of the bundle become thick- 

 walled, forming a more or less continuous zone of connective tissue. 

 There is some development of a somewhat inactive interfascicular 



