PISUM SATIVUM 353 



The Stem. — Since the vascular transition involves the first 

 three internodes of the axis, as well as the hypocotyl, it is desirable 

 to summarize the vascular anatomy of these internodes in order to 

 compare them with the stem pattern which occurs in the fourth 

 internode. 



The First Internode. — The first internode has a wide cortical 

 region limited outwardly by an epidermis which has a thin cuticle 

 and numerous stomata, and inwardly by an endodermis with well- 

 defined Casparian thickenings. As maturation proceeds, large 

 schizogenous lacunae develop in the cortical parenchyma so that 

 there are usually four such cavities located between the cortical 

 bundles. The cortical bundles consist of two fibrous strands lying 

 in the plane of the polar bundles of the stele and two fibrovascular 

 bundles which lie in a plane at right angles to them. (Fig. 178.) 



The stele is bounded by a single-layered pericycle; and abutting 

 this layer are six well-defined groups of phloem fibers, two subtend- 

 ing the polar bundles, the other four being arranged in pairs lateral 

 to the metaxylem. The sieve tubes, companion cells, and phloem 

 parenchyma lie centrad to the groups of phloem fibers. The pri- 

 mary xylem is organized in two distinct regions which differ in 

 the manner in which the proto- and metaxylem are differentiated. 

 The centrally located primary xylem consists of two crescentic 

 bands whose convex surfaces may be in contact at the center of 

 the axis, with the four tips of the crescents directed toward the 

 polar bundles of the stele. (Fig. 179, A.) The development of 

 the primary xylem in this region is exarch, the protoxylem cells 

 constituting the four points of the two bands, while the metaxylem 

 comprises the central primary xylem strand. Parenchyma occupies 

 the space between the protoxylem arms at each pole of the plate. 

 Some variation may occur with respect to the degree of confluence 

 of the two metaxylem bands and there may be a narrow zone of 

 centrally located parenchyma cells between them. 



The two polar bundles are endarch and collateral in their orienta- 

 tion, constituting the median bundles of leaf traces. One is the 

 median bundle of the first trifid bract which diverges at the second 

 node, and the other is the median bundle of the second bract-like 

 leaf which arises at the third node. At the upper limits of the first 

 internode, the metaxylem bands described above are somewhat 

 separated laterally, so that there is a narrow central pith parallel to 

 the long axis of the primary xylem strand which is continuous 



