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Anatomy of the Transition Region of Pisum sativum

1931; University of Chicago Press; Volume: 92; Issue: 4 Linguagem: Inglês

10.1086/334212

1940-1205

J. H. Gourley,

Genetic and Environmental Crop Studies

1. The transition from root structure to stem structure in Pisum sativum is not complete until the third internode is reached. This anomaly consists in the occurrence of exarch bundles in the first and second internodes, and in a protostelic situation in at least a portion of the first internode. 2. There is a transition in the region of the cotyledonary node from the triarch arrangement of the root to that of six characteristic vascular bundles in the first internode. These consist of four elongated lateral bundles lying on either side of the small central pith and in the direction of the long axis of the elliptical stem. They appear as two bundles since the metaxylem elements are in contact. All of the bundles are exarch, a smaller bundle lying at either end of the lateral groups, completing the series. Both of the latter are endarch. To the sides of each lateral group are two fiber groups lying about equidistant from the vascular strands, and at either end of the long axis is another fiber group, establishing the complement of six fiber groups within the stelar region. 3. The two smaller vascular groups are the leaf traces that supply the first two alternate leaf bracts. The lateral ones serve in part to supply leaf traces for the fourth and fifth nodes. 4. After the first leaf trace diverges from the stele at the second node, portions of the lateral exarch bundles are laid down progressively nearer together, until they appear as one endarch bundle above the leaf gap. The protoxylem elements occur inward and the metaxylem elements are laid down toward the outside face, producing a new endarch leaf trace. Two exarch bundles remain on the opposite side of the stem in the second internode. At the third node the same rearrangement occurs, so that in the third internode all bundles are endarch and the permanent stem structure is accomplished. 5. There are four cortical bundles in this species, one fibrous one opposite each leaf trace and a fibrovascular group opposite each lateral bundle. The fibrous ones diverge into the blades in toto, and the arrangement is established in the nodes above by divergences from the fiber groups which lie to either side of the leaf gap. The vascular ones are the stipular traces. At the second and third nodes, branches from them supply the rudimentary stipules or bracts and they end free in the first true stipules at the fourth node. Bundles diverge out of the stele in the second internode at right angles to the leaf traces to supply the next node above. This arrangement is repeated at each of the succeeding nodes.