VASCULAR DEVELOPMENT IN THE TRANSITION REGION OF POPULUS DELTOIDES BARTR. EX MARSH. SEEDLINGS

Seed and developing seedlings of Populus deltoides Bartr. ex Marsh., through 2 wk old, were embedded in Paraplast or Spurr's resin and serially sectioned at 7 or 2 μm, respectively, to examine vascular development in the root-hypocotyl transition region. The procambial template is well established in the dormant embryo. Protophloem forms a continuous system connecting the root with the cotyledons but protoxylem is restricted primarily to the cotyledons. With the onset of germination there is rapid elongation of both the root and hypocotyl. The basipetal progression of protoxylem from an initiating center at the cotyledonary node is correlated with the establishment of a second initiating center in the collet at the base of the hypocotyl. The relation between procambium, initiating layer, and metacambium, and their derivatives is similar to that described for the cottonwood shoot. The primary vascular system of cottonwood should be considered a single unit, both conceptually and functionally. The vascular pattern of the root may be, in part, determined by a basipetal stimulus from the cotyledonary node.