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Leaves of cottonwood (Populusdeltoides, Bartr.) plants grown under constant conditions are initiated at relatively uniform rates. Because of this uniformity, the plastochron index (PI), developed by Erickson and Michelini (1957), can be applied to quantitatively identify the developmental stage of the vegetative shoot. In a preliminary test, the PI was shown to be linearly related to time, with plastochron intervals ranging from 2.00 to 2.76 days. In a second test, leaf plastochron indexes (LPI) were calculated for a series of trees representing five size classes. Statistical models were developed for each size class, relating each of the dependent variables (leaf length, leaf area, leaf dry weight, number of vessels per internode, and number of vessels per petiole) to LPI. These models were used to demonstrate that the PI and LPI can serve two useful purposes when applied to developmental studies of woody plants: (1) to adjust plants of different developmental stages to a standardized morphological time scale, and (2) to predict developmental processes and events from simple, nondestructive measurements.

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An actively growing cottonwood bud was embedded in epon‐araldite and serially sectioned at 2 μm. The sections were analyzed microscopically with the optical shuttle system of Zimmermann and Tomlinson, and all data were quantitatively recorded relative to the apex and to leaf plastochron index (LPI). Analysis of the sections revealed an acropetally developing procambial system organized according to a precise phyllotaxy. Six procambial strands could be recognized and followed long before the leaf primordia that they would enter were evident at the apex. Origin of these strands coincided with developmental events both in the parent trace and its primordium and in the antecedent leaf on the same orthostichy. Once a primordium and its trace attained a certain stage of development, trace bundles began to develop basipetally from the primordium base. These trace bundles appeared to be the earliest progenitors of wood formation in cottonwood. It was concluded that the concept of residual meristem and its corollary, the hypothesis that acropetally developing procambial strands determine the inception sties of new primordia, apply to the cottonwood apex.