PETIOLE DEVELOPMENT AND XYLEM DIFFERENTIATION IN XANTHIUM REPRESENTED BY THE PLASTOCHRON INDEX

Petiole development and formation of xylem vessels have been investigated in Xanthium leaves from early ontogeny to maturity. Kinetics of growth was presented in terms of absolute and relative elemental rates of elongation. The process of vascularization was assessed by the number of differentiated xylem vessels. The leaf plastochron index (LPI) developed by Erickson and Michelini (1957) was used for designating the various stages of development. An exponential increase in petiole length was observed between the LPIs -3 and +4 indicating a constant relative rate of 0.20 or 20% increase per day. After cessation of lamina elongation at LPI 8, petiole elongation continued for an additional 5 day period, to LPI 9.5. Relative elemental rate analysis revealed that the basipetal pattern of elongation was maintained throughout the leaf development. At a specific plastochron age, the only growth was due to the petiole elongation. Leaves which ceased elongating had not completed their internal development, since the process of xylem formation continued for several plastochrons, or about 8 days. The highest rate of xylem formation was ten vessels per day at LPI 5. On the average, about five xylem vessels differentiated per day in the middle portion of a Xanthium petiole. Mature petioles contained an average of 218 xylem vessels. About 12 canals of schizogenous origin preceeded the development of the vascular tissue. THE PETIOLE is a leaf organ which connects the lamina with the stem. Although its tissues are comparable to the primary tissues of the stem, a considerable variation exists in the distributon of vascular bundles (Esau, 1964). From the physiological point of view, the transport of inorganic and organic molecules between the blade and the stem appears to be the main function. Frequently the petioles bend and elongate in such a way that the blades become favorably oriented toward light to facilitate energy absorption for photosynthesis. Special situations may arise in which petiole tissue will initiate a new plant by means of a vegetative reproduction. Petioles of isolated primary leaves of young bean plants, under suitable circumstances, form roots without addition of exogenous growth substances (Varga and Humphries, 1974). Relatively little has been written about the development of intact petioles in a normal undisturbed situation. The available information 'Received for publication 15 September 1980; revision accepted 24 November 1980. We are grateful to Dr. A. Sievers from Botanisches Institute der Universitat Bonn and Dr. Ch. Fuchs from Museum National D'Histoire Naturelle, Paris for sending us articles by Hilderbrandt, Van Tieghem and Vuillemin. We also thank Dr. Eugene F. McIntyre from the University of Pennsylvania and Alexandra Maksymowych for reading the manuscript. Special thanks to Dr. Barbara F. Palser from Rutgers University for helpful suggestions concerning the terminology of petiole anatomy. on petiole anatomy is fragmentary. Little attention has been given to ontogenetic and quantitative aspects of the problem. As a result, there is scarcity of data on the rates of xylem formation relative to specific stages of leaf development. Isebrands and Larson (1977) investigated vascular anatomy of cottonwood (Populus deltoides) petioles and the relationship between vascular bundles in the petiole and specific portions of the lamina. They found that acropetally differentiating subsidiary bundles provided a vascular continuity between the stem and specific portions of the leaf lamina. Bundles continuous with the central leaf trace were functionally related to the tip region of the lamina, while subsidiary bundles continuous with the lateral leaf traces were functionally connected to specific veins of the middle and basal parts of the lamina. More recently, Esau and Charvat (1978) studied differentiation of primary xylem vessels in petioles of Phaseolus vulgaris L. This investigation was confined to ultrastructural aspects of differentiation, such as the thickening of the primary wall, development of secondary wall depositions, and perforations in the end walls. Our investigation is limited to two aspects of petiole development, basic growth kinetics and formation of xylem vessels. The relative elemental rates of elongation and rates of xylem formation have been estimated during the