AN ANALYSIS OF LEAF ELONGATION IN XANTHIUM PENSYLVANICUM PRESENTED IN RELATIVE ELEMENTAL RATES

MAKSYMOWYCHT ROMAN. (Villanova U., Villanova, Pa.) An analysis of leaf elongation in Xanthium pensylvanicum presented in relative elemental rates. Amer. Jour. Bot. 49(1): 7-13. Illus. 1962.-Xanthium plants were grown vegetatively, and leaves, whose developmental stages were specified by a previously described leaf plastochron index (L.P.I.), were marked with India ink along the midrib and photographed during 3 successive days. The relative elemental rates of elongation, d(dX/dpl),/dX were estimated during the whole course of development. The pattern of elongation was not constant but was changing with increasing plastochron age of the leaf. The elements of a young leaf of L.P.I. 0.75 elongated with a constant relative rate. In older leaves, the d (dX/dpl) /dX values were progressively declining toward the tip of the lamina. After L.P.I. 6.3 the only increment in length was due to the elongation of the elements of the petiole. The pattern of growth distribution is discussed in terms of relative elemental rates with respect to cell division and cell elongation in various portions of the lamina and is correlated with the basipetal trend of tissue differentiation in the developing Xanthium leaf. THERE are many aspects and possible methods of studying growth and development of a leaf. Some of these may involve cell division and early histogenesis, and more basic ones may comprehend growth in length and expansion in area. One common approach in these studies may involve measurements and analysis of these measurements (Richards and Kavanagh, 1943). The implication may be that the process of expansion in area or leaf elongation involves cell division and cell elongation. In a way, this may be an indirect method to study cell division and tissue differentiation in a developing organ. The value of quantitative experimentation was well understood by Sachs (1873) in his classical experiments on root growth studies. By putting a series of marks on the root, he was able to differentiate between the growing and non-growing region, which also led to establishing of the zones of cell division, cell elongation and cell maturation in the primary root. A similar but more refined approach was used by Erickson and Sax (1956a, 1956b). Continuous photographic records from roots marked with lamp-black particles provided data for elemental growth rates, and cell counts from macerated tissues gave basic data for computation of rates of cell division in a growing primary root of Zea mays. Goodwin and Avers (1956) obtained photographic records of continuous files of epidermal cells of Phleum pratense and estimated relative elemental rates of root elongation and cell division. These methods gave a dynamic interpreta1 Received for publication April 24, 1961. This investigation has been supported by a research grant from the National Science Foundation, N.S.F. G-9834. The author is indebted to Professor David R. Goddard and Professor Ralph 0. Erickson of the University of Pennsylvania for the research facilities which were used in this project and also to Professor Ralph 0. Erickson for suggestions concerning the methods of analysis and for readina the maqnmirrnt tion of growth and development of the primary root. Avery (1933) originated some quantitative studies on leaf development, also emphasizing growth of different portions of the tobacco leaf, and Richards and Kavanagh (1943) attempted an elaborate system of analysis of the relative ("specific") growth rates in area using Avery's data. From their work, one general conclusion may be reached, that various portions of the leaf grow at different rates depending upon the developmental stage of that leaf, thus revealing inherent complexity associated with leaf growth. Maksymowych (1959) attempted an analysis of leaf development in Xanthium pensylvanicum presenting the developmental processes in terms of the absolute and relative rates of leaf length, rates of cell division and tissue differentiation of the lamina. From data on leaf elongation, it was possible to estimate rates of elongation for the entire leaf and to indicate that the lamina stops elongating about 1.5 plastochrons before the petiole. The question under consideration in present studies deals with elongation of each minute portion of the leaf. One would like to know whether each element of the leaf grows at a constant rate throughout the whole course of development, and, if not, what is the growth distribution along the midrib of the leaf. A series of marking experiments was undertaken in the expectation that the derived rates would provide some indication as to the pattern of elonga. tion of each portion of the leaf. Since elongation involves at least 2 basic processes, cell division and cell elongation, the implication was that the growth distribution along the midrib and the petiole would provide a better understanding of leaf development and processes associated with it. METHODS-Seeds of Xanthium pensylvanicum Wallr. (synonym X. saccharatum Wallr.) were planted at 3-wk intervals in flats of soil, thoroughly watered and incubated at 38 C. This method gave