The effects of mechanical stimulation and etiolation on the collenchyma of Datura stramonium.

WALKER, WALDO S. (Grinnell College, Grinnell, Iowa.) The effects of mechanical stimulation and etiolation on the collenchyma of Datura stramonium. Amer. Jour. Bot. 47(9): 717-724. Illus. 1960.-In an effort to determine the effect of mechanical stimulation on collenchyma tissue, plants of Datura stramonium L. were placed on a mechanical agitator and subjected to intensive shaking for 9 hr. per day for 40 days. Measurements indicated that such stimulation greatly increased the amount of wall thickening per cell, as observed in transverse section. Measurements also indicated that such stimulation may inhibit collenchyma cell elongation. A second group of Datura strcmonium plants was placed in total darkness to determine the effect of such treatment on the quantity of wall thickening in the collenchyma tissue. Measurements indicated that when plants were placed in the dark for extended periods a great reduction of wall thickening resulted. It is suggested that reduction of wall material was due to its utilization as substrate for respiratory processes which occur in the plant under such extreme conditions. The composition and structure of the collenchyma cell walls are discussed. HABERLANDT (1914) theorized that the mechanical tissues of plants may adapt directly to external conditions. He also stated that it is possible that external forces act as stimuli that tend to accentuate the qualitative and quantitative development of the stereome, as long as they do not exceed certain limits of intensity. This has been shown to be true of the collenchyma of celery when plants were subjected to mechanical stimulation (Venning 1949; Walker, 1957). Collenchyma cell walls increased in thickness and the amount of collenchyma revealed in cross-section measurements increased in relation to the size of the petiole after stimulation. The stem of the celery plant is much shortened and the petioles are greatly enlarged, elongated, and the most conspicuous organ of the plant body. There is no lignified tissue present in the petioles of celery other than the major xylem elements. The first portion of the present study was undertaken primarily to determine the effect of mechanical stimulation on the collenchyma in the stem of Datura stramonium L., which contains considerable supporting tissue other than the collenchyma. If such treatment resulted in a response, secondary objectives were to measure the effect in both longitudinal and cross section and to determine at what stage of growth of the plant the effects were most pronournced. The second portion of the investigation was devoted to a determination of the effect of etiolation on the cell walls of collenchyma. The objective of this portion of the study was to determine if "carbohydrate starvation" would result in the re-utilization of material deposited in the cell walls of the collenchyma. In other words, is it possible that cell-wall 1Received for publication February 9, 1960. The author wishes to express his appreciation to R. L. Hulbary for his interest, advice and helpful guidance throughout the progress of this investigation. materials may function as reserve foods under such conditions? Although collenchyma is not the only mechanical tissue found in the stem and petioles of D. stramonium, it is present in considerable quantity in a continuous cylinder which occurs immediately inside the epidermis of the stem and petioles. The cells of the collenchyma occur in layers. From 4-6 layers of cells may be present in the mature internodes of the stem. Collenchyma cells are characterized by unevenly thickened walls. The thickenings in D. stramonium occur mainly at the corners of the cells or at the juncture of 3 cells, as observed in transverse section. The end walls of the collenchyma cells are generally transverse and lack excessive thickening (fig. 1). As the collenchyma cells mature in this species, lacunae between the cells become more and more distinct. Wherever a lacuna occurs, it is completely surrounded by heavily thickened walls of the ad. jacent cells. Although these lacunae may be small in a young internode, they may become as wide as several of the adjacent cells as the internode matures. In all sections observed, the wall or portion of the wall which bordered a lacuna was heavily thickened (fig. 2). The general habit of the mature D. stramonium plant is one of sympodial branching. The plant first produces 7 or 8 internodes followed by the production of a solitary, terminal floral apex. The production of the floral apex initiates activity in the 2 uppermost axillary buds. The sympodial branching is then continued throughout the remaining ontogeny of the plant. These experiments were conducted prior to the formation of the terminal floral apex. MATERIALS AND METHODS.-Effect of agitation.Inbred lines of diploid and tetraploid Datura stramonium were used for this study. Seeds from these lines, obtained from The Committee (Genetics