LIQ UIDAMBAR STYRACIFLUA (HAMAMELIDACEAE). I. HISTOLOGICAL BASIS OF COMPRESSIVE AND

Quantitative changes in cell pattern in the pith, cortex, cortical collenchyma, and epidermis were followed in developing internodes of Liquidambar to examine the cellular basis of compressive and tensile stresses in organized shoot growth. Initially, the highest rates of cell multiplication occur in the pith, followed successively by the epidermis, cortex, and cortical collenchyma. As internodes enter the phase of maximum elongation growth, mitotic activity begins to shift acropetally, accompanied by pronounced changes in cell pattern. The highest rates of cell multiplication now occur in the pith and cortex and continue until the cessation of internode growth. Concomitantly, reduced rates of cell division in peripheral tissues result in rapid increases in rates of cell elongation in the cortical collenchyma and epidermis. Attention is focused on the role of continued cell division in developing internodes with emphasis on differences in rates of cell multiplication between inner and outer tissues affecting patterns of tissue stress. For example, rapid and sustained increases in cell number in the pith, accompanied by growth of readily extensible pith cells, result in the development of compressive forces driving the growth of internodes. Conversely, continuing divisions in less extensible collenchyma and epidermal cells can relieve threshold tensile stresses resulting from the continuous stretching of these tissues by the developing pith. The concept that the passive extension of peripheral tissues, especially the epidermis, control the rate of internode elongation is viewed as an oversimplification of the interacting role of compressive and tensile forces in organized growth and development. The existence of physical stress in elongating intemodes of higher plants was recognized and investigated in considerable detail by several botanists in the 19th century. Both Hofmeister (1859) and Sachs (1865) observed that the separated outer and inner tissues of growing shoots spontaneously changed their dimensions upon isolation from each other. The epidermal and cortical tissues always became shorter and the pith longer than the original length of the excised segment. They also observed that whenever segments of growing stems were split longitudinally, the split halves immediately curved outward reflecting the spontaneous expansion of the pith and contraction of the peripheral tissues. Kraus (1867) confirmed the existence of compressive and tensile forces in elongating intemodes and demonstrated the simultaneous development of radial and tangential stresses in transverse sections of developing stems. The widespread occurrence of longitudinal stresses between the inner and outer tissues of elongating stems led Kraus to conclude that the inner tissues, especially the pith, provide the driving force for growth, whereas the peripheral cell layers determine the rate of elongation growth.