Tissue stresses in growing plant organs

Rapidly growing plant organs (e.g. coleopties, hypocotyls, or internodes) are composed of tissues that differ with respect to the thickness, structure, and extensibility of their cell walls. The thick, relatively inextensible outer wall of the epidermal cells contains both transverse and longitudinally oriented cellulose-microfibrils. The orientation of microfibrils of the thin, extensible walls of the parenchyma cells seems to be predominantly transverse. In many growing organs (i.e. leafstalks), the outer epidermal wall is supported by a thickened inner epidermal wall and by thick-walled subepidermal collenchyma tissue. Owing to the turgor pressure of the cells the peripheral walls are under tension, while the extensible inner tissue is under compression. As a corollary, the longitudinal tensile stress of the rigid peripheral wall is high whereas that of the internal walls is lowered. The physical stress between the tissues has been described by Sachs in 1865 as 'tissue tension'. The term 'tissue stress'. however, seems to be more appropriate since it comprises both tension and compression. Hitherto no method has been developed to measure tissue stresses directly as force per unit cross-sectional area. One can demonstrate the existence of tissue stresses by separation of the tissues (splitting, peeling) and determining the resulting strain of the isolated organ fragments. Based on such experiments it has been shown that rapid growth is always accompanied by the existence of longitudinal tissue stresses.