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In turgid multicellular organs, it is convenient to differentiate between the two kinds of tensile forces acting in cell walls as a result of turgor pressure. The primary forces occur both in situ and in cells isolated from the organ, whereas the secondary forces occur only in situ. The latter are an unavoidable physical consequence of the variation in mechanical parameters of tissues forming layers or strands. The most rigid tissue is under maximal tensile force, whereas the least rigid is under maximal compressive force. These forces cause tissue stresses (that is, certain tissues are under tensile stress, whereas others are under compressive stress in the organ). The primary and secondary forces result in primary and secondary stress in cell walls, respectively. The anisotropy of the primary stress is a function of cell shape. For instance, in cylindric cells the anisotropy expressed as the ratio of longitudinal to transverse stresses is 0.5. The anisotropy of the secondary stress is a function of the compound structure of the organ. For example, in the epidermis of sunflower hypocotyl, the longitudinal secondary stress is much higher than the transverse stress. The primary and secondary stresses are superimposed, and, as a consequence, the stress anisotropy in the outer thick walls of epidermal cells is greater than 1. These outer epidermal walls transmit most of the tissue stress. When the epidermis is peeled but remains turgid, only primary stress remains, but loading of the peel can reestablish the original stress anisotropy. We studied the effect of stress anisotropy changes on the orientation of cortical microtubules (CMTs) in the sunflower hypocotyl epidermis. We showed that changes in stress anisotropy cause the CMT orientation to change in the direction of maximal wall stress. In situ, the relatively high tensile tissue stress in the epidermis causes maximal stress in the longitudinal direction and relatively steep CMT orientation. When the tissue stress is removed from the epidermis by peeling, the CMTs tend to reorient toward the transverse direction, which is the direction of maximal stress in the primary component. On application of external longitudinal stress, to substitute for tissue stress, CMTs tend to reorient in the longitudinal direction. However, a relatively high rate of plastic strain is caused by the stress applied to the peel in an acid medium. This produces a less steep orientation of CMTs. It appears that the change in stress anisotropy orients the CMT in the direction in which the stress is maximal after the change, but there is also some effect of the growth rate on the orientation.

Tissue stresses, which occur in turgid herbaceous stems, both elongating and non-elongating. and tree growth stresses (TGSs) which occur in woody stems, are similar in that (i) they form self-equilibrating patterns of stresses (tensile and compressive) in stems, and (ii) the asymmetric, graviresponsive change in the pattern tends to bend the stem. The longitudinal tensile tissue stress (TS) which occurs in the outer layers of turgid stems is a few times higher than the osmotic pressure of cell sap in such a layer. Usually it is considered that TSs originate from the differential growth of tissues in a stem; however, physical analysis of a turgid stem model has shown that TSs are an unavoidable physical consequence of the variation in structural characteristics of cell layers or vascular strands in turgid stems. The model applied to the sunflower hypocotyl gives forces which fit well to those measured. The structural characteristics are sufficient to explain fully the TSs which exist in turgid stems. Differential growth is not necessary in this respect. Examination of the model shows also that the longitudinal elastic strain of all cell walls in a turgid stem is the same at a given stem level regardless of wall thickness, i.e. the structure-based TSs compensate for the variation in turgor-induced wall stress in single cells with variable diameter and wall thickness. The importance of this compensation for the anisotropy of wall stresses is presented. The forces which generate TSs exert bending moments which are high but they sum mutually to zero in a vertical stem. In gravistimulated turgid stems of Reynoutria, the TSs decrease considerably on the lower side while those on the upper side remain unaltered. The consequences of this asymmetric change for gravitropic bending are analysed. Tree growth stresses arise in a process by which new cells added by the cambium to the secondary xylem tend to shrink longitudinally (except compression wood) during maturation of the cell walls. The pattern of TGSs is characterized by tensile or compressive stress in the peripheral or the core wood, respectively. An asymmetrical pattern of TGSs due to asymmetrical deposition of wood results in a bending moment which equilibrates the bending moment caused by the weight of a lateral branch. In response to a gravity-derived stimulus the TGS pattern may be modified by asymmetric formation of reaction wood which differs histologically from normal wood: tension wood in many arborescent angiosperms, and compression wood in conifers. The formation and functioning of the reaction wood is discussed.

During five rocket flights (TEXUS 18, 19, 21, 23 and 25), experiments were performed to investigate the behaviour of statoliths in rhizoids of the green alga Charo globularia Thuill. and in statocytes of cress (Lepidium sativum L.) roots, when the gravitational field changed to approx. l0(-4) g (i.e. microgravity) during the parabolic flight (lasting for 301-390 s) of the rockets. The position of statoliths was only slightly influenced by the conditions during launch, e.g. vibration, acceleration and rotation of the rocket. Within approx. 6 min of microgravity conditions the shape of the statolith complex in the rhizoids changed from a transversely oriented lens into a longitudinally oriented spindle. The center of the statolith complex moved approx. 14 micrometers and 3.6 micrometers in rhizoids and root statocytes, respectively, in the opposite direction to the originally acting gravity vector. The kinetics of statolith displacement in rhizoids demonstrate that the velocity was nearly constant under microgravity whereas it decreased remarkably after inversion of rhizoids on Earth. It can be concluded that on Earth the position of statoliths in both rhizoids and root statocytes depends on the balance of two forces, i.e. the gravitational force and the counteracting force mediated by microfilaments.