Tissue 8

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Abstract
The longitudinal and transverse forces involved in tissue stresses (TSs) and the stresses themselves, were determined for the hypocotyl of Helianthus annuus L. using Poisson ratios and data from measurements of (i) the forces necessary to restretch the strips of outer tissue (OT) to their initial length in situ; (ii) the external compressive force and (iii) the external osmotic pressure required to prevent the extension of isolated inner tissue (IT) immersed in water or mannitol solutions. These three independent measurements gave a similar magnitude for the longitudinal forces (tensile in the OT, compressive in the IT), 0.37 N in the apical region. More than 80% of the tensile force is transmitted by the epidermis, resulting in a longitudinal TS three times higher than that due to turgor pressure in this layer. The longitudinal compressive TS in the IT amounts to approximately one-third of turgor pressure in this tissue. Transverse TSs in the OT and in the IT were calculated from the longitudinal forces. In the OT, the tensile, circumferential TS is one-fifth to one-sixth of that in the longitudinal direction. In the IT the compressive TS in the transverse direction amounts to nearly half of that in the longitudinal direction.

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