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Summary
A number of experiments have been reported in which several types of test materials (such as oats coleoptiles; castorbean, bean and lupin hypocotyls; pea stems; castorbean, bean, lupin, winter squash and sunflower roots) were submitted to geotropic or phototropic stimuli, or unilaterally treated with AIA.
The results agree in showing that the curvatures determined in the aforesaid organs by the uneven growth of two opposite sides is considerably inhibited when the organ has been enriched with AA. This inhibiting action varies according to the concentration of AA in the organ, such a result being evidenced either through the observation of organs treated with AA solutions of different concentrations, or through a comparison of the behaviours of organs, submitted to the same treatment, some grown in the light, and some in the darkness.
Plasmolysis times have been recorded through a large number of experiments, the results being in accordance with those set forth in a previous paper (Tonzig and Trezzi 1950 a), since they prove that any positive or negative variation in the organ's auxinic contents — be such variations obtained through tropistic stimulations or through an unequal treatment with AIA — bring about a similar variation, with a corresponding increase or decrease in the plasm's structural viscosity. In all cases, the AA tends to reduce the plasm viscosity, thus remarkably impairing the action of the growth-promoting substances.
As a preliminary outline, some data have been reported whence it can be prove that the AA contents is lower — while the growth-promoting substance contents is higher — in the branches with a positive geotropic reaction (down bending forms) than in those with a negative reaction (erect branches) of the corresponding normal forms. The ratio of AA to growth-promoting substances is consequently considerably altered, a condition wich probably accounts for the fact that plasm viscosity is remarkably higher in bending than in erect branches. The higher viscosity rate should correspond to a heavier dispersion of the plasmatic colloids, and the opinion is hypothetically set forth that such a condition may bring about a higher sensitivity in those branches which, like the roots, show actually a positive reaction to the geotropic stimulus.