Experiments in the analysis of the positive and the negative geotropic reactions of primary roots

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Abstract
In the present work the influence of moist air, of sand and of several solutions on the geotropic behaviour of primary roots is studied. The course of the geotropic movement is the result of a concerted action of positive and negative reactions the intensity and duration of which differ in roots of various species. In pea roots the negative movement appears only during a short stage of development. No direct relation exists between the speed of elongation and the appearance of the negative reaction. Primary roots of Zea mays and of Pisum arvense are indifferent to thigmotropic stimuli. The negative movement has, at least in pea roots a smaller mechanically effective force than the positive movement. Therefore the negative reaction does not appear in sand because it cannot overcome the mechanical resistance of the granular medium. In liquid media pea roots react in another way than in air: here the negative reaction begins later, but it has then more influence on the course of the geotropic curvature. The influence of different cations on the geotropic behaviour and on the elongation of the roots can be understood as a combined action of the osmotic effect and of the specific ionic permeability. In pea roots the negative reaction, which appears during the time from 3 to 6 hours after the induction also depends on a definite level of turgor. Primary roots of Zea mays, which grow in a relatively large angle to the vertical line do not lose their geotropic sensibility. They react like plagiotropic organs. In pea roots relations exist between the development of the positive and the negative reaction and the presence of the cotyledons and the tip of the root. Both reactions are induced at the same time by the gravitional stimulus. Their reaction times, however, are different. The root tip is necessary for the induction of both reactions. The negative curvature also appears when the tip is cut off before the end of the reaction time. The course of the geotropic movement of primary roots is compared with the geotropic behaviour of rhizomes. As a possible explanation of both kinds of reactions a two-hormone-hypothesis is discussed.