Identifier: lecturesonphysio00sach (find matches)
Title: Lectures on the physiology of plants
Year: 1887 (1880s)
Authors: Sachs, Julius, 1832-1897
Subjects: Plant physiology
Publisher: Oxford : The Clarendon Press
Contributing Library: NCSU Libraries
Digitizing Sponsor: NCSU Libraries

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tip of the root again penetrates through themeshes into the damp saw-dust, at once bending downwards again geotropically,however, to repeat the same process. In this way a root (w /;/) may fairly sew itselflike a needle and thread passing up and down between the meshes of the netting. It is certainly, as numerous further experiments showed, only the differencein the moisture of the surrounding air which produces these phenomena : aqueous n6 LECTURE XL. vapour is continually being evaporated from the damp saw-dust, and the air in itsimmediate neighbourhood is saturated with it; a little distance from it, however, theair is relatively drier, and the root apices thus become curved, as the experiment shows,so that they are concave on the damper side. In the experiment described the hydrotropism of the root has to overcome itsgeotropism ; but if the seeds are allowed to germinate on the surfaces of a dampblock of peat (T, Fig. 405) which is fastened to the axis ^ of a klinostat and slowly
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Fig. 405--Cubical block of pea A of the klinostat rotated, geotropism, as we already know, is rendered ineffectual. In this casethen the roots also grow closely apphed along the damp surfaces, it is true, butif it accidentally happens that the apex penetrates into the turf itself it then goes ongrowing as in /, 0, q, continually deeper into the turf, and, according to circumstances,probably even again comes out from it, as at /,;/. In this, however, there is yet another cause acting on the roots ; for I havefound that growing root-apices, when they are pressed on the one side by a solidbody, behave like tendrils (only much more tardily) and become curved so as to beconcave on the touched side. This is perhaps also the proper place to mention that the power which roots POSITIVE AND NEGATIVE IIVDROTROnSM. 717 have of penetrating into the soil, and of continuing their progress in it, does notdepend exclusively on their geotropism, but that the contact-stimulus and hydro-tropism just desc

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