Lectures on the physiology of plants . ther the positions. heliotropic influence coincides withthe geotropic one, or each strives to give the organ difterent directions, so that, asa matter of fact, a middle direction results. In order to be correctly understoodin what follows, I may premise that in the case of plants growing in the open theheliotropic effect is to be supposed as if a vertical ray of light fell on the plant froma luminous point in the zenith; since just as we must suppose all the influence of thegravity of the earth to proceed in a straight line only from the centre of gravity
Lectures on the physiology of plants . ther the positions. heliotropic influence coincides withthe geotropic one, or each strives to give the organ difterent directions, so that, asa matter of fact, a middle direction results. In order to be correctly understoodin what follows, I may premise that in the case of plants growing in the open theheliotropic effect is to be supposed as if a vertical ray of light fell on the plant froma luminous point in the zenith; since just as we must suppose all the influence of thegravity of the earth to proceed in a straight line only from the centre of gravityof the earth to the plant, so we may also imagine the effects of the light reflectedfrom the whole sky as produced by a single resultant ray. We shall now make useof this supposition in what follows. The well-known Liverwort, Marchantia poljnnorpha, forms broad shoots whichare green on the upper side, and on the lower side colorless and provided withnumerous root-hairs: these shoots are also seen to be strictlv dorsiventral structures. Fig. 400.—Roots of Vicia Falia growing in soil behind a pane ofglass, at first in the normal, then in the inverted, and then again in 1 position. The aracted with respect the different THE ANISOTROPV OF MARCHANTIA. 709 as regards their internal organisation. In the open, with unlimited Hght on all sides,these shoots grow with their lower surfaces close to the soil, into which the longroots penetrate deeply. Subsequently they produce shoots quite different in shape,long stalks as much as 10 cm. in height, each of which supports cither a lobeddisc with male- organs, or a parachute-like structure with female organs. TheStalks of these shoots are orthotropic and, when growing in the open, are directedperfectly upright. Transverse sections show however that they are not properlyradial in organisation, but have become radial by the rolling together of the twolateral margins of a narrow flat shoot, much as in the case shown at Fig. 398 is to this
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectplantph, bookyear1887
