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Show 13() RELA'fiON BETWEEN CHAP. YIII. and in many others, it was interesting to notice how p;rad.ually the stems began to circumnutate as the light waned in the evening. We have therefore many kinds of gradations from a movement towards the light, which must be considered as one of circumnntation vr;ry slightly modified and still consisting of ellipse or circles,-though a movement more or loss strongly zigzag, with loops or ellipses ~ccasion~lly form.ed,-~o a nearly straight, or even qmte strmght, hehotropw course. A plant, when exposed to a lateral light, th~ugh this may be bright, commonly moves at . first m a zigzag line, or even directly ~rom the l_Jght; and this no doubt is due to its cu cumnutatmg at the time in a direction ithcr opposite to tho sonrce of the lio·ht or more or less transversely to it. As soon, b ' . . hO\revcr as the clircction of the cu cumnutatmg move· ment ne~rly coincides with that of the entering l!ght, the plant bends in a straight course towards the l1ght, if this is bright. 'rhe course appears to be rende~ecl more and more rapid and rectilinear, in accordance \\'lth the d.eoTee of brightness of the light-firstly, by the lono·er 0 axes of the elliptical figures, which th.e phtnt continues to describe as long as the light remams 'vel{ <lim being directed more or loss accurately ~o" aro its s' ource and by each succcss1.v e eu ·1 pse be-mab . de· Rcribed ne' arer to the 11. g1 1 t. S ccon dl }r ' if th. e hght is only somewhat dimmed, by the a?celeratwn ~~r1 increase of the movement towa,rds lt, and by e retardation or arrestment of that from the light, ;.OJ~~ lateral movement being still retained, fo~· the 1g will interfere less w1. t l t at rvrht ana1 e s 1 a movemen ' b. .0 , to its direction, than with one in its own dtroctwn. * In his paper, 'Ueber orthotrope uwl plagiotrope Pflanzen-tl1eilc • (' Arl,eitcn des. B;} /tn~l: in Wurzburg,' Bantl 11' e CaAP. VIII. CIRCUMNU'l'ATION AND HELIOTROPISM. 437 The result is that the course is rendered more or less ziO'zag and unequal in rate. Lastly, when the light is \ery bright all lateral movement is lost; and the whole energy of the plant is expended in rendering the circumnutating movement rectilinear and rapid in one direction alone, namely, towards the light. The common view seems to be that heliotropism is a quite distinct kind of movement from circumnutation; and it may be urged that in the foregoing diagrams we see heliotropism merely com bin eel with, or superimposed on, circumnutation. But if so, it must be assumed that a bright latent! light completely stops circumnutation, for a plant thus exposed moves in a straight line towards it, without describing any ellipses or circles. If the light be somewhat obscured, though amply sufficient to cause tho plant to bencl towards it, we have more or less plain evidence of stillcontinued circumnutation. It must further be assumed that it is only a lateral light which has this extraordinary power of stopping circumnutation, for we know that the several plants above experimented on, an<l all the others which were observed by us whilst growing, continue to circumnutate, ho·wever bright the light may he, if it comes from above. Nor should it be forgotten that in the life of each plant, circumnutation precedes heliotropism, for hypocotyJs, epicotyls, and petiol~s circumnutate before they have broken through the ground and have ever felt the influence of light. :Ve.are therefore fully justified, as it seems to us, in hehevmg that whenever light enters laterally, it is the IS79), Sachs hns discussed the h~~ner in which geotropism and e lotl'opism are affected by differences in the angles at which the organs of plnnts stand with respect to the direction of th f' incident furce. |