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Show 486 CONCLUDING REMARKS AND CHAP. IX. ~ut obliquely towards one side where only n, little light entered. These. results seem to imply the presence of some matter In the upper part which is acted on by light, and which transmits its effects to the lower part. It has been sho"·n that this transmission is independent of the bending of the upper scl1SitiYe part. vVe have an analogous case of transmission in Drosera, for when a gland is irritated, the basn,l an(! not the upper or intermediate part of the tentacle bends. The flexible and sensitive filament of Dionrca likewise transmits a stimulus, without itself bendincr; as does the stem of Mimosa. Light exerts a powerful influence on most vege· table tissues, and there can l>c no doubt that it generally tends to check their growth. But when the two sides of a plant are illuminated in n sLightly different degree, it docs not necessarily follow that the bending towards the illuminated side is caused by changes in the tissues of the same nature as those which lead to increased growth in darkness. We know at least that a part may bend from the light, and yet its growth may not l>c favoured by light. This is the case with the radicles of Sinapis alba, which are plainly apheliotropic ; nevertheless, they grow quicker in darkness than in light.* So it is with many aerial roots, according to vViesnor ;t but there arc other opposed cases. It appears, therefore, that light does not determine the growth of apbcliotropir parts in any uniform manner. We should bear in mind that tho power of bending to the light is highly beneficial to most plants. There * Francis Darwin, 'tiber das Wachsthum negativ heliotropischer Wurzeln': 'Arbeiten des Bot. Inst. in Wi.irzburg,' B. ii., Heft iii., 1 880, p. 521. · r t ' Sitzh. clerk. AimLl. rll'r" ~.,. sensch' (ViL:nna), 1880, P· 12. CrrAP. IX. SUMMARY OF CHAPTER. 487 is therefore no improbabiLity in this power havino- been specially acquired. In several respects liO'ht se~ms to act on plants in nearly the same manna~ as it do ,. on animals by means of the nervous system.* vVith seedlings the effect, as we have just seen, is transmitted from one part to another. An animal may be excited to move by a very small amount of light; an(l it has been shown that a difference in the illumination of the two sides of the coty]ccl_ons of Phn1aris, which could not be distinguished by the human eye, sufficed to cause them to bend. It has also been shown that there is no close parallelism between the amount of light which acts on a plant and its degree of curvature; it was indeed hardly possibLe to perceive any difference in the curvature of some see<llings of Phalaris exposed to a light, which, though dim, was very much brighter than that to which others had been exposed. The retina, after being stimulate(l by a bright light, feels the effect for some time; and Phalaris continued to bend for nearly half an hour towards the side which had been illuminated. The retina cannot percei n· a dim light after it has been exposed to a brio·ht one· h ' and plants which had been kept in the daylight during tho previous c.by and morning, did not move so soon towards an obscure lateral light as did others which hau been kept in complete darkness. Even if light does act in such a manner on the growing parts of pln,nts as always to excite in them a tendency to bend towards the more illuminated side-a supposition contradicted by the foregoing experiments on seedlings and by all apheliotropic * ~achs has made some striking remtl.l'k8 to the same effi ct with respfct to the various stimuli winch excite movement in plants. See his paper 'Ueber orthotrope Ull!l plagiotrope Pflanzent hcile,' 'Arh. des. Bot. Inst. in Wiirzburg,' 1879, B. ii. p. 282. |
