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Show 490 CONCLUDING REMARKS AXD CHAP. IX. np, and that nearest to tho light sinking down, or both twisting laterally.* vVo nHty, also, suspect that tho extreme sensitiveness to light of tho upper part of the sheath-like cotyledons of tho Graminem, and their power of transmitting its effects to the lower p<n't, are specialised arrangements for finding tho shortest path to tho light. vVith plants OTOwing on a bank, or thrown prostrate by tho wind, the manner in 'vhioh the leaves move, even rotating on their own axes, so that their upper surfaces may bo again directed to the light, is a striking phenomenon. Such facts are rendered more striking when we remember that too intense a light injures tho chlorophyll, and that tho leaflets of several Loguminosre when thus exposed bend upwards and present their edges to the sun, thus escaping injury. On the other hanf1, the leaflets of Averrhoa and Oxalis, when similarly o:xposetl, bend down wards. It was shown in the last chapter that heliotropism is a modified form of circumnutation; and ns eYcry growing part of every plant circumnutn,tes more or l:ss, we can understand how it is that the power of bcndmg to the lio·ht has boon acquired bv such a multitude b • . n of plants throughout the vegetable kmguom. lO manner in which a circumnutating movement-yutt is one consistin ('f of a succession of irregular elhpscs ' . 0 or loops-1s gradually converteu,, m· t o a I·e ctilin. car course towards tho lio·ht, has been alrcn.dy exphune~l. First, we have a subc cessi·O n of elli·p ses w1' th th.e u lono·er axes directed towards the light, each of whwh b * Wiesner hns made remarks to nearly the snmo effect with rcsrcct to lrtwcs : • Die undulirmde N utntiou dor Internodien,' p. 6, ex-tracte( l from B. lxxvii.. (IS7S). Sitb. tler k. Almd. cler WJsSOJISCh, Wiou. CHAP. IX. SUMMARY OF CHAP'rER. 401 is described nearer and nearer to its source; then tho loops arc drawn out into a strongly pronounced zigzag line, with hero and there a small loop still formed. At the same time that the movement towards the light is increased in extent and accelerated, that in the opposite direction is lessened and retarded, and at last stopped. The zigzag movement to either side is likewise gradually lessened, so that finally the course becomes rectilinear. Thus under the stimulus of n. fairly bright light there is no useless expenditure of force. As with plants every character is more or less variable, there seems to be no great difficulty in believing that their circumnutating movements may have been increased or modified in any beneficial manner by the preservation of varying individuals. The inheritance of habitual movements is a necessary contingent for this proceRs of selection, or tho survival of the fittest; and we have seen good reason to believe that habitual movements are inherited by plants. In the case of twining species the circumnutating movements have been increased in amplitude and rendered more circular; the stimulus being hero an internal or innate one. vVith sleeping plants the movements have been increased in amplitude and often changecl in direction; and here tho stimulus is the alternation of light and darkness, aided, however, by inheritance. In the case of heliotropism, tho stimulus is tho unequal illumination of the two sides of the plant, and this determines, as in the foregoing cases, the modification of the circumnutating movement in such a manner that the organ bends to the light. A plant which has been rendered heliotropic by the above means, might readily lose this tendency, judging from tho cases already given, as soon as it became useless or |