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Show 454 SENSITIVENESS TO LIGHT. CHAP, IX. it is no doubt a great advantage to them in their struggle for life to expose their cotyledons to the light as quickly and as fully as possible, for the sake of obtaining carbon. It has been shown in the first chapter that the greater number of seedlings circum. nutate largely and rapidly; and as heliotropism con· sists of modified circumnutation, we :1l'e tempted to look at the high development of these two powers in seedlings as intimately connecterl. Whether thoro are any plants which circumnutate slowly and to a small extent, and yet are highly heliotropic, we do not know; but there are several, and there is nothing surprising in this fact, which circumnutate largely and are not at all, or only slightly, heliotropic. Of such cases Drosera rot~tndijolia offers an excellent instance. The stolons of the strawberry circumnutate almost like the stems of climbing plants, and they are not at all affected by a moderate light; but when exposed late in the summer to a somewhat brighter light they were slightly heliotropic; in sunlight, according to De Vries, they are apholiotropic. Climbing plants circumnutate much more widely than any other plants, yet they are not at all heliotropic. Although the stems of most seedling plants are strongly heliotropic, some few are but slightly helio· tropic, without our being able to assign any reason. This is the case with the hypocotyl of Cassia tara, and we were struck with the same fact with some other seedlings, for instance, those of Reseda odomta. With respect to the degree of sensitiveness of the more sensitive kinds, it was shown in tho last chapter that seedlings of several species, placed before a north-ea.st window protected by several blinds, and exposed !l1 the roar to the diffused liD"ht of the room, moved with unerring certainty towa;ds the window, although CHAP. IX. SENSITIVENESS TO LIGHT. 455 it was impossible to judge, excepting by the shadow cast by an upright pencil on a white card, on which side most light entered, so that the excess on one side must have been extremely small. A pot with seedlings of Phalaris Oana1·iensis, which had been raised in darkness, was placed in a completely darkened room, at 12 feet from a very small lamp. After 3 h. the cotyledons were clou btfull y curved towards the light, and after 7 h. 40 m. from the first exposure, they were all plainly, though slightly, curved towards the lamp. Now, at this distance of 12 feet, the light was so obscure that we could not see the seedlings themselves, nor read the large Roman figures on the white face of a watch, nor see a pencil line on paper, but could just distinguish a line made with Indian ink. It is a more surprising fact that no visible shadow was cast by a pencil held upright on a white card; the seedlings, therefore, were acted on by a difference in the illumination of their two sides, which the human eye could not clist! nguish. On another occasion even a less degree of h~ht acted, for some coty leclons of Phalaris became shghtly curved towards the same lamp at a distance of 20 feet; at this distance we could not see a circul~ r d~t 2·29 mm. ('09 inch) in diameter made with Indtan mk on white paper, though we could just see a dot 3·56 mm. (·14 inch) in diameter; yet a clot of the former si~e appears large when seen in the light.* 88 W? next tned ~ow smal~ a beam of !ight would act; ~h1s bears on hght serving as a gmde to seedlino-s whilst they emerge through fissured or encumber~d ground. A pot with seedlings of Phalaris was covered d * ~~rasburger says (' Wirkung 1~!8 lohtes auf Schwarmsporen' 1 ' p. 52), that the spores dr HC'Cmatococcus moved to a light which only just sufficed to allow middle-sized type to be read. |
