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Show 456 SENSITIVENESS TO LIGHT. CHAP. IX. by a tin-vessel, having on one side a circular hole 1·23 mm. in diameter (i.e. a little loss than the 'I'uth of an inch) ; and the box was placed in front of a paraffin lamp and on another occasion in front of a window; and both times the seedlings were manifestly bent after a few hours towards tho little hole. A more severe trial was now made ; little tubes of very thin glass, closed at their upper ends and coated with black varnish, were slipped over tho cotyledons of Phalaris (which had germinated in darkness) and just fitted them. Narrow stripes of tho varnish bar! been previously scraped off one side, through which alone light could enter; aml their dimensions were afterwards measured under the microscope. As a control experiment, similar unvarnished and transparent tubes were tried, and they did not prevent the eotyledons bending towards tho h ght. Two cotyledons were placed before a south-west window, one of which was illuminated by a stripe in the varnish, only ·004 inch (0·1 mm.) in breadth and ·OIG inch (0·4 mm.) in length; and the other by a stripe ·008 inch in bre~clth and ·06 inch in length. Tho seedlings were exammcd after an exposure of 7 h. 40 m., and wore found to be manifestly bowed towards tho light. Some other cotyledons were at the same time treated similarly, ex· cepting that the little stripes were directed not to the sky, but in such a manner that they received only t~e rliffused light from the room; and these cotyledons cltd not become at all bowed. Seven other cotyledons were illuminated through narrow, but compurat~vely long, c1 e are d stn.p es I. n t h e varm· s1u. - name l.y , m breadth between ·01 and ·026 inch, and in length betwoen.·l5 and ·3 inch · and th ese all became bowed to the sHle, by which li~·ht enterecl thron gh tho stripes, w~otheif' th• ese were dbi rected towards the sky or t o one SlUG 0 CHAP. IX. SENSITIVENESS TO LIGII1'. 457 the :oom. That light passing through a hole only ·004 inch in breadth by ·016 in length, should induce curvature, seems to us a surprising fact. Before we knew how extremely sensitive the cotyledons of Phalaris were to light, we endeavom·ed to trace their circumnutation in darkness by the aid of a small wax taper, held for a minute or two at each observation in nearly the same position, a little on the left side in front of the vorti.cal glass on which the tracing was made. The seedlings were thus observed seventeen times in the course of the day, at intervals of from half to three-quarters of an hour; and late in the evening we were surprised to find that all the 29 cotyledons were greatly curved and pointed towards the vertical glass, a little to tho left where the taper had been held. The tracings showed that they had travelled in zigzag lines. Thus, an exposure to a feeble light for a very short time at tho above specified intervals, sufficed to induce well-marked heliotropism. An analogous case was observed with the hypocotyls of Solanum lyeopersicum. We at first attributed this result to the after-effects of the light on each occasion ; but since reading Wiesner's observations,* which will be~eferrecl to in the last chapter, wo cannot doubt that a_n Intermittent light is more efficacious than a continuous one, as plants are especially sensitive to any contrast in its amount. The cotyledons of Phalaris bond much more slowly towards a very oLscure lio·ht than towards a bri()'ht b b ~ne. Thus, in tho experiments with seetllings placed ~n a ~ark room a~ 12 feet from a very small lamp, they ere JUst perceptibly and doubtfully curved towards it after 3 h., and only slightly, yet certainly, after 4 h. • 'Sitz. der k. Aku<l. dcr Wissensch.' (Vienna), Jan. 7880, p. 12. |