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Show 474 THANSl\UT'l'ED EFFECTS OP LIGll'l'. CuAr. IX. Anot~er t1:ial w~s mncl~ in. a different manner, namely, hy banclagmg With stnps of tm-fOJ l, about ·2 in breadth, the upper part,. but n~t tho aetna~ sm:nmit: of ~ight moderately yonng seedlmgs a little over hH.lf an mch m hmght. The RnmmitR and the .basal parts were thus .loft fully exposed to a lateral light dnrmg 8 h.; an upper mtermecliate zone being protected. With four of those seedlings the summits were expo ed for a length of ·05 inch, and in two of them this part became cmved towards the light, but the whole lower part remained quite upright; whereas the entire length of the other two sc~edlings became slightly curved towards the light. 'l'he summits of the four other seedlings were exposed for a length of ·04 inch, and of these one remained almost upright, whil t the other three became considerably curved towards the light. The many free seedlings in the same pots were all greatly curved towards the light. From these several sets of experiment!::l, including those with the glass-tubes, and those when the tips were cut off, we may infer that the exclusion of light from the upper part of the cotyledons of Phalaris prevents the lower part, though fully exposed to a lateral light, from becoming curved. The summit for a length of ·04 or ·05 of an inch, though it is itself sensitive and curves towards tho light, has only a Rlight power of causi11g the lower part to bend. Nor has the exclm;ion of light from the summit for a length of ·1 of an inch a strong influence on the curvature of the lower part. On the other hand, an exclusion for a length of between ·15 and ·2 of an inch, or of the whole upper half, plainly prevents the lower and fu lly illuminatccl part from becoming curved in the manner (see Fig. 181) which invariably occurs when a free cotyledon is exposed to a lateml light. ·with very young seedlings the sensitive zone s~ems to extend rather lower down relatively to their height than m o!der seedlings. We must therefore conclude that when seeclhngs are freely exposed to a lateral light some influence is transmitted from the upper to the lower part, causing the latter to bend. Thi:s conclusion is supported by what may be see~ to occur on a small scale, especially with young cotyledons, wrthout a.ny artificial exclusion of the light ; for they bend beneath tho earth where no li. ght can enter. Seeds of Pha1 a n·s wer· e covered with a layer one-fourth of an iuch in thickness of very fi?t sand, consisting of extremely minute grains of silex coated wJtl CnAP. IX. TRANSMITTED EFFECTS OF LIGHT. 475 oxide of iron. A layer of this sand, moistened to tho same degree as that over the seeds, was spread over a glass-plato; and when the layer was ·05 of an inch in thickness (carefully measured) no light from a bright sky could be seen to pass through it, unless it was viewed through a long blackened tube, and then a trace of light could be detected, but probably much too little to affect any plant. A layer ·1 of an inch in thickness was quite impermeable to light, as judged by the eye aided by the tube. It may be worth adding that tho layer, when dried, remained equally impermeable to light. This sand yielded to very slight pressure whilst kept moist, and in this state did not contract or crack in the least. In a first trial, cotyledons which had grown to a moderate height wore exposed for 8 h. before a paraffin lamp, and they became greatly bowed. At their bases on the shaded side opposite to the light, well-defined, crescentic, open furrows were formed, which (measured under a microscope with a micrometer) were from ·O~ to ·03 of an inch in broad th, and these had evidently been left by the bending of the buried bases of the cotyledons towards tho light. On the side of the light the cotyledons were in close contact with the sand, which was a very little heaped up. By removing with a sharp knife the sand on one side of the cotyledons in the line of the light, tho bent portion and the open furrows were found to ext end down to a depth of about ·1 of an inch, where no light could enter. The chords of the short buried arcs formed in four cases angles of 11°, 13°, 15°, and 18°, with the perpendicular. By the following morning these short bowed portions had stmightened themselves through apogeotropism. In the next trial much younger cotyledons were similarly treated, but were exposed to a rather obscure lateral light. After some hours, a bowed cotyledon, · 3 inch in height, had an open furrow on the shaded side · 04: inch in breadth; another cotyledon, only ·13 inch in height, had left a furrow · 02 inch in breadth. But the most curious case was that of a cotyledon which ha~ just protruded above tho ground and was only · 03 inch in herght, and this was found to be bowed in the direction of the light to a depth of · 2 of an inch beneath the surface. From ;hat ':e kn?w of the impermeability of this sand to light, the ~per Illummated part in those several cases must have doternuned the curvature of the lower buried portions. But an apparent cause of doubt may be suggested : as the cotyledons are continually circumnutating, they tend to form a, minute |
