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Show -170 TRANSl\1I'l'TED EFFECTS OF LIGII'l'. CnA!'. IX. ct~rvatnre could not h.ave been aided in tho least by the weight of the upper part, which acted at right angles to the plane of curvature. It wil~ be sh~wn that when tho upper halves of tho coty· l ~don~ of Pbalans and A v<:-na were enclosed in little pipes of tm-fo1l ~r of blackened glass, in which case tho upper part was mechm~wally prevented from bonding, tho lower and unenclo ed part chd not bend when exposed to a lateral ]i O'ht· and it occurred to us that this fact might be due, not to tho dxclnsion of th? Eght from the upper part, but to some necessity of the hendmg gradually travelling down the cotyleclom;, so that unless the upper part first became bent, tho lower could not l:end, however much it m.ight be stimulated. It was necc snry for our purpose to ascertam whether this Dotion was true, and it was proved false; for the lower halves of se reml cotyledons became bowed to tho light, although th eir upper halves were enclosed in little gln.ss tubes (not blackened), whiel1 prevented, as fttr as wo conld judge, their bending. Nevertheless, as the part within tho tube might possibly bend a very little, fino rigid rods or flat splinters of thin glass were ('em en ted with shellac to one side of tho upper part of 15 cotyledons ; and in six ca cs they were in addition tied on with threads. They were thus forced to remain quito straight. Tho result was that tho lower halves of all became bowed to tho light, but generally not in so groat rt degree as tho conosponding pttrt of tho free seedlings in tho same pots; n.nd this may perhaps be accounted for by some ~light degree of injmy l1aving boon caused by a consider· able surface having been smeared with shellac. It may be <tddod, that when the cotyl edons of Phalaris and Avena are acted on by apogeotropism, it is tho upper part which begins first to bond; n.nd when this part was rendered rigid in the manner just described, the upward curvatme of the basal part was not thus prevented. To test our belief that tho upper part of the cotyledons of Phalaris, when exposed to a latemllight, regulates the bending of the lower part, many experiments were tried; but most of our lirst attempts proved useless from various causes not worth specifying. Seven cotyledons had their tips cut off for lengths varying between ·1 and ·16 of an inch, and these, when left exposed all day to a lateral light., remained upright. In another :=>et of 7 cotyledons, the tips wore cut off for a length of only about ·05 of an inch (1·27 mm.) and these became bowed towards CHAr. IX. TRANSl\U'I''l'BD EFFhCTS OF LIGHT. 4 71 a lateral light, but not nearly so much as tho many other seedlings in the same pots. This ln.ttor case shows that cutting off the tips does not by itself injure tho plants so seriously as to prevent heliotropism; -but we thought at tho time, that such injnry might follow when a greater length wa.s cut off, n.s in the first set of experiments. Therefore, no more trials of this kind were made, which we now regret; a.s we afterwards found that when the tips of three cotyledons wore cut off for a length of ·2 inch, and of four others for lengths of ·14, ·12, ·1, and ·07 inch, and they were extended horizontally, tho amputation dicl not interfere in tho least with their bending vertically upwards, through the action of apogeotropism, like unmutiln.ted specimens. It is therefore extremely improbable that the amputation of the tips for lengths of from ·1 to ·14 inch, could from the injury thus caused have prevented tho lower part from bending towards the light. We next tried the effects of covering the upper part of the cotyledons of Phalaris with little cn.ps which were impermeable to light; the whole lower part beillg loft fully exposed before a south-west window or a, bright pn.raffin lamp. Some of the caps were made of extremely thin tin-foil blackened within; these had the disadvantage of occasionally, though mrely, being too heavy, especially when twice folded. The basal edges could bo pressed into close contact with the cotyledons ; thouc:rh this again required care to prevent injuring them. Nevertheless, any injury thus caused could be detected by removing the caps, and trying whether the cotyledons were then sensitive to light. Other caps were made of tubes of the thinnest glass, which when painted black served well, with the ono great disn.dvantage that the lower ends could not bo closed. But tubes wore used which fitted the cotyledons almost closely, and black paper was ~laced on the soil round each, to check tl.te upward reflection of hght from the soil. Such tubes wore in one respect far hotter t?an caps of tin-foil, as it was pussible to cover at the sn.mo time some cotyledons with tmnsparent nnd others with opaque tubes; and thus our experiments could be controlled. It should be kept in mind that young cotyledons were selected for trial, and that these when not interfered with become bowed down to the ground towards the light. We will begin with the gln.ss-tubes. The summits of nino cotyledons, differing somewhat in height, were enclosed for rather less than half their lengths in uncoloured or transparent |