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Show 124 DISTURBED PERIODIC MOVEM:EN'rS. . CuAP. II. For instance, the cotyledons of lJeta vul,r;ar·i.~, 8olanttm lycope?'sicum, Gerinthe mafor, and Lupinus luteus, when placed in darkness, moved down during the afternoon and early night, instead of rising as they would have dono if they had been exposed to the light. All the individuals of tho Solanum did not bebave in the same manner, for the cotyledons of one circumnutated about the same 1:1pot between 2.30 and 10 P.M. The cotyledons of a seedling of Oxalis corniculata, which was feebly illuminated from above, moved downwards during tho first morning in the normal manner, but on the second morning it moved upward&. The cotyledons of Iot~ts J ar:obceus were not affected by 4 h. of complete darkness, but when placed under a double skylight and thus feebly illuminated, they quite lost their periodical movements on the third morning. On tho other hand, the cotyledons of Gucu1·bita ov,fera moved in tho normal manner during a whole day in darkness. Seedlings of Githago segetum were feebly illuminated from ~tbove in the morning before their cotyledons had expanded, and they remained closed for the next 40 h. Other seedlings were placed in the dark after their cotyledons had opened in the morning and these dirl. not begin to close until about 4 h. had elapsed. The cotyledons of Oxalis rosea sank vertically downwards after being left for 1 b. 20m. in darkness ; but those of some other species of Oxalis were not affected by several hours of darkness. The cotyledons of several species of Cassia are eminently susceptible to changes in the degree of light to which they are exposed : thus seedlings of an unnamed S. Brazilian species (a large and beautiful tree) were brought out of the hothouse and placed on a table in tho middle of a room with two north-east and one north-west window, so that they were fairly well illuminated, though of comse less so than in the hot-house, the clay being moderately bright; and after 36 m. the cotyledons which had been horizontal rose up vertically and closed together as when asleep; after thus remaining on the table for 1 h. 13m. they began to open. The cotyledons of young seedlings of another Brazilian species and of G. neg leota, treated in the same manner, behaved similarly, excepting that thoy did not rise up quite so much: they again became horizontal after about an hour. . Here is a more interesting case: seedlings of Gass'ia tora Ill two pots, which bad stood for some time on the table in tho room just described, had their cotyledons horizontal. One pot was now exposed for 2 h. to dull sunshine, and tho cotyledons Oru.P. II. SENSlTIVENESS OF COTYLEDONS. 125 remained horizontal; it was then brought back to the table, and after 50 m. the cotyledons had risen 68° above the horizon. The other pot was placed during the same 2 h. behind a screen in the room, where the light was very obscmo, and the cotyledons rose 63° above the horizon ; the pot was then replaced on the table, and after 50 m. the cotyledons had fallen 33°. These two pots ~itb seedlings of tho same age stood close too·ether, and were exposed to exactly the same amount of light, yet tho cotyledons in the one pot were rising, whilst those in the other pot were at the same time sinking. This fact illustrates in a striking manner that their movements are not governed by the actual amount, but by a change in the intensity or degree of the light. A similar experiment was tried with two sots of seedlings, both exposed to a. dull light, but different in degree, aud the result was the same. The movements of tho cotyledons of this Cassia are, however, determined (u.s in many other cases) largely by .habit or inheritance, independently of light; for seedlings which had been moderately illuminated during tho day, were kept all night and on the following morning in complete darkness; yet the cotyledons were partially open in the mornin(l' ~nd remained open in tho dark for about 6 h. Tbe cotyledon~ m another pot, similarly treated on another occasion, were open at: A.M.' and remained open in the clark for 4 h. 30m,, after which time they began to close. Yet those same seodlinO's when b b . b, . roug t m the middle of the day from a moderately bright mto only a moderately dull lip;ht raised, as we have soon, their cotyledons high above tho horizon. Sensitivene8s of Gotyledo11s to contact.-This subject does not P?sse~s much interest, u.s it is not known that sensitiveness of this !dnd IS of any service to seedling plants. We have observed cases m only four genera, though we have vainly observed tho coty~ edo~s of many others. The genus Cassia seems to be pre-eminent ~n t?Is l'espect: .thus, th~ cotyledons of .a. tom, whm~ extended 3 onzontal~y, we1e both lightly tapped w1th a very tlnn twig for m., and m the course of a few minutes they formed together an angle of 90°, so that each had risen 45°. A sino·le cotyledon of anothe.r seedling was tapped in a like manner fo~ 1m., and it io~o 27°m 9 m.; and after eight addit.ional minutes it had rison 0 more; the opposite cotyledon, which was not tapped hardly moved at all. The cotyledons in all t.hese cases beca~o hori-zont. a.l agam· m· 1e ss than half an hour. The pulvinus is the most s~nSltive part, for on slightly pricking three cotyledons with a |
