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Show 312 MODIFIED CIRCUMNUTATION. CuAr. n In our entire list of seedlings, there are 30 genera, belonging to 16 Families, the cotyledons of which in some of the species rise or sink in the evening or early night, so as to stand at lea_st _60° above or beneath the horizon. In a laro-e maJonty of the genera, namely, 24, the movement is a rising one; so that the same direction prevails in these nyctitropic movements as in the lesser periodic ones described in the second chapter. The cotyledons move downwards during the early part of the night in_ only 6 of ~he genera ; and in one of them, Oanna~1s, the curvmg clown of the tip is probably due to epmasty, as Kraus believes to be the case with the leaves. The down· ward movement to the amount of 90J is very decided in Owalis Valdiviana and sensitiva, and in Geranium rotundifolium. It is a remarkable fact that with Anoda Wrightii, one species of Gossypium ~nd at least 3 species of Ipomooa, the cotyledons wh1lst young and light sink at night very little or not at all; although this movement becomes well pronounced as soon as they have grown large and heavy. _Although the downward movement cannot be attnbuted to the weight of the cotyledons in the several cases which were investiO'ated, namely, in those of tho Anoda, Ipomrea purpurea and bona-now, nor in that of I. coc· cinea, yet bearing in mind that cotyledons are con· tinually circumnutating, a slight cause might at £rst have determined whether the great nocturnal move· ment should be upwards or downwards. vVe may therefore suspect that in some a b on·g m· a1 me mber of the groups in question, the weight of tho cotyledons first determined the downward du. ectw· n. The fact ohf the cotyledons of these species not s·m 1 n·n g d ow nmuc whilst they are young and tender, seems opposed to the belief that the greater movement wh e n tl wy are CHAP. VI. SLEEP 01~' COTYLEDONS. 313 grown older, has been acquired for tho sake of protecting them from radiation at night ; but then we should remember that there are many plants, the leaves of which sleep, whilst the cotyledonR do not; and if in som~ cases the leaves are protected from cold at night whilst the cotyledons are not. protected, so in other cases it may be of more importance to the species that the nearly full-grown cotyledons should be better protected than the young ones. In all the species of Oxalis observed by us, the cotyledons are provided with pulvini; but this organ has become more or less rudimentary in 0. corniculata, and the amount of upward movement of its cotyledons at night is very variable, but is never enough to be called sleep. We omitted to ascertain whether the cotyledons of Geranium rotundijolium possess pulvini. In the Leguminosre all the cotyledons which sleep, as ,far as we have seen, are provided with pulvini. But wit~ Lotus Jacobteus, these aro not fully developed durmg the first few days of the life of the seedlinO' an~ the cotyledons do not then rise much at nighOt.' ~Ith Tri.(oliurn strictum the blades of the cotyledons rise. at mght by the aid of their pulvini ; whilst the petwle of one cotyledon twists half-round at the same time, independently of its pulvinus. .As a g~~eral rule, cotyledons which are provided Wlth pulvmi continue to rise or sink at night dm·ing a much longer period than those destitute of this orO'an In th "I s latter case the movement no doubt dependso on· alternately greater growth on the upper and lower side of the petiole, or of the blade or of both preceded probably by the increased turg:scence of the growing cells. Such h . movements generally last for a very ; ort period-for instance, with Brassica and GithaO'o or 4 or 5 nights, with Beta for 2 or 3, and wi~h ·'' i,l,:,: )1' .. ") |