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Show 562 SUMMARY AND CHAP. XU. lcdons should be compared, and it will bo seen that they are essentially alike. Ordinary circumnutation is converted into a nyctitropic movement, firstly by an increase in its amplitude, but not to so great a degree as in the case of climbing plants, and secondly by its hein a rendered periodic in relation to the altcrnationsb of day and night. But there is frequently a <listinct trace of periodicity in the circumnutating movements of non-sleeping leaves and cotyledons. r_rhc fact that nyctitropic movements occur in species rlistributed in many families throughout the whole vascular series, is intelligible, if they result from the modification of the universally present movement of circumnutation · otherwise the fact is inexplicable. In the seventh chapter we have given the case of a Porlieria, the leaflets of which remained closed all day, as if asleep, when the p~ant vvas kep~ dry, apparently for the sake of checkmg. evapor~twn . ~omethino · of the same kind occurs w1th certam Grammcm. b . At the close of this same chapter, a few observatwns were appended on what may be called tho embryology of leaves. The leaves produced by youn~ shoots on f'nt-down plants of Melilotus tcmrica slept hko those of <-~, Trifolium whilst the leaves on tho older branches on the sam~ plants slept in a very different m~nner, Proper to the o-enus; and from the reasons ass1g~ecl we are temptedb to look at t 1u .s case as o~e of. revcrswn to <:t former nyctitropic habit. So agam wlth D~sn~: cli'l~m gyrans the absence of small lateral loafl?~s very young ~lants, makes us suspect that the Imme- <liate proaen1· tor of thi·s speC·i es d'd t possess latera 1 l no 1 . lcaflcts abn d that their appearance m· an. a1m ost dru e 1- mcntar'y cond1. t1. 0n at a somew h a t m ore (a dvance age is the result of reversi.O n to a t n' £o l 'I a te predec.e ssor.. However this may be, tho rapid circumnutatmg or OKAP. XII. CONCLUDING REMARKS. 563 gyrating movements of the little lateral leaflets . I ' . , soe1n to be due prox1ma~e y to tho pulvinus, or organ of movement, not hav_mg be n reduced nearly so much as the blarle, dunng the successive modification~ through which the species has passed. We now come to the highly important class of movements due to the action of a lateral light. When stems, leaves, or other organs are placed, so that one side is illuminated :nore b~ig~tly t~an t~c other, they bend towards the hght. This heliotropiC movement manifestly results from tho modification of ordinary circumnutation; and every gradation between the tw'o movements could be followed. vVhcn the light was dim, and only a very little brighter on one siclc than on the other, tho movement consisted of a succession of ellipses, directed towards tho light, each of which approached nearer to its source than the previous one. When the difference in the light on the two sicloH was somewhat greater, tho ellipses were flrawn out into a strongly-marked zigzag lin , and when mu ·h greater the course became rectilinear. We have reason to believe that changes in tho turgescence of the cells is the proximate cause of the movement of circumnutation; and it appears that when a plant is unequally illuminated on the two sides, the always changing turg·escencc is augmented alono- one side . 0 ' a.nd 1s weakened or quite arrested along tho other ~Ides. Increased turgescence is commonly followed by mcreased growth, so that a plant which has bent itself towards the light during tho day would be fixed in this positi?n were it not for apogcotropism acting during the mght. But parts proviclcd with pulvini bcncl, as Pfeffer has shown, towards tho light; and here growth d.oes not come into play any more than in tho ordinary Cifcum.nutating movements of pulvini. 2 0 2 |