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Show 444 l\IODU IED CIRCUMNUTA TION. CIIAP. vw. the action of apogeotropism \vas quite eliminated, Nevertheless, they did not rise nearly so much at night, as when subjected to apoo·ootropism. Is it not possible, or even probable, that leaves and cotyledons, which have moved upwards in the evenin O' 0 through the action of apogeotropism Lluring countle s generations, may inherit a tcnJency to this movement { We have soon that tho hypocotyls of several Legu· minous plants have from a remote pm·jod inherited a tendency to arch themselves ; and we know that the sleep-movements of leaves are to a certain extent inherited, independently of the alternations of light and darkness. In our observations on the oircnmnutation of tho e cotyledons and leaves which do not sloop at night, we met with hardly any distinct cases of their sinking a little in the evening, and rising again in the morn· ing,-that is, of movements the reverse of those just discussed. vVo have no Joubt that such cases occur, inasmuch as the letw s of many plants sleep by sinking vertically downwards. How to account for the few oases which were obsorveJ must be loft doubtful. ~rho young leaves of Cannabis sativa sink at night between 30° and 40° beneath the horizon; and Kraus attributes this to epinasty in conjunction with tl~e absorption of water. Whenever epinastic gro~vth JS vigorous, it might conquer diaheliotropism .m the evening at which time it woulJ bo of no Import· ance t; the plant to keep its leaves hori~ontal. The cotyledons of Anoda rVrightii, of one vanety ~f Gossypium, and of several species of Ipomrea, remam horizontal in the evening whilst they are very young i as they grow a little olJer they curve a littlo down· wards, and when large and heavy sm· 1c so mu ch that they come under our definition of sleep. In the case of Cu.H. VIII. P ARAHELIO'l'ROPISM. 445 theAnoda and of some species of Ipomooa, it was proved that the downward movement did not depend on the weight of the cotyledons ; but from the fact of the movement being so much more strongly pronounced after the cotyledons have grown large and heavy, we may suspect that their weight aboriginally played some part in determining that the modification of the circumnutating movement should be in a dmvnward direction. The so-called Diurnal Sleep of Leaves, or Paraheliotropism.- This is another class of movements, dependent pn the action of light, which snpports to some extent the belief that the movements above described are only indirectly due to its action. We refer to the movements of leaves and cotyledons which when moderately illuminated are diaheliotropic; but which change their positions anc.l present their edges to th" light, when the sun shines brightly on them. Those movements have sometimes been called diurnal sleep, but they differ wholly with respect to the objec gained from those properly called nyctitropic ; and in some cases the position occupied during the day is the reverse of that during the night. It has long been known* that when the sun shines brightly on the leaflets of Robinia, they rise up and present their edges to the light; whilst their position at night is vertically downwards. We have observed the same movement, when the sun shone brightly on the leaflets of an Australian Acacia. Thdos e of .Amphica?·rncea rnonoica tumed their edo·es to the sun· r b ' an an analogous movement of tho little almost rudimentary ~!leaflets of Mirnosa albida was on one occasion so rapid that It could be distinctly seen through a lens. The elong·ated uni- Ft ' Oua e, first leaves of Phaseolus Roxbur·ghii stood at 7 A.M. at 20° above the horizon, and no doubt they afterwards sank a little lower, At noon, after having been exposed for alJout 2 h. to .~ Pte~er ~ives the names and datos of sevr.ral ancient writers in his e enodtschen Bewegungen,'l875, p. G2. |