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Show 418 MODIFIED CIRCUl\'I~UTATIO:N. 0 1JAP. VIII. CHAPTER VIII. MoDIFIED CmcuMNOTATION: 1\ion~DIENTS EXCtTED nY LrGu'r'. Distinction between h eliotropism and tho effects of light on the pcriouicity of the ruovomouts of leaves-H eliotropic movements of Beta, Solnnum, Zea, and Avonu- H oliotropie movements towards an obscure light in Apios, Bmssica, Phnlnris, 'l' roproolum, and Cassia -Aplleliotropic movements of t endril:; of Bignonin.-0[' flowerpeduncles of Cyclamen- Burying of the pods-Heli otropi~m nnd apheliotropism modified form ::; of circumnutation-Steps by which ono movement is eonv<'rteu into the ol,her-'J'ransversal· beliotropismus or diaholiotropism, influenced by epiunsty, the weight of the port and apo:;eotropism-Apogeotropism overcome d nring the middle of the day by dia1wliotropism- Etl'ects of tho weight of the blades of eotyledous-So-callccl diurnal slecp-Cillorophyll injured by intense light-Movements to avoid iutcn ~c light. SACHS first clearly pointed out the important dif· ference between the action of light in modifying the periodic movements of leaves, and in causing them to bend towards its source.* The latter, or heliotropic movcmen ts are determined by the direction of the light, whilst periodic movements are affected by changes in its intensity and not by its direction. The periodicity of the circumnutating movement often continues for some time in darkness, as we have seen in the bst ehapter ; whilst heliotropic bending ceases very quickly when the light fails. N cvcrtheless, plants which hare ceased through long-continued darkness to move pe· riodically, if re-exposcd to the light are still, accordwg to Sachs, heliotropic. . . Apheliotropism, or, as usually designated, ncgatno * 'Physiologic Veg.' (French Translation), lSGS, PP· 42, 5l7, &c. CHAP. VIII. l\fOVEl\'IENTS EXCITED BY LIGII'l'. 419 ~elio~ropism, implies tha~ a plant, when unequally ~llummated o~ the two sides, Lends from the light, mstead of, as m the last sub-class of cases, towanls it. but apheliotropism is comparatively rare, at least in <: well-marked degree. There is a third and large 811 bclass of cases, namely, those of "~rn-msversal-Hcliotropismus" of Frank, which we will here call diaheliotropism. Parts of plants, under this influence, place themselves more or less transversely to the direction whence the lig~t proceeds, and are thus fnlly illuminated. There IS a fom·th sub-class, as far us the fin <ll cause of the movement is concerned; for the leaves of some plant~ whe~ exposed to an intense and injurious amou~t ?f hght duect themselves, by rising or sinking or tw1stmg, so as to be less intensely illuminated. Such movements have sometimes been called dim·n·11 sleep. _If th?ught advisable, they might be calJ~ll parahelwtropiC, and this term would correspond wi th our other terms. It will be _shown in the present chapter that all the ~ovements mcluded in these four sub-classes conSist of m?dified circumnutation. We do not pret~nd to s~ythat If a part of a plant, whilst still growing, did not cucumnutate-though such a supposition is most improbable- it could not bend towards the li o·ht · but '"' am tt f f: . . 0 ' '«o of a ~r o ~ct, heliotropism seems always to consist . mod~.fied cucumnutation. Any kind of movement ~n relatwn to light will obviously 1Jc much facilitated . Y each part circumnutating m· bending successively Ill all d' f h Irec IOns, so that an already existino- movement as only t b · d · 0 be 1 ° e mcrease In some one direction, and to th te~sened or stopped in the other directions, in order ahit should become heliotropic, apheliotropic &c. as t e case b ' ' V t. may e. In the next chapter some obser-a Ions on th · . e sensitiveness of plants to light, their 2 b; 2 |