OCR Text |
Show 450 SENSITIVENESS TO LIGHT. CnAJ>. IX. is strengthened by the fact that with Phalaris and Avena the first true leaf, which is bright green and no doubt decomposes carbonic acid, exhibits hardly a trace of heliotropism. r.rhe heliotropic movements of many other seedlings probably aid them in like manner in emerging from the ground ; for apogcotropism by itself would blindly guide them npwards, against any overlying obstacle. Heliotropism prevails so extensively among the higher plants, that there are extremely few, of which some part, either the stem, flow er-podnnclo, petiole, or leaf, does not bend towards a lateral light. D1·osera rotundifolia is one of the few . plants the leaves of which exhibit no trace of heliotropism. Nor could we see any in Dioncea, though the plants \I'Crc not so carefully observed. Sir J. Hooker exposed the pitchers of Sarracenia for some time to a lateral light, but they did not bend towards it.* We can understand the reason why these insectivorous plants should not be heliotropic, as they do not live chie:Hy by decom· posing carbonic acid ; and it is much more important to them that their leaves should occupy the best position for capturing insects, than that they should be fully exposed to the light. Tendrils, which consist of leaves or of other organs modified, and the stems of twining plants, are, as Mohllong ago remarked, rarely heliotropic; and here again we can see the reason why, for if they had moved towards a lateral light they woulU. have been drawn away from their supports. But some tendrils are apheliotropic, for instance those of Bignonia capreolata * Aecording to F. Kurtz(' Vorhn. ndl. dC's Bot. V ereins dor Provinz Brandenburg,' Bd. xx. IR78) the leaves or pitchers of Darling-tonia Oal~fornir·a ~re stiJ;~~ Apheliotropic. We fa1led to 1 . h this movement in a p!ai~t w uc we possessed for a short tJmc. CuAP. IX,. SENSI'fiVENESS TO LIGHT. 451 and of Smt'law as per a ; and the stems of some plants which climb by rootlets, as those of the Ivy and Tecoma radicans, are likewise apheliotropic, and they thus find a support. The leaves, on the other hand, of most climbing plants are heliotropic; but we could detect no signs of any such movement in those of Mutisia clematis. As heliotropism is so widely prevalent, and a~ twining plants are distributed throughout the whole vascular series, the apparent absence of any tendency in their stems to bend towards the light, seemed to us so remarkable a fact as to deserve further investigation, for it implies that heliotropism can be readily eliminated. vVhen twining plants are exposed to a later~llight, their stems go on revolving or circumnutatmg about the same spot, without any evident defiec~ion towards the light; but we thought that we .might detect some trace of heliotropism by comparmg the average rate at which the stems moved to and from the light during their successive revolutions.* Three young plants (about a foot in height) of Ipomrea CBJrulea and four of I. purpurea, growing in separate ~ts, we~e placed on a bright day before a north-east wmdo~ m a r~om otherwise darkened, with the tips ofth~u revolvmg stems fronting the window. When the tip of each plant pointed directly from the window, an~ when again towards it, the times were recorded. ibis was continued from 6.45 A.M. till a little after P.M. on June 17th. .After a few observations we concluded that we could safely estimate the time *Some erroneous statements :h·!nf~rt~nately gi vcn on this H ~. t, tn The Movements and abJtsofCiimbing Plants' 1875 pp, 28, 32• 40, and 53 CondJ usion~ tere drawn from a~ insufficient rmmber of observations, for we diu nut then know nt how unPqual a rate the stoma and teudl'ils of climbing plants sometimes travel in different lJf~rts of the same rovolution. 2 G 2 |