OCR Text |
Show ~ 44R MOD IF IBD CIRCUMNU'l'ATION. CI!AP. \'[[[. have watched for a fitting opportunity and to hftve traced the movement of tbe loaves whilst they were fully exposed to the sunshine, we did not ascertain whether paraholiotropi~m always consisted of modified circumnutation ; but this certainly was the casr with tho A verrhoa, ancl probably with the other species, <ts their ]raves werr rontinually rircnmnutating. CIIAP. IX. SENSITIVENESS ~ro LIGHT. 449 OHAP~rER IX. SENSITIVENESS OF PLANTS TO LTGU'l': ITS TRANSli1JTTED EFFEC1'S, Uses of heliotropism-Insectivorous and climbing plants not heliotropic -Same organ heliotropic ut one ago and not at another- Extraordinary sensitivenes::; of some vi ants to light-The efl'ccts ofligb t do not correspond with its intensity- Effects of previous illumination -Time required for the action of' light-After-effects of ligiJtApogeotropi~ m acts as soon as light fails-Accuracy with which plants bend to the light-This dcpo11dent on tho illumination of one whole ::;ide of the part-Localised sensitiveness to light and its transmitted effects-Cotyledons of Phalaris, manner of beudingResults of the exclusiou of light from their tips-Effects transmitted beneatl1 the surface of the ground-Lateral illumination of tLe tip uetermincs the direction of the curvature of the ba::,o- Cotyledons of Avena, curvature of basal part due to the illumiuation of upper part-Similar results with the hypocotylfl of Brassica and Beta-Radicles of Sinapis Apheliotropic, due to the sensitiveness of their tips-Condudiug remarks and summary of chapter-Means abyp hwehliiocthro pciisrmcu. rnnutation has beeu converted iuto heliotropism or No one can look at the plants growing on a bank or on the borders of a thick wood, and doubt that the young stems and leaves place themselves so that the leaves may be well illuminated. They are thus enabled to decompose carbonic acid. But the sheath-like cotyledons of some Graminere, for instance, those of Phalaris, are not green and contain very little starch; r:om which fact we may infer that they decompose httle or no carbonic acid. N overtheless, they arc extremely heliotropic; and this probably serves them in another way, namely, as a guide from the buried seeds through fissures in the ground or through overlying m!l8Bes of vegetation, into the light and air. This view 2 G |