CUAl-. IX. SEXSITIVENESS TO LIGHT. 449 



CHAPTEE IX. 



SEXSITIVENESS OF PLANTS TO LiGHT: ITS TKANSMITTED EFFEOTB. 



Uses of heliotropism — Insectivorous and climbing plants not heliotropic 

 — Same orsan heliutropio iit one age and not at another — Extra- 

 ordiiiiiry sensitiveness of some plants to light — The effects of light do 

 not correspond with its intensity— Effects of previous illumination 

 — Time requiied for the action of light— Aft(;r-effects of light — 

 Apogeotropi»m acts as soon as light lails — Accuracy with which 

 plants bend to the light — This dependent on the illumination of 

 one wliole side of the part — Localised sensitiveness to light and its 

 transmitted efttcts — Cotyledons of 1 halaris, munner of bending — ■ 

 Results of the exclusion of light from their tips — Effects trans- 

 mitted beneutli the surface of the ground — Lateral illumination of 

 tlietip determines the direction of the curvature of the base— Coty- 

 ledons of Avena, curvature of basal part due to the illumination of 

 upper part — Similar results with the hypocotyls of Brassica and 

 Beta — Radicles of Simipis apheliotropio, due to the sensitiveness of 

 their tips — Cnnduding remarks and summary of chapter — Means 

 by which ciroumnutation has been converted into heliotropism or 

 aphelioti-opibm. 



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 coty- 

 ledons of some Gramineae, for instance, those of Pha- 

 laris, are not green and contain very little starch ; 

 from which fact we may infer that they decompose 

 little or no carbonic acid. Nevertheless, they are ex- 

 tremely heliotropic ; and this probably serves them iu 

 another way, namely, as a guide from the buried seeds 

 through fissures in the ground or through overlying 

 masses of vegetation, into the light and air. This view 



