Chap IX. SENSITIVENESS TO LIGHT. 440 



CHAPTEE IX. 



Sensitiveness op Plants to Liqiit : its transmitted efpeotb. 



Uses of heliotropism — Insectivorous and climbing plants not heliotrojiio 

 — Same or^an heliutropic at one age and not at another — Kxtra- 

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

 not correspond with its intensity— Eifects of previous illumination 

 ■ — Time required for the action of light— After-effects of light — ■ 

 A pogootropi.jn acts as soon as light fails — Accuracy with which 

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

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

 transmitted eflects — Cotyledons of 1 halaris, manner of bending — 

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

 mitted beneatli the surface of the ground — Lateral illumination of 

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

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

 upper part — Similar results with the hypocotyls of Brassiea and 

 Beta — Eadioles of Sinapis apheliotropic, due to the sensitiveness of 

 their tips — Concluding remarks and summary of chapter — Means 

 by which circumnutation has been converted into heliotropism or 

 apheliotropism. 



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 Graminese, 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 in 

 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 



