GROWTH OP PLANTS. 269 



through, whilst those which give light are stopped 

 (18T, 295). 



702. Chemical action is caused in the cells of the 

 leaves by the agency of these chemical rays of light, 

 which enable the vital power as it is called, to form 

 organic matter : hence it is frequently said that Light 

 stimulates vegetation. Chemical action is also aug- 

 mented by Heat, which, though it cannot alone effect 

 those changes usually produced by light, considerably 

 assists them, and is therefore classed with light as a 

 stimulus. 



703. Electricity also, that marvellous power, in 

 many respects so similar to light and heat, the effects 

 of which we frequently see in lightning and thunder 

 storms, is believed to exert great influence on vege- 

 tation ; but though there certainly appears much 

 probability of the truth of this, it is as yet not proved 

 by direct and unexceptionable experiments. 



704. When the young plants appear above the 

 surface of the ground, their mode of growth is 

 changed : they then require air and light, and being 

 no longer preserved by the soil from the action of 

 light, are immediately able to effect the decomposi- 

 tion of carbonic acid. 



705. The roots of a plant require little more than 

 moisture. The water which they obtain from the 

 soil contains those saline and gaSeous matters which 

 the plants want, whilst the leaves require in addition 

 to these matters the influence of light, and all these 

 they obtain by growing up into the air. 



2a* 



