24 INTRODUCTION. 



SECT. IV. 



ON LIGHT, THE EFFECT IT HAS UPON PLANTS. 



On entering into the subject of the effect of 

 light on plants, it may be well to state something 

 of the usually- received theory, relative to the 

 question ; on the proper observation of which, 

 practice is indubitably dependent. The chief 

 constituents of plants are, by chemists, termed 

 carbon and hydrogen ; these occur in all their 

 parts, and are essential to their existence; of these 

 two bodies, carbon occupies the first position, 

 being in reality the true food of all vegetable 

 matter ; and without entering into the question 

 of its derivation, whether from the earth or the 

 air, a point of contention among some who profess 

 to teach the matter, I will merely draw attention 

 to its action on plants, or rather to their mode of 

 consuming it, in order to show the effect of light 

 on vegetation, and the necessity of a proper sup- 

 ply : Liebig says " the roots and other parts of a 

 plant, possessing the same property, constantly 

 al)sorb water and carbonic acid, this power is in- 

 dependent of solar light. During the night, car- 

 bonic acid is accumulated in all parts of their 



