1 63 EFFECTS OF LIGHT AND HEAT PAKT n. 



of synthetic chemistry, hundreds of substances pre- 

 cisely similar to those produced by vegetables. Al- 

 though it may be inferred from this that chemical 

 action is the same within the vegetable as it is in the 

 inorganic world, yet it is accomplished within the plant 

 under the control of the occult principle of plant-life. No 

 mere physical powers are capable of forming directly out 

 of inorganic elements, the living organism whose pas- 

 sage through the cycle of germination, growth, repro- 

 duction and decay, serves so pre-eminently to distinguish 

 between it and inert matter. Plants, indeed, borrow mate- 

 rials from the inorganic, and powers from the physical 

 world, to mould them into living structures, but both are 

 returned at death to the great storehouse of nature. 



All other circumstances being the same, the vigour 

 and richness of vegetation are proportionate to the 

 quantity of light and heat received. The functions 

 of light and heat are different, but their combined and 

 continued action is indispensable for the perfect deve- 

 lopment of vegetation. Light enables plants to de- 

 compose, change into living matter, and consolidate, 

 the inorganic elements of carbonic acid gas, water, 

 and ammonia, which are absorbed by the leaves and 

 roots, from the atmosphere and the earth ; the quantity 

 of -carbon consolidated being exactly in proportion to 

 the intensity of the illumination, which accounts for 

 the darker green tint of the tropical forests. Light 

 acting in its chemical character is a deoxidizing prin- 

 ciple, by which the numerous neutral compounds com- 

 mon to vegetables are formed. It is the principal agent 

 in preparing the food of plants, and in all the combina- 

 tions and decompositions the law of definite quantitative 

 proportions is maintained. It is during these chemical 

 changes that the specific heat of plants is slowly evolved, 

 which, though generally feeble, is sometimes very sen- 

 sible, especially when the flowers and fruit are forming, 



