44 LIGHT, AND THE NUTRIMENT OF PLANTS. 



exhaust the soil. Their importance, therefore, at the commencement 

 of vegetation, is now observable. Humus, as a product of decayed 

 vegetables, could not then have existed for their support. But their 

 decay has furnished the soil with vegetable matter, now seen to be 

 essential to vegetation. The leaves, fruits and seeds, but not the 

 roots of the vegetables of a former world, are now abundantly found 

 in coal formations. The roots of trees of later periods are however 

 seldom found wanting when taken from below the surface. 



Jls nutriment for plants no substance can be serviceable or neces- 

 sary to their growth which is identical with, or even similar in its 

 composition. Hence the sugar, starch, gum, etc., which are the pro- 

 ducts of vegetables, cannot be assimilated. Aqueous solutions of 

 these substances are imbibed by the roots and conveyed to the various 

 parts of the plants ; but they are not assimilated, and therefore afford 

 no nutriment. This is remarkable, since the form of these bodies is 

 most convenient for assimilation and as they contain, in nearly the 

 same proportions, the elements of woody fibre. 



The effects of light on vegetation is seen by the arrest of their power 

 of decomposing carbonic acid by its absence. A true chemical process 

 of oxydation is said then to commence by the action of the oxygen of 

 the air on the substances of the leaves, fruits and blossoms and equally 

 on those of the dead or live plant. The green parts of plants contain- 

 ing volatile oils, which are changed into resins by the absorption of 

 oxygen, should, of course, absorb more than those without such oils ; 

 and so also with those containing astringent principles, as those of 

 nut-galls, in which nitrogen exists. This has been proved to be so. 

 Some, indeed, are sour in the morning, as the leaves of sorrel from 

 the absorption of oxygen during the night, are tasteless at noon and 

 quite bitter at night. These are deprived of their oxydation in the 

 day time by combining a part of it in their composition. The time 

 required for the leaves to change their color by the effects of the at- 

 mosphere likewise indicates the oxygen they absorb. Those con- 

 tinuing the longest green attract the least oxygen in a given time. 

 Hence many retain a durable green color and are found to absorb only 

 about 0.86 of their volume of oxygen, while the poplar absorbs eight 

 and the beech nine and a half times their volume of this element ; and 

 these are remarkable for the rapid change of the color of their leaves. 

 They are in a state of oxide, and thus the brown leaf of the oak no 

 longer possesses tannic, nor that of the poplar, balsamic properties. 

 This is true also in respect to the wood of trees. 



On the departure of (lay the carbonic acid and the water absorbed by 

 the roots, cease to be decomposed ; they are dissolved in the juices and 

 afterwards escape through the leaves. Carbonic acid is also absorbed 

 by the roots, generally, through the water of the soil; they also absorb 

 air in like manner. The processes of emission and absorption have no 



