66 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



greatest illuminating power of the spectrum, it can scarcely be doubted 

 that light is the agent here concerned ; more especially as the place of 

 greatest heat is in the red ray, and that of greatest chemical power is 

 in the blue, both of which rays were found to be quite inert in the ex- 

 periment just quoted. It must not be supposed from this experiment, 

 however, that the yellow ray, and those immediately adjoining it, are 

 the only sources of this power in the Solar spectrum ; since it proves 

 no more than that, when the leaves were exposed to a highly carbonated 

 atmosphere, they could only decompose it under the influence of these 

 rays. It is certain, from other experiments, that plants will grow, in 

 an ordinary atmosphere, under rays of different colours ; and it appears 

 that the amount of carbon they severally fix, bears a constant proportion 

 to the illuminating powers of the respective rays. 



87. Although this fixation of carbon by the decomposition of carbonic 

 acid, is the most universally dependent, of all the processes of the Vege- 

 table economy, upon the influence of Light, yet it is not the only one, 

 especially among the higher Plants, in which that agent becomes an 

 important condition. Of the whole quantity of moisture imbibed by the 

 roots, and contained in the ascending sap, a large proportion is exhaled 

 again by the leaves ; a small part only being retained (together with 

 the substances previously dissolved in the whole) to form part of the 

 fabric. Now upon the rapidity of this Exhalation depends the rapidity 

 of the absorption ; for the roots will not continue to take up more than 

 a very limited amount of fluid, when it is not discharged again from the 

 opposite extremity (so to speak) of the stem. The loss of fluid by the 

 leaves appears to be a simple process of evaporation, depending in great 

 part upon the temperature and dryness of the surrounding air ; this 

 evaporation, however, does not take place solely, or even chiefly, from 

 the external surface of the leaves, but from the walls of the passages 

 which are channeled-out in their interior. Into this complex labyrinth, 

 the outer air finds its way through orifices in the cuticle, which are 

 termed stomata; and through these it comes forth again, charged with 

 a large amount of vapour communicated to it by the extensive moist 

 surface, with which it comes into contact in the interior of the leaf. Now 

 the stomata are bounded by two or more cells, in such a manner that they 

 can be opened or closed by changes in the form of these ; and this alte- 

 ration is regulated by the amount of Light, to which the leaves are sub- 

 jected. When the stomata are opened under the influence of light, the 

 external air is freely admitted to the extended surface of moist tissue 

 within the leaf, and a rapid loss of fluid is the result ; more especially 

 if the temperature be high, and the atmosphere in a dry state. On the 

 other hand, if the stomata be closed, the only loss of fluid that can take 

 place from the internal tissue of the leaves, is through the cuticle ; the 

 organization of which seems destined to enable it to resist evaporation, 

 so that the exhalation is almost entirely checked. The influence of 

 light upon this important function is easily shown by experiment. If 

 a plant, which is actively transpiring and absorbing under a strong sun- 

 shine, be carried into a dark room, both these operations are almost 

 immediately checked, even though the surrounding temperature be higher 

 than that to which the plant was previously exposed. 



