492 RESPIRATION. 



The process of the combination of atoms into a solid body which we are now 

 considering, for example, the formation of cellulose, is a performance of work 

 combined with the fixation of sensible heat and with the transformation of kinetic 

 into potential energy. But whence do the colourless cells derive their sensible heat 

 and kinetic energy? When carbonic acid is decomposed and sugar or some other 

 carbohydrate is formed in a green cell, a sunbeam becomes imprisoned and fixed. 

 But this is not the case in cells devoid of chlorophyll, especially in those working 

 in darkness under the ground. The protoplasm of these cells derives the sensible 

 heat and kinetic energy which it consumes or renders latent from the sun, not 

 directly, but by very indirect methods. It obtains them by a portion of the 

 material conveyed to it becoming decomposed, by whose synthesis in the green 

 cells above-ground the kinetic energy of the sun's ray has been changed into 

 potential, and in this way the potential energy becomes again changed into kinetic, 

 and the latent heat transformed into sensible heat. The materials which the green 

 cells manufacture out of inorganic food would be merely an accumulated dead 

 capital lying unused if they were to remain in the condition in which they had 

 been formed. They must be turned to account, dissolved, transformed, and dis- 

 tributed; the impelling forces necessary for this are obtained by a portion of the 

 material manufactured ia the green cells undergoing a process which is exactly 

 the opposite of that carried out in their formation. At the very time when 

 carbonic acid is split up, oxygen given out, a carbohydrate formed, and heat 

 rendered latent thereby, carbohydrates are being decomposed, oxygen taken up, 

 carbonic acid excreted, and heat liberated. Of course this process of decomposition 

 cannot extend to the whole mass of the materials manufactured by the green cells. 

 It would indeed be absurd if in one part of the plant those materials became 

 again disorganized and changed into air and water which in another part had 

 been compounded of these same elements. As a matter of fact, this process of 

 decomposition is limited to but a part of the materials produced in the green 

 cells, and the whole process may be most correctly represented thus: one portion 

 of the materials formed from inorganic food in the green cells is employed in 

 the further growth of the plant body; but this further growth only becomes 

 possible if the other portion supplies the forces necessary for the carrying on of 

 the building. The one process is therefore just as important as the other; they 

 mutually supplement each other, and this supplementing is one of the most im- 

 portant life-processes of plants. 



It has been stated that in order to obtain the necessary impelling forces 

 oxygen is taken in, the molecules it attacks are decomposed, and carbon dioxide 

 is liberated. This process is therefore an oxidation, a burning of organic material, 

 and is to be placed in the same category as the burning of carbohydrates, which 

 occurs in animal bodies in respiration. It is called respiration in plants also, 

 although here we do not find special localized respiratory organs as is usually 

 the case in animals. In plants all the living parts can breathe, and to them 

 the atmospheric air, that is to say, the oxygen contained in it, obtains access — 



