gg DESCRIPTION OF THE ACTION OP LIGHT ON LEAVES. 



being filled with water holding carbonic acid in solution, part of which acid has bees 

 derived from the air, and part brought through the spongioles from the soil, decompo- 

 sition takes place a decomposition accomplished under the law of the definite action 

 of light; tha; if the rays increase in brilliancy, the chemical result goes on with more 

 rapidity, and if there be a diminution, the chemical result correspondingly declines ', 

 that this action goes forward at a maximum under the influence of the yellow ray, the 

 orange and the green coming next in rank, and the others following in the order of their 

 illuminating power ; that from this circumstance, the extreme violet and extreme red 

 seem to possess little activity, and the tithonic rays appear to be in no manner engaged, 

 or engaged only in an indirect way, after the same manner as radiant heat. As fast as 

 carbonic acid, dissolved in the vegetable juices, is disposed of, new quantities are taken 

 up, some little coming with the ascending sap from the ground, but the great part being 

 supplied from the air; for, through the air, by diffusion, gases pass with great rapidity, 

 and percolate through their films of water (Ap., 80). By aerial currents, by the move- 

 ment of the leaf an extensive and continually renewed contact with new portions of 

 air is established ; from this the carbonic acid is taken, which is dissolved in the watery 

 juices circulating. Brought under the luminous influence, it undergoes decomposition, 

 its carbon and a portion of its oxygen being appropriated (Ap., 818), and a volume of 

 nitrogen equal to the volume of oxygen thus appropriated, evolved along with the remain- 

 ing oxygen. There is, therefore, a removal of water and carbonic acid from the air; a fixa- 

 tion of carbon, hydrogen, oxygen, and nitrogen in the plant, and an evolution to the 

 air of nitrogen and oxygen. From the constant appearance of the former of these 

 bodies, we are led to suspect that the light acts primarily on some azotized body, the 

 destruction or eremacausis of which is essential to the total action (A p., 824) ; while 

 all this is going on, chlorophyl is abundantly formed, and so long as the process is ac- 

 complished, the leaves retain their green colour. 



387. On several occasions it has been said, that th - phenomenon here described an- 

 swers to a true digestive, and not to a respiratory process ; the older chemists and bot- 

 anists confounded it with the latter function ; but it is obvious that it does not answer 

 to respiration, either in mode of operation or in result. Respiration is an oxydizing 

 process, the object of which is to maintain the animal machine at a fixed thermomet- 

 ric point a result which implies direct combustion or burning; an animal, rigorously 

 speaking, burns carbon like a locomotive engine. But in this action, on the contrary, 

 carbon is reduced from carbonic acid, and there should be a descent of temperature in- 

 stead of an elevation, a large amount of heat being absorbed heat which is furnished, 

 under natural circumstances, by the sun, along with his light. The continued supply 

 of heat in this way prevents us from discovering that reduction of temperature which 

 should befall the leaf; and, besides this, it is exposing to the open atmosphere its broad 

 surface, and any thermometric disturbance is at once compensated by external agen- 

 cies. It is possible, indeed, that the decomposing process could not go on, save under 

 the conjoint presence of heat and light, though the specific function which each of 

 these agents discharge may be different. The action of leaves in the sunshine bears, 

 therefore, no sort of analogy, either in manner or in result, to the respiratory processes 



