194 PART III. PHYSIOLOGY. [ 45 



process which require special notice : the part played by the 

 mineral food, the action of light, the function of chlorophyll. 



With regard to the first point, it appears that the process in 

 question cannot be performed unless potassium-salts are supplied 

 to the plant. There is no reason to believe that this metal takes 

 any direct part in the process ; but it has an indirect, though none 

 the less well-marked effect upon it (see p. 192). 



The importance of exposure to light is briefly this. The chem- 

 ical process represented in the foregoing equation is one which 

 involves the doing of work ; for, from the simple and stable mole- 

 cules C0 2 and H 2 0, a more complex and less stable molecule CH 2 

 is produced. Work cannot be done without energy, and the plant 

 cannot evolve in itself the energy necessary. It avails itself, there- 

 fore, of the kinetic or radiant energy of the sun's rays. Hence 

 the importance of exposure to light is that the plant, by absorbing 

 the light-rays, obtains the energy required for the chemical work 

 which has to be done. 



Next, as to the function of chlorophyll. The function of chloro- 

 phyll is to serve as the means by which the rays of light are 

 absorbed, and their energy made available for the performance of 

 the chemical work by the protoplasm with which the chlorophyll 

 is associated. When light which has passed through a solution of 

 chlorophyll is examined with a spectroscope, the spectrum is seen 

 to present certain dark bands, known as absorption-bands, in the 

 red, yellow, green, blue, and violet, the band in the red being the 

 most conspicuous. These bands indicate that certain of the rays 

 of the solar spectrum do not pass through the chlorophyll, but are 

 arrested and converted into another form of energy. It is this 

 energy which, in the living plant, the chlorophyll places at the 

 disposal of the protoplasm for the construction of an organic 

 molecule out of carbon dioxide and water, as expressed in the fore- 

 going equation. Protoplasm without chlorophyll is incapable of 

 making use of the kinetic energy of the rays of light for the per- 

 formance of this chemical work. 



The product of this process of carbon-assimilation is (as indi- 

 cated in the foregoing equation) a non-nitrogenous organic sub- 

 stance having the composition of a carbohydrate. A leaf which 

 is actively assimilating carbon under the influence of light is 

 generally found to contain relatively large quantities of carbo- 

 hydrate, in the form either of sugar or starch. 



The performance of this process can be readily demonstrated. 



