CHAPTER XIII. 



PHYSIOLOGY OF THE LEAF (continued} CARBON 

 ASSIMILATION, OR PHOTO-SYNTHESIS. 



Grand function of the leaf Fixation of carbon Experimental 

 proof Contrast between assimilation and respiration Water- 

 cultures Conditions of assimilation Amounts of carbon- 

 dioxide employed Evolution of oxygen Bubble counting 

 Rays of light concerned Spectrum of chlorophyll Starch 

 Quantities formed. 



THE grand function of the typical leaf, to which all its 

 other functions must be regarded as subsidiary or acces- 

 sory, is that of carbon-assimilation i.e. the fixation of 

 the carbon of the carbon-dioxide gas always existing in 

 small quantities in the air, which carbon is then worked 

 up into the structure of the plant ; and, as we have seen, 

 the leaf is to be conceived of as a complex piece of 

 physiological machinery for carrying out this function in 

 the best manner. 



The essentials of the process of carbon-assimilation are 

 the fixation of carbon from the carbon-dioxide of the air, 

 and its addition to and assimilation into the substance of 

 the plant, thus increasing its dry weight. This can only 

 take place in green organs, especially the leaves, con- 

 taining chlorophyll, and only in them when they are 

 exposed to light of sufficient intensity. Moreover, the 

 actual place where the process occurs is in the little 



