VIII] SOURCE OF THE CARBON 85 



no mechanism or agency which can convert minerals like 

 the salts of calcium, magnesium and potassium, &c, above 

 referred to, into bodies like sugar, starch and acids, &c, 

 which contain carbon in their molecular structure ; and 

 even if we did the relative weights of the solid sub- 

 stances dissolved in the collected liquids are so much 

 greater than those of the traces of minerals in the water 

 delivered by the vessels to the mesophyll-cells, that even 

 if minute traces of some carbon-compound did exist in 

 the latter, the fact would not enable us to escape from 

 the following conclusion. 



Somewhere and somehow the mesophyll-cells must 

 have added the sugar or other carbon-compounds to the 

 efferent liquid. They must have taken the water from 

 the afferent pipes, and possibly made some use of the 

 traces of minerals therein, but they must have added the 

 sugar or other carbon-compound to the water collected 

 from these pipes. They cannot possibly have converted 

 any part of the water or minerals into carbon, and the 

 question therefore resolves itself into, Whence did the 

 mesophyll-cells obtain the carbon necessary to explain 

 the presence of such relatively large quantities of carbon- 

 aceous bodies, such as sugars, delivered up to the efferent 

 system ? Only one source of this carbon is possible viz. 

 the carbon which always exists in the surrounding atmo- 

 sphere, and we shall see later on that we now know 

 that that is the source : the chlorophyll-corpuscles absorb 

 carbon-dioxide from the air, tear the carbon from it, and 

 build it up with oxygen and hydrogen into the sugars, 

 &c, in question. 



Before we can understand this, however, it is necessary 

 to examine the other tissues of the leaf somewhat more 

 closely. 



