METASTASIS. 179 



chlorophyll, and only in sunlight. Those parts of ordinary 

 plants which are destitute of chlorophyll are entirely want- 

 ing in the power of assimilation, and likewise the chloro- 

 phyll-bearing portions are unable to assimilate in darkness. 

 Carbon dioxide is probably decomposed into carbon oxide 

 and free oxygen : C0 2 = CO + 0. At the same time water 

 is decomposed into hydrogen and oxygen : H a = 2 H 4- 

 0. The free oxygen atoms are exhaled, and by the union 

 of carbon oxide and hydrogen, starch is in most cases 

 formed ; this appears as minute granules imbedded in the 

 chlorophyll-bodies (Fig. 43, p. o2). In some plants no 

 starch is formed in the chlorophyll, but oily or sugary mat- 

 ters which have nearly the same chemical significance. 

 Assimilation is thus a deoxidizing process. Both water and 

 carbon dioxide contain large quantities of oxygen, while in 

 starch it is much less ; consequently, in the formation of the 

 latter from the former, there must be a surplus of oxygen. 

 This may be shown as follows : 



19 rn _J12CO 1 starch 



1-s <--Oa = } 19 O) 



\\* X (- = 24 O set free, I = C,, H ao O lo + 2 H 2 O 



19 TT n J 1 *'-') 



13Ha -J24H.. 



Here twelve molecules of carbon dioxide and twelve mole- 

 cules of water produce one molecule of starch and two mole- 

 cules of water (water of organization), while twenty-four 

 atoms of oxygen are set free and permitted to escape from 

 the cells into the surrounding air or water. 



II. METASTASIS. 



233. Its General Nature. The chemical changes just 

 described, which constitute assimilation, take place only in 

 chlorophyll-bearing plants, or parts of plants, and in these 

 only in the sunlight. In cells which are destitute of chloro- 

 phyll, and in the chlorophyll-bearing ones in the absence of 

 light, other chemical changes take place ; these, while differ- 

 ing much among themselves, agree in always being processes 

 of oxidation, and changes of one organic compound into an- 

 other. To these chemical changes, in order to distinguish 



