METASTASIS.^ 179 



chlorophyll, and only in sunliglit. 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 curboa oxide 

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

 is decomposed into hydrogen and oxygen : H^ = 2 H + 

 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, j). 52). 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 : 



12C0. =J13C0 I starch 



{5l:t =: 34 set free. }■= C^H^oOio + SHjO. 

 13H.0 = ]l-^> J 



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), wliile 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 



