142 PHYSIOLOGY OF THE DOMESTIC ANIMALS 



is without action on starches. It therefore closely resembles animal 

 gastric juice. 



Still another analogy may be traced between the animal and vege- 

 table kingdoms. Under certain circumstances plants develop heat, as in 

 germinating seeds, or in flowers during fecundation. Sugar is the sub- 

 stance in such cases whose combustion sets free heat. It exists in 

 germinating seeds, and disappears during germination, from the action 

 of a diastatic ferment analogous to the glycogen ferment in animals. 

 The analogy is, however, not perfectly complete, as the plants manu- 

 facture their starchy material from inorganic materials ; animals must 

 obtain it ready-made. 



In the dark the processes of assimilation of plants are entirely 

 suspended. Then carbon dioxide is given off, and oxygen is absorbed, 

 for the processes of respiration or oxidation still continue. During the 

 day the carbon dioxide, which is constantly absorbed by the roots and 

 leaves, is in the leaves broken up into oxygen, which is set free, while 

 the carbon remains tixed. At night C0 2 is also absorbed by the roots, 

 but is exhaled from the leaves without undergoing change : for, as we 

 have found, for its deoxidizing purposes chlorophyll requires the assist- 

 ance of sunlight and heat. It is also possible that a part of the C0 2 

 which is set free during the night is not only derived from C0 2 absorbed 

 from the leaves and roots, but also is the result of oxidation by a part 

 of the oxygen which is absorbed. 



2. The Animal Cell. The relationship which we have traced 

 between the chemical processes of plants and the atmosphere and soil 

 around them is reversed in the case of animal cells ; for, while green 

 plants absorb the inorganic constituents of the earth and atmosphere, 

 and from them build up complex, inorganic compounds, the oxygen of 

 the atmosphere in the animal permits of the reduction of its complex tis- 

 sues and constituents. For the green plants the atmosphere forms one of 

 their chief foods; for animals it is the great agent which permits their 

 tissue changes, on which all liberations of energy depend. In green 

 plants the chief vital phenomenon is the liberation of oxygen ; in ani- 

 mals it is the absorption of oxygen. In plants the liberation of oxygen 

 is an index of increase in weight ; in animals the absorption of oxygen 

 leads to a loss of weight. That the animal cell may retain its com- 

 position unaltered it must be supplied with its tissue-constituents. 

 Unlike vegetable cells, the animal cell is incapable of manufacturing 

 these tissue-constituents from inorganic elements. The most that the 

 animal cell may do is to transform a member of one class of its con- 

 stituents into another member of the same group. Thus, the animal cell 

 may transform the albuminoid matters contained in vegetable cells into 

 albuminous bodies which are peculiar to animals. It may transform the 



