INTRODUCTION. 3 



reactions in the plant world are largely endothermal and 

 require for their completion the constant expenditure of ex- 

 ternal energy. This energy is derived from sunlight and 

 through its agency chlorophyll-bearing plants are able to effect 

 a remarkable condensation, viz. : that of carbon dioxide with 

 water accompanied by liberation of oxygen. In its simplest 

 terms this condensation may be represented as 



CO 2 + H 2 O = H 2 CO + O 2 ; 



that is, formaldehyde and oxygen result. 



Formaldehyde is the first member of the series containing 

 the monosaccharoses and may be the actual starting point in 

 their elaboration by the vegetable cell. Of the mechanism of 

 further transformations by the plant we know but little; it is 

 likely that many of the following changes are brought about 

 by the action of soluble ferments or enzymes, which will be 

 referred to in a subsequent chapter. With the completion of 

 this synthesis a large amount of kinetic energy of the solar 

 rays is transformed into the potential energy of protein, fat 

 or carbohydrate. In the oxidation of the plant as fuel or 

 food the opposite change is accomplished, and the stored up 

 energy in complex organic molecules is liberated as heat, elec- 

 tricity or muscular motion. These reactions are so charac- 

 teristic for plants and animals that we are apt to lose sight of 

 others which also take place. In plants there is a respiration 

 process as in animals, in which oxygen is absorbed and carbon 

 dioxide liberated, and in the dark this may be readily observed, 

 since then it is not obscured by the much more prominent re- 

 duction process. Indeed, this respiration may be followed in the 

 light in the case of those plants which are free from chloro- 

 phyll. Further than this there are parasitic plants which, free 

 from chlorophyll, must depend on other plants for their nour- 

 ishment; they consume organic and not inorganic materials, 

 and in this behavior resemble animals completely. 



Then it must be remembered that the activity in the animal 

 is not wholly oxidation or degradation. It is well known that 



