24 INTRODUCTION. 



albuminous, nitrogenous bodies, all of which are also found in the 

 animal body, and which represent the essential food-stuffs of the 

 animal world. In doing so, I am aware that I am trespassing to 

 a certain extent upon what will follow in subsequent chapters ; but 

 as I shall deal with the chemistry of animal life more exclusively 

 in the present work, it has been deemed best to consider briefly the 

 principal syntheses which are effected by plants before proceeding to 

 a more detailed study of the subject proper. 



Synthesis of the Carbohydrates. It has been pointed out 

 that during exposure of chlorophyl-bearing plants to sunlight 

 the carbon dioxide of the air is decomposed, with liberation of 

 oxygen. The volume of gas thus set free is equivalent to the 

 volume of carbon dioxide that is decomposed. At the same time a 

 reduction of water takes place, as is apparent from the observation 

 that a larger amount of hydrogen is found in the plant than is 

 necessary to form water with all of the oxygen that is present at the 

 same time. It thus follows that one-half of the oxygen per volume 

 must be derived from carbon dioxide, and the other from water, 

 according to the equation : 



CO 2 + H 2 O : : r + 20, 

 2 volumes. 2 volumes. 



in which r represents one atom of carbon, one atom of oxygen, and 

 two atoms of hydrogen, which have been retained by the plant. A 

 combination of these atoms in one molecule, how r ever, would repre- 

 sent one molecule of formic aldehyde, CH 2 O. Should this be 

 actually formed in the plant, we would at once have a probable 

 explanation of the manner in which the carbohydrates are con- 

 structed, as these are polymeric compounds of formic aldehyde, or 

 their anhydrides. In fact, it is possible artificially to effect the 

 synthesis of many of the carbohydrates which are found in the 

 living world by starting with this simple aldehyde. Formic alde- 

 hyde, it is true, has not as yet been isolated as such from the leaves 

 of plants, as its existence here is probably only momentary, but its 

 oxidation-product, formic acid, has frequently been obtained. It is 

 thus extremely probable that this is actually the starting-point in the 

 elaboration of the carbohydrates by the plant. As to the exact 

 manner in which the aldehyde is derived from carbon dioxide and 

 water, we are as yet uncertain ; but it appears from the interesting 

 researches of Gautier and Timiriazeff that the chlorophyl first 

 decomposes water, under the influence of sunlight, and forms a 

 hydride of chlorophyl, which is colorless, and that this product 

 subsequently reduces carbon dioxide, with liberation of oxygen and 

 restitution of the green chlorophyl. These changes may be rep- 

 resented by the equations : 



(1) x + H 2 O = a;H 2 + O. 



(2) *H 2 + CO, := x + H a CO + O. 



