INTRODUCTION xv 



yet its relation to the study of plants can scarcely be less vital 

 than is that of physiological chemistry to studies of animal life. 



The common conception that plant life and animal life are 

 antithetical or complementary to each other has much to justify it. 

 Animals breathe in oxygen and exhale carbon dioxide; while 

 plants use the carbon dioxide of the air as a part of the raw mate- 

 rial for photosynthesis and exhale oxygen. Plants absorb simple 

 gases and mineral compounds as raw food materials and build 

 these up into complex carbohydrates, proteins, fats, etc.; while 

 animals use these complex compounds of plant origin as food, 

 transforming parts of them into various other forms of structural 

 material, but in the end breaking them down again into the 

 simple gases and mineral compounds, which are expelled from the 

 body through the excretory organs. Thus it would seem that the 

 study of the chemistry of plant life and of animal life must neces- 

 sarily deal with opposite types of phenomena. 



But one cannot advance far into the study of the biochemistry 

 of plants and animals before he discovers marked similarities 

 in the chemical principles involved. Many of the compounds are 

 identical in structure, undergo similar changes, and are acted upon 

 by similar catalysts. Plant cells exhibit respiratory activities, 

 using oxygen and giving off carbon dioxide, in exactly the same 

 way that animal organisms do. The constructive photosynthetic 

 processes of green plants are regulated and controlled by a pig- 

 ment, chlorophyll, which is almost identical with the blood pig- 

 ment, haematin, which regulates the vital activities in the animal 

 organism, differing from the latter only in the mineral element 

 which links the characteristic structural units together in the 

 molecule. Many other points of similarity in the chemistry of 

 the life processes of plants and animals will become apparent as 

 the study progresses. It is sufficient now to call, attention to 

 the fact that these vital processes, in either plants or animals, 

 are essentially chemical in character, and subject to study by the 

 usual methods of biochemical investigations. 



The protoplasm of the cell is the laboratory in which all the 

 changes which constitute the vital activities of the plant take 

 place. All of the processes which constitute these activities 

 assimilation, translocation, metabolism, and respiration involve 

 definite chemical changes. In so far as it is possible to study each 

 of these activities independently of the others, they have been 



