4 TEXTBOOK OF PLANT PHYSIOLOGY 



famous in the history of plant physiology. This connection 

 between agriculture and plant physiology provides the latter 

 repeatedly with new vitality for its future development. 



To the casual observer the life of a plant may appear more 

 simple than that of the visibly more active animal. From the 

 biochemical point of view, however, the transformations within a 

 plant are far more varied and complex than those in the animal. 

 Plants alone are able to manufacture organic compounds from such 

 elementary substances as carbon dioxide, water, and the mineral 

 salts. This important synthesis requires a considerable amount 

 of energy, which plants obtain from sunlight. 



Plant physiology deals, therefore, in the first place, with the 

 study of the synthetic mechanisms, which are characteristic of 

 plants and are the initial steps in their nutrition. This phase of 

 the subject matter will be discussed in the first part of the book. 



The synthesis of organic compounds from inorganic constitu- 

 ents takes place in the leaves, wherein a continual exchange of 

 gases with the surrounding atmosphere takes place. But owing 

 to their structure, leaves necessarily evaporate water, which seems 

 to be indispensable for their normal activity. The absorption and 

 release of water, therefore, are of great importance in the life of a 

 plant. The second part of the book deals with water relations. 



In the third part the detailed steps in the nutrition of plants 

 will be discussed. It will be concerned primarily with the absorp- 

 tion and utilization of compounds prepared by primary synthesis 

 and the production of new masses of living matter. This will lead 

 to the study of the mechanism of respiration and the liberation of 

 energy. Since the essential features of nutrition and respiration 

 are common to both plants and animals, we shall come in close 

 contact with problems of general physiology. 



The utilization of nutritive substances results in the growth, 

 movement, and reproduction of organisms. These again are 

 general physiological problems, but in plants they are character- 

 ized by considerable specificity. These and related questions are 

 discussed in the fourth and last part of the book. An advance 

 much beyond a description of the phenomena occurring has not 

 been made in this phase of plant physiology. Still, because of 

 their great theoretical importance, it has been deemed necessary 

 to treat them, even in this brief text, in considerably greater detail 

 than is customary. 



