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CHAPTER I V « --^ 



THE FIELD OF PLANT PHYSIOLOGY 



In order to understand the mechanism of a plant in its entirety many 

 separate phases of the dynamic activity of plants have been recognized and 

 studied as individual processes. In his efforts to interpret these individual 

 processes and their inter-relationships the plant physiologist is confronted with 

 many problems: What is the mechanism by w^hich v^^ater, gases, and solutes 

 enter a plant from its environment? How do such substances pass out of a 

 plant into its surroundings? How are foods and other complex organic com- 

 pounds synthesized in the plant? How are they utilized in the development 

 and maintenance of plants as living systems? What transformations of energy 

 occur within a plant, and what exchanges of energy' take place between a 

 plant and its environment? How are water and solutes transported from one 

 part of a plant to another? How are new tissues constructed? How is the 

 development of one organ or tissue coordinated with the development of other 

 organs or tissues? Why does a plant produce only vegetative organs at cer- 

 tain stages in its life cycle and reproductive organs only at other stages? How 

 are individual plant processes and the development of a plant as a whole 

 influenced by environmental conditions? All of these problems and many 

 other related or subsidiary ones lie within the province of the branch of science 

 which is known as plant physiology. 



For convenience, the study of plant life is subdivided into various branches 

 such as physiology, morphology, anatomy, ecologj^ pathology, genetics, etc. 

 Such a classification is necessarily more or less arbitrary. Neither physiology 

 nor any other phase of plant life can be singled out for study without some 

 consideration of plants from other viewpoints. A particularly intimate inter- 

 relationship exists between the structures and processes of plants. Every 

 physiological process is conditioned by the anatomical arrangement of the tis- 

 sues, and by the size, configuration, and other structural features of the cells in 

 which it occurs. Furthermore, the coordinated development of cells and tis- 

 sues, i.e., of the plant itself, is a complex of physiological processes. Thus the 

 sciences of plant physiology and plant anatomy merge in the study of plant 

 growth. 



