16 Plant Physiology 



undergone profound changes, and the method of these 

 sciences is now the method applicable to a study of all 

 matter. In view, then, of the relationship of cell physi- 

 ology to morphology, on the one hand, and to physico- 

 chemical advances, on the other, a study of the cell has 

 become fundamental for any comprehensive view of gen- 

 eral physiology. 



9. The cell a physiological unit. Representing the pro- 

 toplasmic unit, the living cell is ultimately the seat of all 

 those complex chemical and physical changes, or diverse 

 energy transformations, of the living body. As a unicel- 

 lular organism the cell must act independently in a par- 

 ticular response ; in a multicellular body it responds also 

 as a distinct unit, in unison, however, with many other cells 

 associated together as a tissue. In any case it has been by 

 an investigation of the cell that many of the principles of 

 absorption, digestion, assimilation, excretion, and respira- 

 tion have been demonstrated. The fundamental concep- 

 tions of growth and differentiation, of fertilization and 

 reproduction, were only possible through the development 

 of cell study. 



Every cell passes through a cycle of changes. Each is 

 a seat of many, if not of all, of the physiological processes 

 characterizing the organism as a whole. In the more 

 complex plants and animals diversity of labor among the 

 cells has developed to such an extent that certain cells are 

 restricted, or specialized, with respect to their activities, 

 but all cells must perform certain fundamental functions 

 necessary to growth, development, and differentiation. 



In almost any physiological process, or in the ultimate 

 effects of various stimuli upon the organism, the cell is 



