THE MECHANISM OF THE ACTION 119 



I. THE PHYSIOLOGICAL APPROACH 



A. Definitions 

 We shall restrict the term "growth" to irreversible in- 

 crease in volume. The phenomena involved can be separated, 

 following Sachs, into four growth stages: 



1. Organization, i.e. the formation of organ primordia — 

 the determination of their number and place, (but 

 not the determination of their exact nature) ; 



2. Differentiation, which consists both of the determina- 

 tion of the nature of the organ and of the laying down 

 of most of its cells; 



3. Elongation, i.e. enlargement of the cells formed by 

 differentiation, taking place largely by water uptake; 



4. Maturation, or chemical changes occurring in the 

 completed organ. 



The first two stages are essentially morphogenetic and cor- 

 respond to the zoologist's use of the term growth, especially 

 because in animals (with certain exceptions) cell enlarge- 

 ment plays a relatively minor role. The third stage, elonga- 

 tion, often intergrades to some extent with the second — 

 that is, it may be accompanied by increase in cell number. 

 For our purposes the complication which cell multiplication 

 introduces will be omitted. In nuclear division and the 

 subsequent formation of a primary cross-wall no elongation, 

 that is, no growth in the sense of irreversible increase in 

 volume, is involved. All four stages may be influenced by 

 auxin, but at this point we shall restrict ourselves to a 

 consideration of the third stage, in which the action is 

 probably most direct, and shall return to the effects on the 

 other stages in later chapters. 



B. Nature of Growth 



Increases in cell volume may be due to water uptake, or 

 increase in dry weight, or both. Usually water uptake is 

 the major factor; cell wall formation accompanies it and 

 is responsible for most of the increase in dry weight, while 



