TEACHING AND LEARNING 



any internal classification they may exhibit, and moreover is 

 often made erroneous, or at least misleading, by the presence 

 of very aberrent individuals, the former sorts the data out, so 

 to speak, and arranges them around their center of frequency 

 (mode), or, in some cases, their several modes. It is important 

 in this connection to keep in mind an application of Quetelet's 

 Law, namely, that variables, where there is no disturbing cause, 

 tend to group themselves symmetrically on both sides of their 



Fig. i. — Graph (polygon) of protoplasmic streaming in a species of 

 nltella under varying temperatures, recorded for alternate 



DEGREES. 

 The continuous line is an average of the results of nine students ; the broken line is the 

 result of one student. 



mean, and also that they fall most abundantly nearest the mean. 

 This principle has some important practical, as well as theo- 

 retical, uses in Plant Physiology, as will later appear. 



More than once in this book I have emphasized the desira- 

 bility of giving the educational study of Plant Physiology a more 

 quantitative character than it has had in the past. An impor- 

 tant phase thereof concerns the expression of physiological 

 quantities, or constants, for the different processes. The physi- 

 ologist cannot, like the chemist and the physicist, express his 

 quantities in definite figures or formulae, because plants are 



