118 



PLANT PHYSIOLOGY 



accompanied by the formation of special thickenings in the cell 

 walls, pitting of the wall, the loss of the cell contents, etc., 

 remain unknown at present; but the changes of cell structure in 

 differentiation are associated with the assumption of a definite 

 function by the cell. To perform its function, a cell takes on a 

 special structure. Morphological changes always follow changes 

 of physiological function. 



Fig. 24. 



-Measurement of growth by means of a horizontal microscope (after 



Molisch) . 



•28. Methods of Measuring Growth. General Laws of Growth 

 and Attempts at Their Mathematical Formulation. — An exact 

 study of the phenomenon of growth requires, in the first place, a 

 precise measurement of it. Several methods are available for 

 this purpose. 



The simplest of these consists in measuring the length of the 

 growing organs or of whole plants at definite time intervals by 

 means of a properly subdivided ruler. As the growth of plants 

 on the whole is rather slow, this comparatively crude method 



