PART II 



THE EFFECT OF CHEMICAL AND PHYSICAL 

 AGENTS UPON GROWTH 



CHAPTER X 



INTRODUCTION: ON NORMAL GROWTH 



ORGANIC growth is increase in volume.* It is not develop- 

 ment ; it is not differentiation ; it is not increase in mass, 

 although the latter may often serve as a convenient measure 

 of growth. 



In analyzing the processes of growth in organisms we must 

 recognize at the outset that organisms are composed of living 

 matter and formed substance, and that growth may therefore 

 result from the increase in volume of either of these. The 

 living matter, in turn, is composed of two principal substances : 

 the plasma and the enchylema or cell sap ; so growth may be 

 due to the increase of either of these substances, may result 

 either from assimilation, or more strictly from the excess of 



* Growth has been variously defined.- Thus HDXLEY has called growth 

 *' increase in size," which is essentially the same as my definition. SACHS ('87, 

 p. 404) defines growth as an increase in volume intimately bound up with 

 change of form (" eine mit Gestaltveranderung innig verkniipfte Volumen- 

 zunahme") ; and he illustrates the definition by the example of the growth of 

 a sprout from its beginning to its full development. In this case two phenom- 

 ena are distinguishable : first, increase in volume, and, second, the filling out of 

 the details of form. As SACHS says, these phenomena taken together are gen- 

 erally denominated " development "; and it seems decidedly advantageous to 

 retain this word with its usual signification, and to distinguish the two compo- 

 nent processes by the terms growth and differentiation. 



PFEFFER'S ('81, p. 46) definition differs still more widely from the one pro- 

 posed above. He defines growth as change in form in the protoplasmic body 

 (" die gestaltliche Aenderung im Protoplasmakb'rper ") ; and he goes on to say 

 that increments of volume and mass are not proper criteria of growth. PFEFFER 

 illustrates this statement by the following example : A plant stem or a cell mem- 



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