CHAPTER II 

 THE LIFE CYCLE 



GROWTH AND REDUCTION 



Definitions of growth and reduction. — One of the most charac- 

 teristic and striking features of the living organism is its ability to 

 add to its own substance. In most organisms an enormous increase 

 in size and weight occurs during the earlier part of the life cycle. 

 This is commonly known as growth. But different authorities are 

 not entirely agreed as to what constitutes growth. The differ- 

 ences of opinion seem to center chiefly about the question whether 

 growth consists simply in increase in size, or whether change in 

 form is the essential feature. Davenport,' following Huxley and 

 others, defines organic growth as increase in volume. The plant 

 physiologist Pfeffer ('oi), on the other hand, says that in general 

 all formative processes which lead to a permanent change of form 

 are to be regarded as growth. Most authorities have regarded the 

 addition of material, or of certain kinds of material, or the increase 

 in size as the essential feature of growth. To make change of form 

 the basis of growth is certainly a wide departure from the com- 

 monly accepted meaning of the word, and also fails, I think, to 

 recognize the significance of accumulation of material in the organ- 

 ism. Increase in size or the addition of material may occur without 

 appreciable change in form, and change in form may occur without 

 increase in size or amount of material, and most of those who 

 have attempted to define growth have recognized this fact. The 

 capacity of the organism to add to its own substance and to in- 

 crease in size is evidently closely connected with the fundamental 

 processes of metaboHsm, and even organisms which do not undergo 

 appreciable changes of form do nevertheless grow in the usual 

 sense of the word. 



But any consideration of the problem of growth which does 

 not take into account the process of reduction is incomplete. Under 

 the usual conditions of existence the healthy active organism is not 



' In Davenport's Experimental Morphology ('97, pp. 281-82) a number of the 

 definitions of growth which have been given are cited. 



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