XIII. GROWTH 



353, Volume Relations of Protoplasm. Increase in size, and 

 physiological differentiation of a plant depend upon the increase 

 in size, and capacity for morphological differentiation of the cells 

 of which it is composed. The ultimate volume of a protoplast 

 is limited by the physical characteristics of living matter, and the 

 degree of differentiation it may show in its various organs is influ- 

 enced by forces resident in the chemical relations of these units. 

 A unicellular organism is therefore incapable of attaining a bulk 

 beyond that possible to a body with the viscosity of its proto- 

 plasm, or a degree of complexity beyond that offered by the 

 diverse nature of its nucleus, cytoplasm and plastids. An organ- 

 ism may increase its capabilities in both features if it is composed 

 of a number of fused protoplasts to form a coenocyte with sev- 

 eral nuclei, numerous plastids and a large mass of cytoplasmic 

 material. The bulk and functional development of higher organ- 

 isms must rest, however, upon the multiplication of the cells, and 

 their enlargement to the physiological and physical optimum of 

 size and efficiency. 



354. Purpose of Multiplication of Cells. The cell, in simpler 

 organisms, increases in volume until the approximate limit is 

 reached, when it divides by various methods into two or more 

 cells, which in turn repeat the process. This action gives rise to 

 linear series of cells of the same degree of differentiation. 



The spore or egg cell of the higher plant undergoes division 

 and re-division in such manner as to lead to the formation of a 

 number of protoplasts, some of which lose the power of further 

 division or multiplication, and assume certain functions for the 

 performance of which they show more or less specialized morpho- 

 logical characters. A fraction of the products of division retain 



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