General Physiology of the Plant. 2 1 1 



change of form. Growth is, of course, the result of active 

 constructive metabolism, and as such involves an increase 

 in the dry weight of the plant. We shall glance first at the 

 general conditions cf growth, and then briefly summarise the 

 phenomena of growth. 



Conditions of growth. Growth can take place only in 

 the living plant, the growth of unorganised substances being 

 merely the result of accretion. Growth is, in the first place, 

 due to expenditure of energy taking place under certain 

 conditions of temperature. Again, an adequate food supply 

 in the form of the substances essential to the nutrition of 

 plants is necessary. Water more especially is of importance. 

 In addition, however, to these general conditions, growth is 

 modified by the ability of the protoplasm itself to assimilate 

 and respire, i.e. the anabolic and katabolic powers of each 

 indmdual cell must be taken into account. Now it has 

 been shown by Spencer that the mass of a cell increases as 

 the cube of the dimensions, while the surface increases only 

 as the square. The surface of the cell, however, is the area 

 of nutrition, i.e. the larger the surface, other things being 

 equal, the more nourishment can be absorbed. Again, the 

 same surface is the area of excretion, so that the surface of 

 the cell is at once alimentary and purificatory. In the early 

 life of the cell the anabolism is in excess of katabolism. 

 As, however, that excess gradually makes itself apparent in 

 increase of bulk, Spencer's law comes into operation, and 

 with the increase of size the possibility of the maintenance 

 of the same equilibrium between the anabolic and katabolic 

 process comes to an end. The relative excess of waste 

 products may kill the cell, but much more frequently, as in 

 young cells, restoration of the original conditions takes place 

 by division of the cell into two or more parts. Thus growth 

 of the individual cell brings about growth of the plant as a 

 whole. 



The phenomena of growth are too complex for any 

 detailed treatment here. The most that we have space to 



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