GKOWTH 



315 



the energy which is demanded in the constructive processes. 

 This is evident also from the consideration that the growth 

 of the cells is attended by the growth in surface of the cell- 

 wall, and as the latter is a secretion from the protoplasm 

 a product, that is, of its katabolic activity such a decom- 

 position cannot readily take place unless oxygen is admitted 

 to it. 



Growth, so far as it implies only the formation of living 

 substance, is thus a constructive process. It is, however, 

 intimately associated with destructive metabolism or kata- 

 bolism, the latter being involved in the construction of 

 the increased bulk of the framework of the cell or cells, 

 and being essential to supply the energy needed for the 

 constructive processes. 



When the conditions mentioned are present, the course 

 of the growth of a cell appears to be the following : the 

 young cell, immediately it is cut off from its fellows, absorbs 

 water in consequence of the presence in it of osmotically 

 active substances. With the water 

 it takes in the various nutritive 

 substances which the former con- 

 tains in solution. There is set up at 

 once a certain hydrostatic pressure 

 due to the turgidity which ensues 

 upon such absorption, and the ex- 

 tensible cell- wall stretches, at first, 

 in all directions. The growth of 

 the protoplasm at the expense of 

 the nutritive matter for a time keeps 

 pace with the increased size of the 

 cell, but by and by it becomes vacuo- 

 lated as more and more water is 

 attracted into the interior. Even- 

 tually the protoplasm usually forms 

 only a lining layer to the cell-wall, and a large vacuole 

 filled with cell-sap occupies the centre (fig. 136). The 

 growth of the protoplasm, though considerable, is therefore 



FIG. 136. ADULT VEGETABLE 



CELLS. x 500. (After 



Sachs.) 

 h, ci 11- wall ; p, protoplasm ; 



k k, nucleus, with nucleoli ; 



s s', vacuoles. 



