GKOWTH 



309 



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

 cell-wall, and as the latter is a secretion from the proto- 

 plasm, a product, that is, of its katabolic activity, such a 

 decomposition 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, how- 

 ever, intimately associated with destructive metabolism or 

 katabolism, 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 fellow, 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 FIG. 136. ADULT VEGETABLE 



cell, but by and by it becomes vacuo- g c L h L 8 s j 



lated as more and more water is k, cell-wall; p, protoplasm; 



.-,..,, ... -p-. k k. nucleus, with nu- 



attracted into the interior. Even- c ieoii ; ', 



_J 



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 

 not commensurate with the increase in the size of the cell. 

 The stretching of the cell-wall by the hydrostatic pressure is 



