GROWTH 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 bemg 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 stxetches, 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 ric.136—Apunr Vecrrapie 
cell, but by and by it becomes vacuo- Basha} eee: ee 
lated as more and more water is *, cell-wall; p, protoplasm ; 
attracted into the interior. Even- nae a es ote 
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. 186). 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 
