GROWTH 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 yyy. 136—Apunr VeoRTaBer 
cell, but by and by it becomes vacuo- pate x 500. (After 
* achs, 
lated as more and more water 18 4, ctwall; protoplasm ; 
attracted into the interior. Even- kk, nucleus, with nucleoli ; 
ss’, vacuoles. 
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 
