316 VEGETABLE PHYSIOLOGY 
not commensurate with the increase in the size of the cell. 
The stretching of the cell-wall by the hydrostatic pressure is 
fixed by a secretion of new particles and their deposition 
upon the original wall, which as it becomes slightly thicker 
is capable of still greater extension, much in the same way 
as a thick band of india-rubber is capable of undergoing 
greater stretching than a thin one. The increase in surface 
of the cell-wall ig thus due, firstly, to the stretching caused 
by turgidity, and, secondly, to the formation and deposition 
of new substance upon the old. The latter only is permanent ; 
the former can be removed by irrigating the cell with a 
solution of a substance, such as common salt, which will 
rob it of the water it contains. The constructive changes 
leading to the formation of new protoplasm are attended in 
this process by the katabolie formation of cell-wall and other 
substances, such as the osmotic bodies which are necessary 
to draw the water into the cell. The supply of oxygen 
is needed to allow the protoplasm to undergo these kata- 
bolic decompositions, enabling it thus to prepare the several 
products spoken of, and to gain from such decompositions 
the energy which must be expanded upon the construction 
and reconstruction of the living substance, and used in 
the secondary chemical changes which supervene. 
~ The process of the growth of a cell is limited in its extent, 
though the limits vary very widely in different cases. In 
some, cells grow only to a few times their original dimensions ; 
in others, they may attain a very considerable size. In any 
case, however, we can notice that the rate of growth takes 
a certain course throughout the process; it begins slowly, 
increases to a maximum, and then becomes gradually 
slower till it stops. The time during which these regular 
changes in the rate can be observed is generally spoken 
of as the grand pertod of growth. 
Changes in the shapes of cells arising during growth 
depend upon two factors. The capacity of the cell to yield 
to hydrostatic pressure may be affected differently in different 
directions by the conditions of the cells which surround it. 
