310 VEGETABLE PHYSIOLOGY 
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 is 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 the growth of the cell-walls; 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 katabolic 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 expended 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 varies regularly throughout the process ; it begins 
slowly, increases to a maximum, and then becomes gradu- 
ally slower till it stops. The time during which these 
regular changes in the rate can be observed is generally 
spoken of as the grand period 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. In the merismatic tissue of a growing point 
