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 



