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 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 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 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 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 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. 



