GEOWTH 473 



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 catabolic activity, such a decomposition 

 cannot readily take place miless oxygen is admitted. 



When these conditions are present the course of events 

 appears to be the following ; the yoimg ceU, immediately it is 

 separated from its fellow, absorbs water, and with the water its 

 contained nutrient substances. There is set up at once a cer- 

 tain hydrostatic pressure due to the turgidity, and the extensible 

 cell-waU 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 cell,' but by and by it 

 becomes vacuolated as more and more water' is attracted into 

 the interior. Eventually the protoplasm forms usuaUy only a 

 lining layer to the cell- wall, and a large vacuole fiUed with cell- 

 sap occupies the centre. The growth of the protoplasm, though 

 considerable, is therefore not commensurate with the increase 

 in size of the cell. The stretching of the cell-wall by the hydro- 

 static pressure is fixed by secretion of new particles upon the 

 original wall, which as it thus becomes slightly thicker is 

 capable of stUl greater extensibihty, much in the same way as 

 a thick band of india-rubber is capable of 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 of new cellulose upon the old. The latter only is the 

 growth of the cell- walls ; the former can be removed by irrigat- 

 ing the cell with a solution of a substance, such as common salt, 

 which wiU rob the cell of its contained water. The constructive 

 changes leading to the formation of new protoplasm are attended 

 in this process by the catabohc formation of ceU-wall and other 

 substances, such as the osmotic bodies which are necessary to 

 attract the water into the cell. The supply of oxygen is needed 

 to allow the protoplasm to undergo these catabolic decomposi- 

 tions, enabling it thus to produce these several products and to 

 gain from such decompositions the energy which must be ex- 

 pended upon the construction and reconstruction of the living 

 substance and used in the secondary chemical changes which 

 supervene. 



This process of the growth of a cell is limited in its extent, 

 though the limits vary very widely. In some cases 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 is not constant ; it begins 



