l4 2 BOTANY OF THE LIVING PLANT 



organs (p, I JO). The total mass of the meristematic cells at the 

 apex Of a root or shoot remains relatively constant because the 

 cella on the side of the meristcm remote from the apex are suc- 



rivcly ceasing to divide, and pass into the phase of enlargement. 

 Much the same applies to cambial activity. In an organ of limited 



>Wth, such as a leaf, all the cells soon pass into the later phases 

 of development. 



On entering the phase of enlargement the newly-formed cells under- 

 go a very considerable increase in size, due not so much to further 

 elaboration of protoplasm within them, as to the intake of water. 

 It is to this that most of the increase in size of the growing plant 

 is due. This enlargement of new cells, with the attendant formation 

 of vacuoles, has already been described on pp. 21-22, though the 

 mechanism was not there considered. It was formerly believed that 

 cell-enlargement was due to an increase in osmotic pressure of the 

 cell-sap. This might be produced by the conversion of insoluble 

 into soluble, crystalloidal substances, e.g. starch might be converted 

 to sugar. Experiment has, however, failed to reveal evidence of such 

 increase. It is now believed that the intake of water is due rather 

 to an increase in the plastic extensibility of the walls of the newly- 

 formed cells, although presumably there is a production of additional 

 osmotic substances while the stretching is in progress, so as to counter- 

 balance the tendency to dilution caused by the absorption of water. 

 During the stretching of the cells additional layers of cellulose are 

 deposited on the walls, resulting in an increase in thickness rather 

 than the decrease which would otherwise follow. The final arrest 

 of the stretching may be due to the continued strengthening of the 

 walls in this manner. The stretching of the cell-wall during growth 

 is permanent : plasmolysis of the mature cell does not result in the 

 shrinkage of the cell-walls to the dimensions which they possessed in 

 the newly-formed stage. 



A considerable interval may elapse between the emergence of 

 cells from the meristematic phase and their enlargement. Instances 

 of this are seen in the resting buds of trees. By meristematic activity 

 the various parts of the shoot for the forthcoming year are laid down 

 within the buds during the summer and autumn : only in the sub- 

 sequent spring do the cells undergo enlargement, causing the unfolding 

 of the buds. 



In roots and stems the enlargement of cells is chiefly a matter 

 of elongation. This follows with some regularity on the emergence 

 of the cells from the meristematic phase, so that behind the growing 



