116 



PLANT PHYSIOLOGY 



The enlargement of the cell during the stage of elongation is 

 the result of causes different from those operating during meri- 

 stematic growth. In meristematic cells, synthesis increases the 

 amount of protoplasm; while, in elongating cells, there is an 

 increase in the quantity of cell sap, consisting mainly of water. 

 The question naturally arises: ''What causes water to accumu- 

 late in the cell and to enlarge the cell volume?" The process 



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Fig. 23. — Successive stages in the growth of root cells (after Brown). 



of cell elongation was considered for a long time to be as follows. 

 It was supposed that there appear in the cell osmotically active 

 substances like sugars or organic acids. These substances by 

 their osmotic action cause water to move through the cell mem- 

 branes, resulting in an increased turgor of the cell. The cell 

 wall, still thin and elastic, will begin to stretch under the influence 

 of turgor pressure, like a rubber balloon when air is pumped into 

 it, and the volume of the cell increases. At the same time, there 

 occurs a dilution of the osmotically active substances, resulting 



