286 TEXTBOOK OF PLANT PHYSIOLOGY 



of cell sap, consisting mainly of water. The question naturally 

 arises as to what causes water to accumulate in the cell and to 

 increase the cell volume. The process of cell elongation, which 

 sometimes had been identified with the mechanism of cell growth 

 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 then begin forcefully 

 to attract water, resulting in an increased turgor of the cell. The 

 still thin and yielding cell wall will begin to stretch under the influ- 

 ence of turgor pressure, similar to 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 in a decreased osmotic pressure within the cell and a 

 gradual lowering in turgor. The stage of elongation then ter- 

 minates. 



This idea, appealing in its simplicity and permitting one to 

 compare the growth of living cells with the so-called artificial 

 ones prepared from precipitation membranes, for a long time 

 enjoyed great popularity. Soon after its rise, however, facts began 

 to accumulate, which contradicted it. It was found that osmotic 

 pressure does not decrease in cells as they grow, but remains fairly 

 constant. Moreover, Ursprung's recent researches have shown 

 that in the period of elongation cells do not have a greater but, on 

 the contrary, a lower turgor and at the same time a considerably 

 increased suction tension. With the osmotic pressure unchanged, 

 an increase in suction force can take place only when there is a 

 diminished pressure of the cell membrane (Art. 42). The volume 

 of the cell increasing, this decrease can occur only by a rapid 

 growth of the membrane itself. 



Ursprung's investigations enable us to construct the following 

 mechanism of the growth of cells in the stage of elongation. The 

 increase in amount of protoplasm, characteristic of the embryonic 

 stage, is replaced by an augmented production of cellulose and other 

 substances of the cell wall, thus causing the membrane to grow 

 rapidly. Its pressure on the cell contents now diminishes, and as a 

 result there arises a considerable suction tension, causing water to 

 enter the cell. This suction is so great, that in case of shortage 

 young growing parts may draw water from old, already mature 

 cells. This explains the fact that young shoots of the potato and 

 other tuber and root crop plants can absorb water from the storage 



