GROWTH OF PLANTS 117 



in a decreased osmotic pressure within the cell and a gradual 

 lowering of turgor pressure. The stage of elongation then 

 terminates. 



This idea, appealing in its simplicity and permitting one to com- 

 pare the growth of living cells with artificial ones prepared 

 from precipitation membranes, for a long time enjoyed 

 great popularity. However, facts began to accumulate 

 that 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, or tendency to absorb water. With 

 the osmotic pressure unchanged, an increase in suction tension 

 can take place only when there is a diminished pressure of the 

 cell membrane, as has been explained in Art. 6. The diminished 

 turgor pressure during the increase in volume of the cell may be 

 the result either of a rapid growth of the cell wall or of an increase 

 in its elasticity. 



UrspAmg'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. The pressure of the wall on the 

 cell contents now diminishes, and as a result there arises a con- 

 siderable suction tension, causing water to enter the cell. This 

 suction is so great that in case of water deficit, young growing 

 parts may draw water from older or 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 tissues 

 on which they develop, although the absolute degree of difference 

 of their osmotic pressures is not great. 



Elongation is followed by the third and last period of growth, 

 the stage of internal differentiation. The thin, stretched wall 

 now grows in thickness. The cell has ceased to increase in 

 volume and assumes its final form and size. The homogeneous 

 zone of meristematic cells gives rise to the various tissues of the 

 mature organ, such as the conducting and mechanical elements. 

 The internal factors that control differentiation, frequently 



