GROWTH. 335 



prove that the existence of this hydrostatic pressure, in other 

 words, a turgid condition of the cell, is essential to its growth. 

 It is a matter of common experience that plants will not 

 grow unless they are supplied with water ; on the contrary, 

 under these conditions they wither. This point was made 

 the subject of experiment by Sorauer. He grew a number of 

 Barley-plants in soils containing different proportions of 

 moisture, 10, 20, 40, or 60 per cent, of the amount requisite 

 for the complete saturation of the soil, the other conditions 

 being the same in all cases, and he found that within a certain 

 limit for, as we have already learned (p. 49), an excess of 

 .water is hurtful the more moist the soil the more perfect 

 was the development of the plant. We have here the proof 

 of the importance of water in the process of growth. In the 

 light of de Vries' experiments, we are able to explain So- 

 rauer's results by attributing the imperfect growth of the plants 

 in the relatively dry soils to the fact that in the absence of an 

 adequate supply of water the turgidity of the growing cells 

 could not be maintained. De Vries found, namely, that the 

 growth of branches, peduncles, etc., was more or less dimin- 

 ished when they were placed in solutions of neutral salts 

 (NaCl or KNO 3 ) of sufficient concentration to withdraw 

 water from the growing cells and thus to diminish their 

 turgidity, and that growth was entirely arrested when the 

 solutions were sufficiently concentrated to cause complete 

 plasmolysis in the cells (p. 43). The arrest of growth was not 

 due to any injury done to the cells by the reagent employed, 

 for de Vries observed that after thorough washing in water to 

 remove the salt the organs resumed their growth. 



It appears, however, that turgidity is not in all cases essential to the 

 growth of cell-walls. Strasburger has pointed out that when pollen-tubes 

 are being formed from pollen-grains the pollen-grains are not turgid. 

 He attributes the growth in length of the pollen-tube to the pressure of 

 the contained protoplasm. This is probably true, as mentioned above, of 

 the growth of all cells in its first stage. 



The distension of the cell-wall in consequence of the 

 turgidity of the cell is not, however, growth, for the size of 

 the cell can be diminished by diminishing the turgidity. 



