186 HISTORY OF BOTANY 



values of the different cell contents and proved a relation- 

 ship between osmotic pressure and molecular weight. To 

 De Vries we owe much of our knowledge of plasmolysis, 

 a term introduced to indicate the separation of the 

 protoplasm from the cell wall, when the protoplasmic 

 membrane refused to allow the solute outside the cell to 

 enter the central vacuole, even after it had penetrated 

 the cell wall. In 1884 De Vries showed that the osmotic 

 effect depended not on the dissolved weight of the sub- 

 stance but on the number of molecules dissolved, and that 

 when substances are dissolved in quantities equivalent to 

 their molecular weights they all produce the same osmotic 

 effect. A further discovery was that although this was 

 true of non-metallic compounds it was not true of solutions 

 of metallic salts when these were very dilute, for in these 

 cases the salt underwent partial dissociation and the 

 free ions exercised the same osmotic effect as the entire 

 molecule. This is a matter of considerable importance 

 when we take into account the extremely dilute solutions 

 of minerals in the soil that are presented to the root 

 hairs. 



Sachs in 1865 pointed out that the osmotic transferenc 

 of solutes from the soil to the root hairs and from them to 

 the cortical cells not only induced turgidity in these 

 cells but established an internal pressure in the root which 

 forced the water and its dissolved salts into the wood 

 vessels, and so created " root pressure," and accounted 

 for the phenomenon of " bleeding " from wounds. 

 Several investigators, notably Baranetski, Detmer, and 

 Wieler, were also able to demonstrate that root pressure 

 exhibited both a daily and a seasonal periodicity. 



TRANSPIRATION 



Hales, nearly 150 years before, had noticed that 

 water vapour was given off from a leaf surface much less 

 freely than it was from an equal surface of water, but 



