FACTORS DETERMINING OSIVIOTIC PRESSURES 141 



TABLE 19 THE EFFECTS OF DIFFERENT SOIL WATER CONTENTS UPON THE OSMOTIC PRESSURE 



OF MAIZE PLANTS (dATA OF HIBBARD AND HARRINGTON, I916) 



The solute content of the cell sap is controlled by the specific metabolic 

 processes of the plant and by the absorption of mineral salts by the plant 

 from its environment. The rate of photosynthesis is an important factor in 

 determining the osmotic pressure of plant cells, particularly those of leaf tis- 

 sues. The influence of the inherent metabolic processes of any species upon 

 the kinds and concentrations of various types of soluble organic compounds 

 produced, such as simple carbohydrates, organic acids, amino acids, etc. has 

 an exceedingly important effect on the magnitude of the osmotic pressure in 

 any species. Metabolic conditions and their effects upon osmotic pressures 

 may also be altered by environmental conditions. A well-known example of 

 this is the difference in the osmotic pressures of sun and shade leaves on the 

 same plant. The former almost invariably have the higher osmotic pressure, 

 due presumably to their greater photosynthetic activity. 



The mineral salts which contribute to the osmotic pressures of plant cells 

 are all absorbed from the soil solution, or in aquatic species from the water 

 in which part or all of the plant is immersed. Different species of plants 

 vary greatly in their toleration of high concentrations of mineral salts in the 

 soil. All species can become adjusted within limits to a change in the mineral 

 salt content of the substratum. This adjustment takes the form of an increase 

 in the osmotic pressure of the plant with an increase in the osmotic pressure 

 of the medium from which it obtains its mineral salts (Table 20). 



Other tissues of plants also show an increase in osmotic pressure with 

 increase in the osmotic pressure of the soil solution, but such increases are 

 generally most pronounced in the roots. 



Species indigenous to alkali soil regions usually have relatively high os- 

 motic pressures. Alkali soils are rich in soluble salts, and the high osmotic 

 pressures found in species native to such soils are due to the absorption of 

 relatively large quantities of mineral salts. In fact the highest recorded os- 



