RESISTANCE OF PLANTS 379 



unfit for the cultivation of crop plants. Such saline lands are 

 inhabited by especially salt-tolerant plants, the ''halophytes," 

 which are distinguished from usual plants by anatomical and 

 physiological peculiarities. 



Excessive salinity of the soil is injurious to the plant in two 

 ways. First, the accumulation of salts leads to an increase in 

 the osmotic pressure of the soil solution. Consequently, its 

 water-supplying power is decreased, and the water supply to the 

 plant is reduced. It has already been noted that the suction 

 tension of the roots is not high, and an increase of the osmotic 

 pressure of the soil solution up to 5 or 10 atmospheres, correspond- 

 ing to a 0.6 to 1.2 per cent solution of sodium chloride, creates a 

 serious hindrance to absorption. Experiments show that the 

 total concentration of salts in water cultures must not exceed 

 a few tenths of 1 per cent and must not have an osmotic pressure 

 of over 1 to 2 atmospheres. Watering the soil with salt solutions 

 likewise causes a rapid wilting of the plants as a result of a check 

 to the normal water supply. 



Second, the excessive accumulation of soluble salts has a toxic 

 effect upon the plants. Even such salts as sodium chloride and 

 sulphate, which are widespread in saline soils and which in weak 

 concentrations are neutral, at higher concentrations are toxic and 

 upset the normal physiological functions. Magnesium salts 

 and the alkaline carbonates that usually accompany sodium 

 salts are especially toxic. In the discussion of the mineral nutri- 

 tion of plants, the injurious effect of the various anions and 

 cations has already been noted. Though in sahne soils the salt 

 solutions are usually balanced, at high concentrations their toxic 

 effect becomes apparent. But different plants show varying 

 degrees of tolerance in this respect. 



Under natural conditions, alkah soils are inhabited by a rela- 

 tively rich and diverse vegetation consisting of halophytic plants 

 well adapted to high salt concentrations. According to the 

 investigations of Keller as well as of other authors, the halophytes 

 may be divided into several sharply distinct groups. Plants of 

 one of these groups are very resistant to salts and accumulate 

 them in their cells in large quantities, thereby increasing con- 

 siderably their osmotic pressure. This group is formed mostly 

 of Salicorniaceae, which develop on saline soils on the shores of 

 seas and salt lakes and near salt deposits. The Salieorniaceae 



