382 PLANT PHYSIOLOGY 



growth of the root system of the seedUngs. As a result, plants 

 are produced that remain backward in their development and 

 are seriously injured from drought. 



The accumulation of salts in the cells, resulting in poisoning 

 of the protoplasm, retards the synthetic process, such as photo- 

 synthesis and the function of proteins. The cells accumulate 

 chiefly soluble carbohydrates, amino acids, and such soluble 

 compounds. Their accretion increases the osmotic pressure in 

 the cells, which retards the growth of plants, though it is advan- 

 tageous from the point of view of obtaining water from more 

 concentrated soil solution. 



Salinity of the soil has a singular effect upon the stomatal 

 apparatus of the plant. Under the influence of salts, the process 

 of starch formation is impeded in the guard cells. As has been 

 noted above (Art. 71), the formation of starch contributes to 

 their normal closing when water is deficient. Hence, on saline 

 soils the stomata are open permanently even during drought. 

 This results in an excessive water loss and threatens the plant 

 with desiccation. But at the same time this effect of salts, which 

 contributes to the accumulation of soluble carbohydrates and 

 similar compounds, increases the drought resistance of plants, 

 thereby compensating to a certain degree for the loss of water. 



Salinity of the soil has a great influence upon the filling of 

 grain, interfering with its development and causing it to shrivel. 

 Accumulation of carbohydrates is retarded to a greater degree 

 than is that of protein substances. As a consequence, on saline 

 soils wheat produces grain rich in protein. This increases con- 

 siderably its commercial value. 



90. Resistance of Plants to Fungi. Character of the Inter- 

 relations between the Parasite and the Host Plant. Immun- 

 ity. — Among the factors causing considerable decreases in the 

 yield, besides drought, frost, and alkalinity, there may be men- 

 tioned the attacks of parasitic fungi. Rusts, smuts, root rots, 

 and other diseases cause enormous damage in agriculture, and 

 the struggle with them represents one of the essential problems 

 in present-day agriculture. A detailed discussion of all these 

 diseases cannot be made in this textbook. Only a few of the 

 most important facts of the physiology of fungus diseases will 

 be considered here. A description of the symptoms of diseases 

 and of the f\mgi causing them is the subject of phytopathology. 



