RESISTANCE OF PLANTS 377 



through these cavities into the roots, where it is consuined in 

 respiration. 



Only a few of the crop plants can grow on waterlogged soils. 

 Rice is the most important of these. It not only endures but 

 even requires for successful development flooding of the field 

 with water during the greater part of its vegetative period. 

 Most of the plants, how^ever, can grow on bog soils only after 

 drainage, which causes a lowering of the water table and is 

 accompanied by the penetration of oxygen at least into the upper 

 soil layers. Anaerobic processes are then replaced by oxidation 

 processes, the toxins are destroyed, and the roots of plants can 

 develop normally. All these changes require a long time. Bog 

 soils become fit for cultivation of crop plants only 1 or 2 years 

 after drainage. 



One of the results of the anaerobic fermentation is the accumu- 

 lation of an excessive amount of organic acids and substances that 

 increase the hydrogen-ion concentration. Bog soils are fre- 

 quently very acid. Plants inhabiting them, such as species of 

 Carex and Deschanipsia caespitosa, are characterized by a very 

 high pH optimum of their growth. After drainage of bogs, 

 soil acidity decreases slightly but not sufficiently. Hence, liming 

 of drained bog soils is essential. It improves the reaction and 

 contributes to a more normal decomposition of organic soil 

 compounds. 



With a temporary excess of soil moisture, for instance, in the 

 spring w^hen snow melts or during prolonged rains that make 

 ponds in depressions of the field surface, injury to plants from an 

 excess of water may occur. The results in this case are similar 

 to those occurring in bog soils and are explained by insufficient 

 penetration of oxygen into the soil. Temporary waterlogging 

 retards somewhat the growth of plants and weakens them in 

 their struggle with weeds, which may develop vigorously under 

 conditions of excessive soil moisture. Prolonged waterlogging 

 leads to death of crop plants. 



Lodging of cereals, which also frequently results in considerable 

 losses, is likewise to a certain degree caused by excessive soil 

 moisture as well as by high atmospheric humidity. Lodging is 

 frequently observed in rainy summers and with artificial irriga- 

 tion. The chief cause of lodging is the disturbance of the equi- 



