376 PLANT PHYSIOLOGY 



usual period of drought in the intervals between irrigations and 

 at the same tinie not giving too large amounts of irrigation water 

 at a time. 



88. Injury to Plants from Excessive Humidity. Factors in 

 the Resistance of Bog Plants to Waterlogging. Lodging of 

 Plants, Its Causes and Means of Control. — An excess of water 

 in the soil may be almost as harmful as a deficiency; waterlogged 

 soils can be used for cultivation only after drainage. This is not 

 because water is harmful to the roots; for experiments with 

 growing various plants, even xerophytes, in water culture have 

 proved that this is not true. But when the soil is waterlogged 

 or temporarily flooded, all the capillaries are filled with water, 

 and air cannot penetrate into the soil. The roots are deprived 

 of the oxygen indispensable for their life activity. It has already 

 been emphasized that oxygen is one of the conditions essential 

 for water absorption by the root that supplies water to the aerial 

 parts (Art. 65) and likewise for the absorption of mineral and 

 nitrogen salts (Art. 58). 



Besides this direct influence of oxygen deficiency upon the 

 roots, the filling of the soil capillaries with water and the decrease 

 of aeration in general lead to a series of other very serious conse- 

 quences. The most important of these is the checking of the 

 normal oxidation in the soil produced by the activity of aerobic 

 soil bacteria. Anaerobic processes then begin, chiefly butyric 

 and similar fermentations. Carbon dioxide and other organic 

 acids accumulate in the soil, as well as the products of 

 reduction both of organic and of inorganic substances, many 

 of which are toxic to the roots of plants. American authors 

 term these products ''bog toxins." Crop plants, whose root 

 systems require a loose, well-aerated soil, cannot grow on such 

 soils. Only special bog plants can thrive on them, their roots 

 being specially adapted to these conditions anil unsusceptible 

 to the bog toxins. They have anatomical structures that pro- 

 vide them with oxygen for respiration, even in soils that are 

 completely lacking in oxygen. The stems of bog plants fre- 

 quently possess well-developed intercellular spaces and air 

 cavities that are connected with similar cavities in the stems and 

 leaf peduncles. Oxygen is formed in photosynthesis and, expand- 

 ing with the heating of the leaves by the sun's rays, is forced 



