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TEXTBOOK OF BOTANY 



tices, partly because of the oxidation of organic matter in them and 

 partly by the removal of the surface layers down to the more compact 

 subsoil. The failure of plants to grow well on many abandoned farms is 

 due not so much to lack of essential salts as to the compactness of the 

 soil. The social significance of these facts has but recently been recog- 

 nized by the general public. 



Fig. 128. Development of "knees" from the roots of bald cypress when growing 

 in lowlands annually submerged by floods. Photo by U. S. Forest Service. 



Available oxygen has an immediate effect on respiration, and conse- 

 quently on growth. The roots of some kinds of plants, such as willow, 

 Cottonwood, and cypress, can survive and grow in much lower concen- 

 trations of free oxygen than roots of many other kinds of plants (Fig. 

 128). Oxygen is not necessarily obtained from the soil air by some 

 plants, such as those growing in swamps or in partly submerged 

 areas. Many of these marsh plants have continuous air cavities 

 extending throughout the plant. The green shoots above may well have 

 an excess of oxygen in the daytime, and there is always some 



