28 AERATION AND AIR-CONTENT. 



had a small share in the corrosion of rock surfaces. Raciborski 

 (1905) has confirmed the existence of oxidizing power in the roots 

 of a large number of plants, as have other investigators. Knudson 

 (1920) has recently come to the conclusion that roots do not secrete 

 invertase. 



It was early shown by Saussure (1804) that the shoots of the vari- 

 ous species exhibit different respiration intensities, and that this 

 was true of roots as well. He found that fleshy plants required on 

 an average but 1.1 times their volume of O, while aquatic and marsh 

 plants needed but 1.6 times their volume, in contrast to an average 

 of 6 times for deciduous trees and shrubs. Differences in the respira- 

 tory power of roots were also found by Garreau (1851), but the 

 most comprehensive study was that of Freyberg and Meyer (1879), 

 who showed that the roots of swamp plants require less oxygen than 

 those of land plants, a fact already noted for their shoots by Bohm 

 (1875). The respiration maximum for the roots of mature swamp 

 plants was 78 per cent greater and for the roots of seedlings 50 per 

 cent greater than in land plants. Hoffmann and Sokolowski, and 

 Appleman have determined that varieties of potatoes possess dif- 

 ferent rates of respiration and exhibit corresponding differences in 

 their relation to oxygen. The ability of roots to endure the absence 

 of oxygen has been shown to be most variable, as indicated in the 

 section on anaerobic respiration, and similar results have been ob- 

 tained from field studies, which are discussed in the corresponding 

 section. 



AEROTROPISM. 



Mohsch (1884 : 111) made an exhaustive study of the growth 

 responses of corn and pea roots to various gases and reached the 

 following conclusions: If a growing root is exposed to a certain gas 

 upon one side, so that the latter is present in unlike quantities for a 

 considerable time on the two opposed sides of the root, the root 

 departs from its normal direction of growth in a definite manner. 

 This phenomenon is termed aerotropism. Such an effect by gases 

 upon growing roots has been shown for oxygen, CO2, ether, ammonia, 

 etc. The roots are sensitive in different degrees to different gases. 

 The effect of oxygen is weak, of CO 2 stronger, and of chlorine very 

 strong. If a gas is too strong, the root bends towards the source of 

 the gas (positive aerotropism); with a more moderate amount of 

 gas it bends away (negative aerotropism). With reference to oxy- 

 gen, the facts are somewhat more complex. The positive bending 

 arises from the fact that the concave side is injured and its growth 

 in length is less than on the opposite side. Decapitated roots react to 

 CO2, chlorine, and illuminating gas just as uninjured ones, though 

 in a weaker manner. 



