RESPIRATION AND OXYGEN. 101 



contained in tilled soils is greater than in untilled, the differences are 

 insufficient to explain the value of tillage. Fallow mellow soil un- 



dergoes movements which increase the total amount of air-space. 

 Rolling the soil diminishes its aeration. 



Brizi (1906 : 89) regared the disease called "brusone" as being non- 

 parasitic, at least in part, and probably due to irregular or incom- 

 plete respiration in water or soil low in oxygen. 



Ehrenberg (1906 : 193) noted the case of turnips that had re- 

 ceived so much liquid manure that the plants stood in water up to 

 the crowns. At first they showed no injury, while the water was 

 receding and being absorbed by the soil. Soon, however, a marked 

 wilting developed rapidly among the leaves and finally extended to 

 the petioles. For several days the leaves remained hanging and then 

 gradually began to recover, although certain portions, especially at 

 the tip of old leaves, died off. Later, similar observations were made 

 on sugar-beets and carrots, while corn, grass, and other plants showed 

 no injury from the flooding. The injury to fleshy roots was ascribed 

 to the lack of oxygen, since these require a large amount at the time 

 of their most active development. -^ 



Clements (1907 : 19) regarded air-content as a factor of impor- 

 tance in all soils, but particularly in acid ones, owing to the constant 

 use of oxygen by the roots. The air-content varies inversely as the 

 water-content, and hence water-plants show characteristic modifica- 

 tions called forth in response to a low air-content. Plants which 

 grow in saturated soils or in water apparently do not compete for the 

 latter, though it is probable that a new factor, air-content, enters the 

 problem. 



Hesselmann (1910 : 91) has emphasized the fact that the swamping 

 of pine forests in Sweden is not a question of water, but of oxygen. 

 The water of the moors and swamp forests is almost completely free 

 from oxygen, and the pine forests suffer greatly in consequence. On 

 the other hand, the pines thrive on the banks of spring brooks, where 

 the water contains oxygen as a result of its rapid motion. In north*^ 

 ernmost Sweden the swamping of the soil is gradually increasing 

 through a marked rise in the level of the ground-water. The latter 

 has been deprived of its oxygen through contact with the peat and 

 thus brings about a significant depression of the functioning of the 

 tree roots. Wherever the soil is better aerated or the motion of the 

 water more rapid, so that it absorbs oxygen, the water works no in- 

 jury, even though it had originally been rendered oxygen-free in 

 passing through the moor. 



Hole (1911) pointed out that the Saccharum-Shorea community 

 in India grew in moist but well-aerated soil, while the Erianthus- 

 Terminalia community occurred in soil moist to wet, and hence less 

 aerated and apt to become somewhat water-logged during rains. 

 Shorea was stated to be decidedly sensitive in regard to aeration 



