FACTOR INDICATORS. 87 



"6. As Sal seedlings can be successfully grown in water cultures, the injuri- 

 ous effect of bad aeration is not due to water as such. This probably explains 

 the fact that Sal can grow on the banks of the rivers or even of stagnant lakes, 

 in which the water is kept well aerated by exposure to the air or by the pres- 

 ence of green aquatic plants." 



The significance of aeration in field soils has been emphasized by Howard 

 (1913 : 7, 10) : 



"Important results have been obtained relating to water-logging and drain- 

 age, and it is suggested that these matters are of far greater importance than is 

 generally supposed. Even partial water-logging has been shown to reduce the 

 wheat crop 50 per cent. It is possible that the so-called indigo disease is the 

 consequence of water-logging and a want of cultivation in a wet season, and 

 that the best way of dealing with the situation is by improved drainage and 

 by a more thorough aeration of the soil. I believe the damage done to land in 

 Bihar by water-logging during the monsoon is not even dimly realized. Land 

 can be harmed by water-logging when water does not lie on the surface for long 

 periods and when water-logging would not even be suspected." 



Plants may indicate good or bad aeration. The former are naturally of 

 little importance as aeration indicators, since their impress is due to some 

 other factor or factor-complex. Aeration indicators proper are correlated 

 with a deficiency of soil-oxygen, and are naturally confined to wet soils and 

 water, owing to the inverse relation existing between the amount of water and 

 of oxygen. They may be conveniently arranged in four groups, based upon 

 the kind of response to deficient aeration. In the first two, the species have 

 developed adaptations which enable them to live so successfully in swamps 

 and bogs that the habit is now obligate for the majority of them. The species 

 of swamps regularly possess a special aerating system of air-passages and 

 diaphragms, often supplemented by superficial roots and a marked movement 

 of the transpiration stream. Such indicators are found typically in Equi- 

 setum, J uncus, Heleocharis, Scirpus, Alisma, Sagittaria, Sparganium, etc. 

 Air-passages also occur in some bog-plants, but they are little or not at all 

 developed in the shrubby species, such as Vaccinium, Ledum, Andromeda, 

 Kalmia, Empetrum, etc. In most of these, the aeration devices are subordi- 

 nate to those designed to conserve the water-supply during drought, especially 

 in winter (Gates, 1914). Coville (1911, 1913) has emphasized the importance 

 of good aeration for the successful culture of the blueberry, pointing out that 

 this is secured in nature by the superficial roots as well as by their position in 

 hummocks. It is probable also that mycorhiza plays an important r61e, partly 

 in increasing the available nitrogen, and partly also perhaps in directly com- 

 pensating for the deficit in oxygen. 



The other two groups of aeration indicators consist of plants which grow 

 normally in well-aerated soil. Hence they lack special adaptations for aera- 

 tion, and consequently serve to indicate a lack of oxygen by their growth or 

 distribution. Those which are somewhat tolerant of water-logged and poorly 

 aerated soils respond to reduced oxygen content by decreased growth and 

 reproduction. Intolerant species drop out, and their reduced number or 

 absence serves as an indicator of conditions. Field studies of aeration or 

 acidity have been few in the region concerned here. The most important is 

 that of Sampson (1912 : 51) in the Wallowa Mountains of northeastern Oregon. 



