26 Bergman. — Relation of Aeration to Growth and Activity of 
The Amount of Oxygen required for Growth . 
Deherain and Vesque (6, p. 340) by their experiments established the 
fact that if the roots were deprived of oxygen the plant itself soon perished. 
The amount of oxygen necessary for growth is very small according to 
Wieler, ( 26 ) but varies in different plants. He found the maximum growth 
to take place in Vida Faba with 5-6 per cent, of oxygen. For Helianthus 
3 per cent, was the optimum amount. With other plants a retardation took 
place between 14-16 per cent, according to the plant. Vochting ( 24 , p. 94) 
found that a reduction of pressure to 3 per cent, or below caused the pro- 
duction of hairs on the roots of potato tubers to cease. Wacker ( 25 , p. 85), 
in growing seedlings of Vicia Faba and Helianthus annuus under bell-jars 
in which the atmospheric pressure had been reduced to one-tenth, found 
a slight retardation in the root development as compared with plants under 
full atmospheric pressure. The difference was not great, however, and he 
could not state with certainty that the retardation was due to the reduction 
in the amount of oxygen. 
Recently Cannon and Free ( 4 , p. 178), in experimenting with various 
plants, have shown that they behave very differently in their response to a 
diminution of the oxygen supply to roots. Coleus they found to be injured 
with a small decrease of oxygen below that of normal atmosphere. Nerium , 
on the other hand, is quite resistant to oxygen deprivation and first showed 
injury after 26 days in an atmosphere of pure nitrogen. They also found 
that with Salix ‘entire deprivation of oxygen appeared to be without 
injurious effect 
The Oxygen Content of Various Substrata . 
The oxygen content of the water or other substratum in which the 
plants are growing is an important factor in determining not only the 
growth and activity of the roots but of the entire plant. Whether the 
amount present is sufficient for the plant's needs, and the manner of main- 
taining or replenishing the supply, are other points to be considered. 
Boussingault and Lewy ( 3 ) and von Fodor ( 7 ) found the oxygen content of 
soils to be somewhat lower than that of atmospheric air. Russell and 
Appleyard ( 18 ) also found the same to be true. In soils under usual condi- 
tions there are abundant spaces between soil particles through which air 
can diffuse, so that a supply of oxygen is available to the roots at all times. 
When the soil is saturated, or has water present in excess, the air is 
driven out of the interstices of the soil. The only oxygen then available is 
that in solution in the water. The greater part of a unit volume of soil is 
occupied by soil particles, leaving only a small volume which can be occu- 
pied by water. Water, moreover, contains a relatively small amount of 
oxygen. Accordingly it is evident that much less oxygen is available in 
