44 THE BIOLOGY OF FLOWERING PLANTS 



The effect of placing an ordinary root system in water- 

 logged soil depends on the type of plant used. Plants 

 normally growing in ordinary soil usually suffer badly. 

 Balls (19 1 2), on excavating root systems of the cotton plant 

 in Egypt, found that all roots which had been submerged by 

 irrigation water for ten days were dead, though this region 

 of the soil might be again exploited by new roots produced 

 after the fall of the water-level. Bergman (1920) found that 

 the roots of Impatiens balsamina and of Pelargonium when 

 submerged died off, and that the leaves wilted, went yellow, 

 and dropped off in a few days. After ten days, if the plant 

 survived, new roots developed at the water-level and recovery 

 took place. Ranunculus sceleratus and Cyperus alter ni- 

 folius grew in saturated soil with no ill effects, and the roots 

 grew throughout the submerged soil, these roots being 

 provided with aerating tissue. The injurious effects were 

 also produced in Impatiens and Phaseolus in a soil permeated 

 with carbon dioxide instead of air. Noyes, Trost and 

 Yoder (19 18) found a retardation of growth of roots of 

 radish and lupin grown in soil permeated with carbon 

 dioxide. The fact that carbon dioxide has a narcotic 

 effect must be taken into account here, but similar 

 results have been obtained using the neutral gas nitrogen 

 by Cannon (1920), and Cannon and Free (1920) using 

 helium. The growth of most roots ceased when only 

 1-2 per cent, oxygen was present ; in pure nitrogen the roots 

 of the sunflower die and are replaced by new adventitious 

 roots, short, thick, little branched and with no hairs. In the 

 same conditions the roots of the maize do not die ; their 

 growth rate slows down and then increases again ; a 

 physiological adjustment has taken place. 



The growth of land plants in water culture is a remark- 

 able instance of accommodation. Such plants as the oat, 

 barley and buckwheat may be brought to maturity with 

 their root systems in the highly abnormal medium of a weak 

 solution of salts. To ensure successful development, how- 

 ever, it is necessary to aerate the culture solution frequently 

 by blowing air through it. The effect of aeration comes 



