62 CARNEGIE INSTITUTION OF WASHINGTON. 



absence of a medium such as the soil, which greatly hinders gas diffusion 

 and the convectional stirring of the surrounding atmosphere. The 

 hypothesis which suggests itself is that the diffusional relations are 

 effective in the film of absorbed gas which doubtless blankets the 

 surface of the living material. This hjrpothesis has not yet been 

 confirmed by direct experimental evidence. 



Root Adaptation to Deficient Soil Aeration, by W. A. Cannon atid E. E. Free. 



When a plant of the ordinary sunflower is grown in soil which is 

 normally aerated, and is then placed under conditions in which the 

 normal soil air is replaced by nitrogen, the roots previously developed 

 die. New roots immediately start from the lower end of the stem and 

 replace the previous root-system. If the plant is exposed to conditions 

 involving only low transpiration and no serious strain on the organism, it 

 will survive this change of root-systems and continue to live, apparently 

 healthily, under anaerobic conditions in the soil. The anaerobic roots 

 formed when the oxygen of the soil atmosphere is removed differ from 

 the normal roots in being shorter, thicker, and less branching, and they 

 are almost or completely lacking in root hairs. 



A similar sequence of reactions is observed when sunflower plants 

 are growTi in aerated soil and then transferred to culture solution. All 

 or nearly all of the roots die, but a new set of roots is formed im- 

 mediately, and this new set has morphological characteristics similar 

 to those of the roots produced in anaerobic soil. This is in line with the 

 well-known tendency of the roots of many plants growTi in water- 

 culture to be short and thick and deficient in root hairs. It is also in 

 line with the deficiency of root hairs on the roots of many swamp 

 plants, and suggests that lack of root hairs may be a result or a symp- 

 tom of deficient soil aeration rather than a response to a surplus of 

 water, as has been assumed. 



^^Tien plants of corn (Zea mays) are grown in soil with sufficient soil 

 aeration, and the soil oxygen then removed, the roots do not die, but 

 both the roots and the shoot cease growth for a period of several days; 

 growth of both roots and shoot is then resumed slowly, and continues, 

 though at a lesser rate than is the case when the soil is well aerated. 

 This suggests that some kind of physiological readjustment of the roots 

 to anaerobic conditions occurs when the soil oxygen is removed and is 

 necessary to continued anaerobic existence of the roots. If this be 

 true, the adaptation in the case of the corn root differs from that in the 

 case of the sunflower root mainly in that the physiological changes are 

 not accompanied by perceptible morphological modifications. 



