28 SCIENCE PROGRESS 



tage is gained by the immersion of the whole root in water. 

 The root hairs are relegated to a position of secondary impor- 

 tance. In view of this root-tip activity it is perhaps not so 

 surprising to discover that Knudson (Amer. J. Botany, 1919, 

 6, 309-310) has been able to demonstrate that root caps sloughed 

 off by plants in water culture may retain their vitality, in so 

 far that their cells are plasmolysable, for at least seventy-one 

 days. 



Experimental work on root physiology has been confined 

 almost exclusively to the treatment of root systems, either in 

 soil or in water culture, with gas mixtures containing different 

 concentrations of the atmospheric gases, especially oxygen and 

 carbon dioxide. In experiments on the aeration of soils, it 

 should be borne in mind that the chief difficulty lies in separating 

 the two factors, moisture content and air content. The air 

 content of a soil naturally varies inversely as its moisture 

 content, and under field conditions it is almost impossible to 

 differentiate between effects due to more or less aeration 

 and those following upon a less or greater water content. 

 Another consideration which has been insufficiently emphasised 

 in aeration work is the distinction between lack of oxygen and 

 excess of carbon dioxide in soil atmosphere. Cannon and Free 

 (Science, n.s., 191 7, 45, 178-180) and Cannon (Carnegie Inst. 

 Wash. Year Book, 191 8, 17, 80-81) have attacked the problem 

 from both points of view, but it is more usual to find that 

 investigators consider the question as one merely of " aeration," 

 no further analysis being attempted. Cannon and Free in 

 laboratory experiments found a progressive retardation of 

 root growth with increasing carbon-dioxide content of the air 

 surrounding the roots. The concentration of COg necessary 

 completely to inhibit growth was not the same in all species, 

 roots of a species of Opuntia being unable to grow in 50-75 

 per cent. CO2, whilst those of a Prosopis were not completely 

 stopped by these concentrations. In general artificial aeration 

 was found to favour root growth in soil. Cannon also approached 

 the question from the viewpoint of oxygen concentration. 

 Roots of the same two species were subjected to the influence 

 of artificial atmospheres, wherein oxygen was replaced to a 

 varying extent by nitrogen. The results were supplementary 

 to those of Cannon and Free, Prosopis being less sensitive to 

 low oxygen content than Opuntia. The ecological significance 

 of this specific reaction is obvious, and adequately explains the 

 absence of Opuntia from fine soils which are badly aerated. 

 The experiments also demonstrate the necessity for analysis 

 of the results of aeration trials. 



Cannon (Carnegie Inst. Wash. Year Book, 191 8, 17, 83-85) 

 has succeeded by experimental means in inducing shallow- 



