32 AERATION AND AIR-CONTENT. 



not produced in roots by the one-sided access of such gases as oxy- 

 gen, hydrogen, or carbon dioxid, and their roots are therefore not 

 aerotropic. The evidence is regarded as being decidedly against a 

 belief in the aerotropism of roots. 



Sammet (1905 : 621), at the suggestion of Pfeffer, has investigated 

 the degree to which roots in water, roots in earth, and roots, shoots, 

 and fungal hyphse in saturated air respond to the unequal distribu- 

 tion of various substances. In saturated air the roots responded in 

 different ways. To oxygen they reacted only by positive curvature, 

 while to carbon dioxid, ether, etc., higher concentrations gave posi- 

 tive and lower, negative curvatures. A stream of air produced no 

 reaction in completely saturated air, but in air somewhat less than 

 saturated, hydrotropic curvature occurred. The roots in soil showed 

 combinations of chemotropism and hydrotropism, but when oxygen 

 was applied to the surface of dry soil, the roots curved upward to 

 the surface, showing that aerotropism was stronger than the influence 

 of hydrotropism. Intact and decapitated roots showed the same 

 behavior to chemotropic stimuli, but hydrotropic response was 

 suppressed in the latter. 



Bergmann (1920 : 16) found that, a week or so after the sub- 

 mergence of roots in water, new laterals appeared on the base of the 

 stem at the surface of the water. An examination showed that the 

 submerged roots had died. In the case of plants in soil wet from 

 below, the upper roots made the greatest growth; the lower ones 

 remained alive in Impatiens, but died in Pelargonium. Cannon and 

 Free (1920:62) have found that the roots of sunflower behave simi- 

 larly when placed in an atmosphere of nitrogen. 



Summary. In spite of the conclusions of Bennett and the con- 

 firmation given by the experiment of Schreiner and Reed (1903), 

 the evidence in support of the aerotropic curvature and growth of 

 roots is practically conclusive. While Bennett has succeeded in 

 throwing doubt upon Molisch's results, her own conclusions are too 

 sweeping, owing to the failure to reckon with the oxygen already in 

 the roots of the media used. Most of the experiments were carried 

 on for too brief a period, 10 to 30 hours as a rule, and no account was 

 taken of the inhibiting effect of the amount of the gases used. Mol- 

 isch, Goebel, Ewart, and Sammet have furnished the experimental 

 evidence of aerotropism or oxytropism, while Goebel, Jost, Schenck, 

 and Wilson have observed aerotropic effects in the plants of swamps 

 especially. 



In connection with the comprehensive study of bog and swamp 

 vegetation, preliminary experiments have been made upon the 

 behavior of roots in saturated soil. In three successive series of 

 pot experiments, seeds of sunflower and bean were planted in various 

 positions in glazed pots filled with sandy loam. In two of these an 



