24 



BearLy all such oxygen supply will be by diffu- 

 sion. The compressions/land expansions which take place in 

 the internal air passages of aerial organs under bending 

 by the wind or other forces do not occur in roots. The 

 diurnal temperature changes of the deeper layers of soil 

 are both slow and slight. Changes of atmospheric pres- 

 sure have been shown by Buckingham 1 to have relatively 

 very little effect on the diffusion of oxygen and carbon 

 dioxide into and out of soils, and the effect would be 

 still less important in the long and narrow air passages 

 of roots. It is probable that changes of turgor may 

 enlarge or restrict slightly the internal air spaces and 

 these changes may be frequent and considerable enough 

 to have some effect. It is scarcely probable that they 

 can approach in magnitude the effects of diffusion. 

 Growth movements are similarly insignificant. Continuous 

 gaseous currents such as those which have been observed 

 in Helumbrum and certain other plants^ are impossible 



1. U. S. Bureau of Soils, Bulletin 25_, pp. 26- 

 33, 41-45, 47-50 (1904). 



2. Raffenau-Delile,- Ann. sci. nat. (2) 16 : 

 328 (1841); Lechartier ,- Ann. sci. nat. (5) 8: 364 

 (1867); Ivlerget,- C. R. 77: 146y (1873), 78: 884 (1874); 

 Bathelemy,- Ann. sci. nat. (5) iy : 152 (1874). Palladin 

 suggests a physical explanation based on the phenomena of 

 differential gas diffusion through heated porous plugs, - 

 Pflanzenphysiologie, p. 123 (lyll). 



