180 



SCIENCE 



[N. S. Vol. XLV. No. 1156 



less in normal cases the soil atmospliere is 

 always slightly higher in carbon dioxide and 

 slightly lower in oxygen than is the general 

 atmosphere. Since the diffusion processes are 

 effected very markedly by the average size of 

 the interspaces between the soil particles, the 

 difference in composition between the soil 

 atmosphere and the general atmosphere will be 

 greatest in the soils of fine texture and least 

 in those of coarse texture. The similar effect 

 of the soil water is obvious, especially the 

 effect of the water existing as films about the 

 grains in diminishing the size of the spaces 

 available for gas diffusion. 



The ecological bearing of these facts is 

 manifest. Although deficiency in aeration has 

 frequently been suggested as an agricultural 

 difliculty, or as the reason why certain species 

 do not grow upon soils of heavy texture, it 

 does not appear that this suggestion has had 

 any exact experimental basis, nor does it seem 

 to have been appreciated that different species 

 may have great differences in the oxygen re- 

 quirement of their roots and widely variant 

 responses to differences in soil aeration, re- 

 sponses which appear to be quite as specific 

 and significant as the responses to tempera- 

 ture and to available water which form, the 

 present basis of ecological classification. The 

 importance of root-habits in ecology has long 

 been recognized on the basis of their char- 

 acteristic and specific reaction to the features 

 of the soil environment included under tem- 

 perature and water relations. Thus it has 

 been shown that the general distribution of 

 the cacti as a family is closely related to the 

 response of the roots to the temperature of the 

 soil. It seems probable that soil aeration 

 must be added as a factor of no less impor- 

 tance than temperature and water. Thus in 

 the matter of local distribution of cacti, it is 

 probable that the restriction of this family to 

 habitats which have a relatively well-drained 

 soil, in which the accumulation of carbon 

 dioxide during the season of most active root 

 growth is probably relatively slight, may be 

 owing in part to the inability of the roots of 

 these forms to grow well in an atmosphere 

 charged heavily with carbon dioxide. On the 



other hand, the distribution of Prosopis and 

 Nerium along the river bottoms, and of Salix 

 in swamps, indicates that the presence of a 

 relatively large amount of carbon dioxide in 

 the soil does not act as a limiting factor to 

 these species. Again, Howard^ notes that the 

 general distribution of the gram (chick-pea) 

 as a crop in India is closely associated with 

 the fact that the roots of this species require a 

 relatively large amount of air. It accordingly 

 occurs only, or mainly, where the soil and the 

 system of cultivation provide an amount of 

 air sufficient for its root growth. 



In many semi-arid regions the physiog- 

 raphy is such that there are relatively large 

 and shallow basins without outlet, the central 

 portions of which are flooded during rainy sea- 

 sons but are dry for most of the year. Usu- 

 ally the central flats or playas of such basins 

 have soils composed largely of fine silt or clay, 

 and which puddle easily. It is characteristic 

 of these playas that they are void of plant 

 life during all, or most, of the year, and that 

 no perennials are to be found in the lowest 

 places, even when no excess accumulation of 

 salts in the soil has occurred. It is here sug- 

 gested that the probable reason for the absence 

 of plant life on such playas is directly trace- 

 able to insufficient soil aeration at the time 

 when the soil is suitably moist and of a tem- 

 perature suitable for the growth of plants. 

 It is interesting, also, that around such playas 

 the plants frequently occur in well-marked 

 bands or zones. Where the zones are found 

 there is probably little difference in the avail- 

 able moisture or the soil temperature and it is 

 suggested that the zonal differentiation may be 

 a result of unlike response of the roots of the 

 plants comprising the zones to the atmosphere 

 of the soil. Wliether zonation is to be associ- 

 ated with the relation of roots to the soil 

 atmosphere in places outside the arid regions 

 remains to be seen, but this may well be the 

 case. 



"W. A. Cannon, 

 E. E. Free 



2 Howard and Howard, Bulletin 52, Agricultural 

 Eesearch Institute, Pusa, 1915. 



