ON THE RELATION OF ROOT GROWTH AND DEVELOP- 
MENT TO THE TEMPERATURE AND AERATION 
OF THE SOIL 
W. A. Cannon 
There is great diversity in root development in desert plants. 
This is to be seen even in a single habitat where all of the species are 
apparently subject to similar environmental conditions. At the 
same time the same species, when growing in another habitat which 
differs as much as may be from the first one, may, however, still retain 
each its characteristic type of root-system. From these general facts 
it appears probable that there are environmental conditions, to a 
degree apart from the soil per se, which are held in common by other- 
wise unlike habitats, to which the developing roots react in character- 
istic manner and which thus may be of determinate importance in 
shaping the direction of root development. Among the most striking 
of such common factors, which fortunately lend themselves conven- 
iently to experimentation, are soil aeration and soil temperature. 
Accordingly, in the preliminary experiments looking to a solution 
of the general problem relating to differences in root development, 
the reaction of roots to aeration and to temperature were taken up. 
The reaction of roots to soil aeration was studied as an introduction 
to this work. It is apparent that such root-systems as lie close to the 
surface of the soil, as, for example, that of Fouquieria splendens, and 
especially Opuntia versicolor and other cacti, must hold a very different 
relation to the atmospheric air than such root-systems as are deeply 
placed. And, in fact, the cultures showed that the roots of Opuntia 
had a very definite reaction to an abundant supply of air. The 
possible bearing of these results on the placing of the roots in the soil, 
or on their typical development, will be treated in the concluding 
section of this paper. In the meantime, it will be sufficient to state 
that the results of the direct aeration experiments were not entirely 
consistent, for whatever reason, and they will be repeated at a future 
time. The attention, therefore, was turned, for the time, to a study 
of the reaction of roots to various soil temperatures and to a consider- 
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