ROOT GROWTH AND DEVELOPMENT 
223 
are profoundly influenced by the addition of water. That there is 
active movement of air in air-dry soil, and that the movement is 
strikingly affected by moisture can be easily demonstrated. For ex- 
ample, a tube 30 cm. long, filled closely with air-dry adobe, requires, 
under the conditions of the test, only 0.5 cm. water pressure to cause 
an active air current to pass through. On the other hand, it is 
extremely difficult to add so little water that the air flow is not quite 
stopped. And, of course, if sufficient water is added to puddle the soil, 
it is impossible to force air through. It happens, therefore, with the 
rains of summer and the consequent thorough wetting of the soil, that 
the movements of the soil atmosphere are greatly modified, and the 
ingress and egress of air cut down to a marked degree, or entirely 
stopped. 
It is apparent, from what has already been given, that roots which 
lie close to the surface of the soil are subject to the influence of an 
environment which is quite different from that affecting the deeply 
placed roots. From the standpoint of experimentation it is best to 
assume, also, that it is owing to unlike responses to environmental 
conditions, that the characteristic differences in the mature root- 
systems is largely, if not wholly, due. Thus, so far as the Fouquieria- 
Opuntia species treated in the present paper are concerned, it appears 
that low soil temperature is a factor which limits downward root 
growth, while, on the other hand, this factor affects the root growth 
of Prosopis very little. From observations on the behavior of the 
roots of Opuntia versicolor, growing in tubes, it appears, also that root 
branching in this species is very closely related to good soil aeration. 
Whatever may be the factors that control root branching in Prosopis, 
on the contrary, it is probable that abundant aeration is not an im- 
portant one. Thus, root growth and root branching in Fouquieria- 
Opuntia take place mainly near the surface of the ground where the 
aeration-temperature conditions are favorable, and the root-systems 
of these forms are superficial. As the lower temperature of the deeper 
soil does not inhibit root growth in Prosopis, and as the roots are not 
so restricted in the aeration relation, deep penetration results. And, 
thus, through unlike physiological response there results strikingly 
dissimilar root growth and development. 
As regards the relation of root reaction and root type to species 
distribution, however, it must be admitted that it is a subject upon 
which up to the present very little work has been done. Enough has 
