ROOT GROWTH AND DEVELOPMENT 
2ig 
As regards the general results of growing young plants, and seedlings 
of Fouqiiieria splendens, and of Opuntia versicolor in tubes exposed to 
air temperatures, it can be stated, in brief, that root penetration of i 
m., or over, was obtained in each species in a single season. The air 
temperatures to which the tubes were exposed, and to which they fairly 
closely conformed, ran as high as 35.0° C. 
In the box culture the roots of the same species as above used, 
attained, within a period of 10 weeks, a depth of 47 cm. The tem- 
perature of the vertical soil column was fairly uniform and varied 
from 10.0° C. to about 45.0° C. The long root growth was obtained 
where the temperature was 30.o°-35.o° C. 
At the time the box culture, just referred to, was running, seedlings 
of Fouquieria and of Prosopis were grown in the garden (of the Coastal 
Laboratory) where the surface soil varied in temperature from 10.0° C.^ 
to 22.0° C. Ten weeks after the seeds germinated it was learned that 
the roots of Fouquieria had attained a depth of 15-20 cm., and that 
the roots of Prosopis, on the other hand, were 32 cm., or over, in length.. 
Thus the tube, the box, and the garden cultures, to give no more,, 
indicated that relatively deep penetration of the soil might be expected 
in the cases of Fouquieria and Opuntia if the soil is of a suitable tem- 
perature, but if this is not the case, the roots of these plants remain 
near the surface. On the other hand, the roots of Prosopis, in a 
relatively short time and in relatively cold soil, attained to a consider- 
able depth, which, in fact, was approximately twice that of the roots 
of Fouquieria splendens of the same age and growing under like con- 
ditions. 
In the other tube cultures referred to, the rate of root growth was 
observed day by day, or hour by hour, as the case might be, and 
temperature readings, either of the soil of the tubes, or of the chamber 
where the tubes were placed, were made at the same time. In the 
first of the more exact tube cultures the period of observation extended 
over a period exceeding 10 weeks, during which time a thermographic 
record was kept of the culture chamber, and, also, the soil temperature 
of the tubes was taken. The leading results of this series is shown 
graphically in figure 4. It will be seen, in brief, that the growth rate 
of the roots varies directly with the variation of the air temperature 
to which the tubes were exposed. Beyond this, however, it appears 
that, at parallel temperatures, the roots of Prosopis grew more rapidly 
than did those of the two other species. It is to be seen especially that 
