390 
W. B. MCDOUGALL 
the resting period of the roots studied are not fixed and hereditary, 
since, in 1914, although most of the roots under observation had a 
summer rest, some of the hickory roots did not have; and in 191 5 
there was no summer rest period in any of the roots studied, unless 
it occurred after September i , which would be most unlikely. There- 
fore an external cause of the rest period, when it does occur, must be 
looked for. The two most important factors in the physical environ- 
ment, that vary with the seasons, are temperature and moisture. A 
little study of the results given shows that the lowering of either the 
temperature or the moisture content of the soil retards or stops root 
growth. In 1914 there was very little rainfall from early spring until 
the end of August. The soil thus became progressively drier and 
reached a minimum of water content toward the end of August. The 
rate of root-growth also gradually decreased and ceased entirely in 
most cases some time in July, to begin again only after the heavy rains 
of August 28. In other words, the summer period of rest was only 
during the period of drought. In 1915 there was no period of drought 
and, naturally, no rest period. The hickory roots which did not have 
a rest period during the summer of 1914 were some of the most deeply 
located roots upon which observations were made, and, naturally, 
the soil was not so thoroughly dry at that depth as nearer the surface. 
It is probable that observations on still deeper roots would show all 
roots located where adequate moisture was available growing through- 
out the period of drought. Brown (10) found that in the aerial parts 
of the pine growth is retarded first in the upper portions of the tree 
and may continue for several weeks longer below. It is very probable 
that in the subterranean parts a similar difference between the upper 
and lower portions would be observed on the approach of a critical 
season. 
It seems reasonable to conclude, then, that the summer rest period, 
when it occurs, is due not to any inherent tendency toward perio- 
dicity but to a lowering of the water supply. As to the winter rest 
period, the results show a close relation to temperature. But tem- 
perature to a certain extent controls the water supply, since a lowering 
of the temperature renders absorption increasingly difficult and thus 
reduces the amount of physiological water. In this case, therefore, 
the rest period is due indirectly to temperature but more directly to a 
decrease in the available water supply. 
