450 BOTANICAL GAZETTE [DECEMBER 
It is needless to enumerate the weather conditions which affect the 
rate of evaporation, but it is worth while to note at least one chief 
factor in the control of root absorption. It is well known that root 
absorption is accelerated by moderate increase of ground temperature 
above the freezing point. It has been shown by MacDoucats that 
ground temperatures in the vicinity of New York City at a depth of 
30%" are maximum about 8:00 to 11:00 at night, and minimum 
about 8:00 to 10:00 in the morning. At greater depths the maximum 
and minimum would occur later, but the temperature variations would 
be less marked. Therefore the maximum temperature of the roots 
of birches—which lie mostly within less than 60°™ of the surface of 
the ground—must occur during the night, and the minimum tempera- 
ture during the afternoon. Thus, root absorption and the pressure 
produced by it tend to increase at night. In Ciark’s (I. c.) experi- 
ments on roots severed from the tree, the rule was for root pressure 
to increase during the night and decrease during the day, for the 
whole period in which pressure was strong. ; 
Taking the combined effect at night of increasing root absorption 
and decreased evaporation, there is a decided tendency toward an 
increase of pressure in the stem during the night. As the pressure 
increases the rate of infiltration also increases, tending thus to dimin- 
ish the rate of increase of pressure (fig. 3). After sunrise evaporation 
begins to oppose the rise of pressure, so that about noon prey 
begins to decline. The decreasing activity of the roots at this time 
aids the decline. What pressure might be developed in a birch stem 
by the prolonged action of root pressure, if the modifying influences 
of evaporation and infiltration could be eliminated, is shown by 
C1ark’s record of 193°™ pressure in a birch root severed from the 
stem. This pressure is more than double the pressures usually 
observed in the trunk. - 
Assuming that root pressure is essentially osmotic, the concentra- 
tion of the sap in the root CLARK observed must have been about two 
and a half times that of the sap at the bases of trees I have observed. 
At different times during the sap season, I have evaporated sap ai 
birches and found it to contain o. 5 to 1 per cent. of solids, largely 
s ther 
4MacDoveat, D. T., Soil temperatures and vegetation. Monthly Wea 
Review 31:no. 8. 1903. 
