1914] VINSON—DATE PALMS 325 
and the daily soil temperature one foot below the surface at 
7:00 A.M. are also plotted. 
The temperature factor, as influencing the rate of growth, has 
other components than those expressed by maximum and minimum 
alone, because duration of temperature is of the utmost impor- 
tance. The soil temperature is to a certain extent an index of all 
these, but in this case is modified by still other factors, such as 
€vaporation. Continuous thermographic records were lacking, 
so that it became necessary to construct somewhat arbitrary ones. 
In the dry air and under the clear skies of Arizona the thermo- 
graphic record is subject to relatively few variations from a normal 
form. In general the lowest point of the curve falls about sunrise, 
and the highest at 1:00 or 2:00 P.M. By the use of these points a 
fairly accurate thermographic record for this region can be drawn 
from the daily maximum and minimum temperatures. This alone, 
however, furnishes no usable data for the construction of a curve 
representing the total daily amount of heat received. If we 
assume some empirical temperature as that below which no marked 
growth takes place, and use this as a base line on the thermograph 
sheet, the areas lying above this line represent, at least relatively, 
the heat available for growth. 
The selection of such a base line is not an easy matter, and at 
best must be somewhat arbitrary. The data obtained, however, 
will be relative and consistent on any base line chosen. For the 
Present case 50° F. was selected, because the date palm does not 
seem to utilize temperatures below that, at least to any marked 
degree. Growth after the surface soil temperature reached 50° F. 
Was practically nothing. This is not to be construed as meaning 
that no growth would occur at a uniform temperature of 50° F., but 
under actual climatic conditions the minimum temperature which 
Would accompany a maximum of 50° F. would effectually inhibit 
rowth. It will be noticed in this connection that during the first 
Week of J anuary 1906 growth was entirely inhibited, while during 
the winter of 1907, when the daily maximum was always above 50 
F. and the minimum seldom below 30° F., growth never entirely 
ceased, 
The curve representing the weekly heat-time areas, when plotted 
