410 
Journal of Agricultural Research 
Vol. XXXI, No. 5 
Vinson, in analysis of weekly growth measurements made for 
Forbes at the Cooperative Date Garden at Tempe, Ariz., in 1906 
and 1907, assumed an “empirical temperature” of 50° F., but on an 
incorrect hypothesis, for he writes: 
For the present case 50° F. was selected (for the “ empirical temperature, 
below which no marked growth takes place”). 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 accompanies 
a maximum of 50° F. would effectively inhibit growth. 
This last conclusion is proved erroneous nearly every winter, for 
maximum temperatures of 46° to 50° F. with minima above 32° are 
not rare in the date-palm regions. (See Table II, records of January 
16, 17, 18, 19, in comparison with records for December 7 to 12 in 
Tabie I.) 
During the period covered by Vinson’s article the observed palms 
reached zero growth only during the first week of January, 1906. 
The official weather records for that time show: January 1, maximum 
53°, minimum 29°; January 2, maximum 51°, minimum 19°; Jan¬ 
uary 3, maximum 53°, minimum 18°; January 4, maximum 59°, 
minimum 20°. 
The three days with minima of 19°, 18°, and 20° would alone 
account for the cessation of growth, leaving the real zero point 
undetermined. 
ZERO POINT DETERMINATIONS UNDER LABORATORY CONDITIONS 
Through the courtesy of Frederick V. Coville, botanist of the 
Bureau of Plant Industry, space was secured in a cool chamber 
which he had constructed in one of the Department of Agriculture 
greenhouses in Washington, D. C., where, by means of a small 
refrigerating machine, the temperature could be controlled within a 
range of from 3° to 5° F. Three seedling palms of the Thoory variety 
in 8-inch pots were selected for this work, and after a test of their 
activity at greenhouse temperatures (about 70° to 80°), were placed 
one or two at a time in the cool chamber. This cool chamber occupies 
the position of a greenhouse bench and is about 18 inches deep, 3 
feet wide, and 20 feet long. A quantity of saturated sphagnum on 
its floor insured a rather even condition of high humidity in the air. 
The greenhouse roof above the bench is shaded during the greater 
part of the day, so that the light that reached the palms in the cool 
chamber was considerably reduced in intensity. The temperature 
of the chamber was recorded by a Richard thermograph, the accuracy 
of which was checked by a highly sensitive mercury thermometer 
placed near it. 
The governing by the machine thermostat was somewhat erratic 
and gave a wider temperature range than was desirable. During the 
main period of 32 days (fig. 4) there was an extreme range from 48° 
to 56° F. But the time bdow 50° was only 1.2 per cent of the whole 
period and the time above 55° was negligible. The mean of the 
period was, as closely as can be estimated, 53°. The second period, 
covering five days in which readjustments of the temperature were 
made, gave a range of from 46° to 54°, the last 48 hours showing a 
mean of about 49°. During the third and last period of 48 hours, 
after the thermostat had been again adjusted to a lower temperature, 
the range recorded was from 44.5° to 49.5°. The peaks of highest 
