402 
Journal of Agricultural Research 
Vol. XXXI, No. 5 
THE ZERO POINT OF VEGETATIVE GROWTH 
The accurate determination of the zero point, or specific tem¬ 
perature below which all growth of the date palm would cease, is a 
laboratory problem involving the possibility of the nicest tempera¬ 
ture control and the elimination of the effect of sudden temperature 
variations. With the most favorable conditions for experimenting 
provided, it would probably be found that the zero point is not 
constant for the species but varies with the varieties and with the 
individual plants, so that if a zero point for the species were assumed 
from results of the study of one variety, it might be but a matter of 
time until it would be demonstrated that another variety begins its 
growth at a slightly lower temperature. 
For the purpose of this paper, cessation in pushing up of the inner 
leaves from the bud of a date palm at the normal period of the 
day will be assumed to indicate a dormant condition of that tree. 
De Candolle evidently expected the date tree to enter a resting 
period somewhat independently of temperature conditions, for he 
writes regarding it as follows: 2 
Vegetation probably ceases to be active during the winter, but on a tree which 
retains its leaves this period is necessarily not well defined. ... Its vegeta¬ 
tion ought to be greatly retarded after fruiting. It is the ordinary physiological 
law, furthered by the lowering of the temperature. For how long does this 
resting last? At what average heat will the activity of the tree begin again in 
the spring? This is what is necessary to know, but positive observations along 
this line are lacking. 
The growth records at Indio, Calif., and at Tempe, Ariz., how¬ 
ever, show that growth ceases only (1) under destructive minimum 
temperatures of about 20° F. or lower, and (2) when zero point 
maximum temperatures are reached. With accurately marked 
active leaves under daily observation at the Indio date garden, it 
was determined that slow growth was made when minimum tem¬ 
peratures dropped below the freezing point, even down to 22° or 
21°, provided the day's maximum temperatures were well above 
50°. This proved that such low temperatures, if not prolonged, 
did not in themselves wholly check growth. The short duration 
usual to such minima prevents penetration to the growth center of 
the tree. If, now, with minimum temperatures above the freezing 
point, low maximum temperatures occur, followed by cessation of 
growth, this result may be logically attributed to the low maxima 
and a zero point indicated. 
ZERO POINT RECORDS AT INDIO/ CALIF, 
In the autumn of 1916 records of daily leaf growth were begun on 
four date palms in the United States date garden at Indio, Calif., 
the growth data in the accompanying tables being the average daily 
pushing up from the bud center of five active young leaves. These 
growth records could be coordinated with standard thermometer 
and thermograph records taken in a standard weather bureau shelter 
a short distance from the trees. In these records the writer was 
fortunate in obtaining, in December, 1916, and January, 1917, 
records of temperatures associated with a sharp decline and finally 
* Candolle, Alph. de. g£ographie botanique raisonnee. v. 1, p. 371. Paris. 1855. 
