446 
Journal of Agricultural Research 
Vol. XXXI, No. 5 
area of cell division. A fruiting tree has in addition the bases of 6 to 
even 15 fruit-bearing stalks which continue their elongation well into 
the summer; also, the nourishing of the rapidly growing bunches of 
fruit must be provided for. The work performed in this huge 
growth center is therefore very great and the temperature protection 
afforded by the ascending sap current during the intense heat of the 
desert afternoon is of the greatest importance. 
While attributing a most important influence to factor ( 8 ), the 
ascending sap current, it is evident that that can only be effective 
within the protecting and insulating influence of the protecting 
envelope and the fibrous outer tissues of the trunk proper. 
Without these, the sap current would be brought to the air tem¬ 
perature before it could reach the growth center. It might be com¬ 
pared to the attempt to convey hot water or steam from a heating 
poiler to a radiator m a distant building without the insulating cover¬ 
ings and inclosed conduits which heat engineers provide for such 
service. 
Unusual space is given to this topic because in the idea of the 
regulating value of the ascending sap current in determining the 
temperature of the phyllophore of the palm there appears to be a 
heretofore unrecognized principle in plant physiology, of first impor¬ 
tance in the date palm, the Washingtonia, and other desert endogenous 
plants, and not to be ignored in the economy of exogenous plants, 
both woody and herbaceous. 
DISCUSSION 
EQUALIZATION OF TEMPERATURE IN THE GROWTH CENTER BY TISSUE INSULATION 
The idea of the thermos bottle preserving the temperature of 
either cool or hot contents was anticipated by the date and other 
palms, though unrecognized all these years. In fact this idea clearly 
explains the wonderful adaptation of such palms to the extreme tem¬ 
perature ranges of their environments. The heavy layer of fiber 
surrounding the trunk of the date palm is only relatively less efficient 
than the arrangement of outer and inner flasks with a nonconducting 
vacuum zone between. The excessive heat of the midday air and the 
chill of the frosty night alike penetrate but slowly toward the tender 
embryonic cells in the bud. The strong volume of the sap current, 
absorbed by the roots at deep soil temperatures, ascending through 
the intricate vascular system of the trunk, is able to preserve most 
of its low temperature until it is dispersed in the leaves, because of 
the low conductivity of the protective envelope, the layers between 
it and the surrounding air. Thus the stabilization of temperature 
in the growth center of the date palm is established. 
It is not too much to affirm that but for this protective adaptation 
to the extreme heat conditions of its environment, the death point 
for the growing tissue of the date palm would be reached nearly 
every year in the Sahara, and frequently in its adopted home in 
southern California. This would hold equally true of the Washing¬ 
tonia, the one native palm of California, in which the writer has 
observed temperature differences between the growth center and the 
air similar to those of the date palm. The survival and majestic 
growth of the Canary Island palm, P. canariensiSj at the Furnace 
Qreek Hanch in the Death Valley of California, with almost the 
ivorld’s highest heat record, can be explained on no other hypothesis. 
