Sept. 1, 1925 
Thermostasy of Growth of the Date Palm 
429 
and are discussed by Von Martius {15). The whole subject was 
reviewed at length by John Casper Branner (2) in 1883, and the 
results of his own researches in South America given, in comparison 
with these early theories. Three points in his conclusions have a 
distinct bearing on the subject of this paper: 
(1) This author states (. 2 , p. 467 ): 
I would suggest that this development of the fibro-vascular bundles, always 
in the direction of the apex is due to the light. This apex, at its central point, 
is very pulpy and translucent while its sides are enveloped in the young and 
growing fronds which render the parts surrounding the center more or less 
opaque. 
Fig. 4—Diagrammatic representation of the fibro-vascular bundles 
in the phyllophore of a date palm. Solid lines showing areas in 
which the bundles have completed their elongation; broken lines, 
meristematic areas in which the bundles are in process of forma¬ 
tion and elongation with branches sent out to the new leaves 
Reference should here be made to Plate 5, which shows the reverse 
half of the palm top cut from that portrayed in Plate 4. 
This shows how impossible it would have been for any light to 
penetrate to the center of the cell division which would have been 
most active in the area between 4-7 and 1 -g of the photo scale. 
The area 2-4 + d-h was, at the time of cutting, ivory white and so 
brittle that it cracked, as is shown, with the most careful handling. 
It had been exposed to the air about an hour when photographed and 
