THE INHIBITIVE EFFECT OF DIRECT SUNLIGHT ON THE 
GROWTH OF THE DATE PALM 1 
By Silas C. Mason 
Horticulturist , Office of Crop Physiology and Breeding Investigations, Bureau of 
Plant Industry , United States Department of Agriculture 
INTRODUCTION 
A study of the habits of the growth of the date palm reveals an 
adaptation to the conditions of desert climates to which attention has 
only recently been directed (-5, p. 8, 4). 2 
The lower minimum temperature point for growth has been treated 
(6), leaving still much work to be done on the optimum and maximum 
cardinal points for growth. The protective temperature regulation of 
the growth center and its influence on other temperature reactions, 
making growth possible the whole year, has been shown (7). The 
reaction of the date palm and a number of other palm genera to light 
conditions are considered in the present paper. 
The date palm, along with many other palms, has a diurnal period 
of leaf elongation, in darkness or in the absence of direct sunlight, 
alternating with a complete cessation of growth in bright sunlight. 
This diurnal alternation of growth and rest, occurring inversely to 
the exposure to direct sunlight, is discussed in the following pages. 
The literature on the growth habits of the date palm is very meager 
and the most of it is lacking in definiteness. Branner {1, p. 481) 
states: 
In conclusion I find: (1) That all fronds and spadices originate at the center of 
the phylophore; (2) that the fibrovascular bundle division continues to grow 
until its frond reaches maturity; (3) that the growth of a palm trunk continues as 
long as the bundle divisions of the part are in active connection with living fronds, 
and no longer; and (4) that the growth of palms is therefore an internal growth, and 
the term < *endogen” is not a misnomer as far as palms are concerned. 
The inference that the entire growth of the leaf is made within the 
“ phylophore” (correctly phvllophore) may be drawn from this, but 
is not positively stated. 
A study of date-palm growth involves the questions: (1) In what 
regions of the plant is the growth made? (2) What relation does 
f rowth bear to temperature? and (3) What relation does growth 
ear to light and darkness? 
The experiment p 1 approach to these questions calls for methods 
adapted to the particular structure of the date palm which is distinct 
from exogenous plants generally and also from many other genera 
of palms. 
TERMINAL LEAF GROUPS 
The leaves of the date palm when full grown are distributed 
around the axis very accurately according to the system of phyllo- 
taxv of the genus; but in emergence from the bud they appear to 
follow growth impulses or waves in that from three to five leaves 
emerge together in close contact and with the perfectly formed 
1 Received for publication Sept. 26, 1924; issued October, 1925. 
8 Reference is made by number (italic) to “Literature cited,” p. 468. 
Journal of Agricultural Research, 
Washington, D. C. 
( 455 ) 
Vol. XXXI, No. 5 
Sept. 1, 1925 
Key No. G-478 
