460 
Journal of Agricultural Research 
Vol. XXXI, No. 5 
air, growth was proportionally prolonged after sunrise. In other 
words, growth in the form of leaf elongation is for the date palm 
somewhat inverse to the light intensity. It is most active in dark¬ 
ness, nearly or completely checked in bright sunlight, and partial in 
obstructed light. 
GROWTH REACTION TO ARTIFICIAL LIGHT AND DARKNESS 
In order to be able to determine more definitely the relation of date 
leaf growth to light and darkness it was deemed necessary to provide 
for the exclusion of light at will. In order that otherwise normal 
growth conditions might not be disturbed, a folding dark cell was 
constructed which could be placed around a medium-sized palm in 
the field in a few minutes time. Four panels of light pine framework, 
each 5 feet wide by 12 feet high, were covered with black enameled 
cloth (the black surface within) to prevent excessive heating in the 
sunshine. Three of these panels were hinged together and when 
erected around the tree the fourth side was slipped into rabbeted 
7~/AT£ A7 
T/A7*~ ,4 A* 
Fig. 2. —Daily and hourly growth of the leaf of Mactoum seedling date palm, plotted from auxanometer 
sheets, May 10 to 19,1918 
iambs provided to secure light-tight joints and held in place by 
hinged hasps. With the aid of high stepladders, the top, built like a 
box lid, was placed in position and also secured with hasps. Thus 
in a few minutes the date palm, with its spreading leaves drawn by 
cords into a more erect position (the only disturbance to its natural 
condition of growth) was inclosed in a completely dark cell or chamber 
5 by 5 by 12 feet in size (300 cubic feet capacity). At first the auxa¬ 
nometer and stand were retained inside the dark cell, but it was found 
more convenient for observation to carry the recording wire through 
the house wall to the stand with instrument outside. 
The first test was made on May 22. After a normal gain during 
the night of the 21st and 22d, growth had come to a complete stand¬ 
still at 8 a. m., as shown by the auxanometer tracing, Graph I, 
Figure 3. 
At 10.37 a. m., after 2 hours 37 minutes record of no growth, the 
dark cell was closed and remained in position till 4.55 p. m. As the 
actual auxanometer records are somewhat difficult to interpret, they 
have been transferred to graphs similar to the day and night records 
in Figure 2, but the time in hours is represented by one-half inch 
spaces, while the actual leaf growth is represented in the inch and 
