sept. i, 1925 InJiibitive Effect of Sunlight on Growth of Date Palm 
457 
Frequently we meet with another type of leaf expansion where the apex at 
once passes into a state of rest. This is the case in many lianes where specially 
formed apices, fulfilling particular functions, are produced long before the rest 
of the lamina is completed. The elongation is basipetal also in the long leaves 
of monocotyledons, owing to the development of an intercalary growing zone 
at their bases. 
Attention has been called (7) to the fact that all of this basipetal 
growth was so deeply seated in the bud as to be in complete dark¬ 
ness; quite the reverse of the mode of terminal growth of endogenous 
shoots like the bamboo or in the twigs of exogenous shoots like the 
apple or the pine, where cell division must go on within translucent 
tissue, or in very small twigs in nearly full sunlight. 
PERIODICITY OF LEAF ELONGATION 
Several trees of different ages were selected for study of their 
leaf growth, five of the newest central leaves being designated by 
letters, and the trees by their place numbers in the blocks. 
The method of recording the advance in growth was by driving 
stakes, or for taller trees by erecting little towers. Upon these 
were placed carefully leveled crosspieces, over which a steel square 
enabled the record marks to be made on the ribs of the leaves with 
a thin-bladed knife. A day's growth of as little as 1 mm. could 
be accurately recorded. At first observations were made morning 
and evening, but the fact was soon disclosed that with occasional 
exceptions the expansion or pushing up from below took place almost 
wholly at night. 
As it was not feasible at first to keep these records in close coor¬ 
dination with sunrise and sunset, the observations being made about 
8 a. m. and 4 p. m., only the general facts of the relation of growth 
to fight and darkness were brought out. Two facts, however, soon 
gained prominence, in addition to the main one (that the chief 
growth is made in darkness). The first was that intermittently 
some growth was made during daylight hours. Whether this was 
a holding over or prolongation of the night growth, or was due to 
conditions occurring during the day, could be shown only by securing 
continuous or auxanometer records. The second fact noted was 
that certain leaves, and on some days several of them, were actually 
shorter at night than when the morning reading was made. This 
interesting phenomenon, which has been observed in the growth 
history of a variety of plants and is attributed to decreased turges- 
cence of the cells, will be made the subject of further study. 
AUXANOMETER DATA 
In April, 1918, an auxanometer was installed by making use of an 
anemometer clock having a horizontal recording cylinder revolving 
once in six hours. The spirally threaded spindle for carrying the 
record cylinder laterally to receive the wind velocity record was 
discarded, leaving the cylinder its rotary motion in a fixed position 
on its axis, and a pen-carriage was constructed to move from left to 
right at the pull of the wire actuated by the leaf growth. This clock 
in its case, was placed on a rigid stand by the side of the tree to be 
observed. 
Since the midrib of the date leaf was quite rigid and pushed 
upward by a positive expansion below in the heart of the bud, an 
