296 The Philippine Journal of Science 1913 
to 6 o’clock, and it was found that during the night all zones grow 
which have grown during the preceding twelve hours of day ; but 
that during each day the extreme back part of the region which 
has elongated during the preceding night ceases to increase in 
length. Obviously no zones curve when they do not grow, and 
since the last growth of each zone takes place during the night, 
when it is brought into the day position, it follows that this posi- 
tion is the final one assumed, and that therefore all parts of each 
twig come permanently into the day position. It may be re- 
marked that the growth is in general somewhat more rapid at 
night than it is during the day. However, this would not in 
itself result in final assumption of the day position, if it were not 
that the growing region is shorter during the day. 
An attempt was made to determine the causes of the curvature. 
From the regular succession of day and night positions, it is 
obvious that those conditions subject to diurnal change are in 
one sense the responsible factors. These are primarily the light, 
and in a measure, perhaps, the temperature. Neither of these 
can possibly be a directive stimulus. Light, or light and temper- 
ature combined, act merely as conditions which determine the 
response to other conditions which are directive. A priori the 
curvatures might be assumed under the influence of gravity, or 
of internal directive factors, tendencies known as hyponasty, 
epinasty and rectipetality, or these internal factors and gravity 
might be jointly responsible. As it was impracticable in dealing 
with this kind of a subject to eliminate the action of gravity, 
recourse was had to the scheme of reversing the position of a 
branch. 
Two of the branches already experimented with were long 
enough so that it was practicable to bend them near the bottom 
and bring the actively growing twigs through an angle of 
180° so that what had been the upper side of each twig became 
the lower. This was done at 7.30 in the morning, while the 
twigs were in what is called the day position. Since the twigs 
are not exactly the same shape, and do not stand at the same 
angle toward the horizontal, they were unequally exposed to the 
action of gravity after the position of the branch was reversed. 
Some of them were so placed that epinasty and geotropism, as 
tendencies possibly responsible for the day movement, were 
brought into almost exact conflict. Other twigs fall into such a 
position that the two possible conflicting forces acted obliquely to 
one another. The result during the day was the development of 
a diverse and decidedly complicated lot of curves. By evening. 
