358 The Philippine Journal of Science ms 
and both had decreased in length 0.5 millimeter. During the 
next hour they were exposed to diffused light, and both increased 
in length 0.5 millimeter. At the end of this hour they were 
both straight. During the next two hours they were exposed to 
direct sunlight, became much wilted, and decreased in length 
0.5 millimeter. At 2 o’clock they were in partial shade and 
were slightly wilted ; one had increased in length 0.5 millimeter, 
while the other had decreased the same amount. From 2 o’clock 
until dark they were in the shade. By 3 o’clock they had become 
straight. By 5 o’clock they were curved upward and had re- 
gained their longest previous length, having exactly the same 
length as at 8 o’clock in the morning. During the following 
three hours both shoots were curved upward and apparently 
turgid; one increased in length 1 millimeter, while the other 
remained stationary. 
It will be noticed that these shoots, like the previous ones, 
made no growth on the second day between 8 o’clock in the 
morning and dusk, but during the elongation in the afternoon 
regained only the length they had had in the morning. The 
elongation which took place between 1.30 and 6 in the afternoon 
of the preceding day was accompanied by a recovery from wilt- 
ing and was probably also a return to a previous early morning 
length. 
The response of the shoots of the second plant to direct sun- 
light was very striking. During the first period of sun they 
decreased in length, while one hour of diffused light was sufficient 
to restore their original length. Later in the day, while again 
exposed to the sun, they again decreased in length. 
It is interesting to note in this connection that MacDougal 7 
has found that Opuntia discata might shorten during the night 
as the result of increased acidity and heightened transpiration, 
as is shown in the following quotation: 
A number of mature joints of this succulent were placed in bearing 
with precision auxographs of a type devised by the author in 1902, in 
which both expansions and contractions, magnified 20 to 50 times, are 
recorded. The changes of size of a mature joint, such as those indicated 
in March 1913, consist mainly of swelling by increased absorption of 
water made possible by disintegration of acids in the sap. This does not 
proceed at an even rate, but is most rapid in the first half of the day, 
being greatest from 11 a. m. until 2 p. m. During the remainder of the 
day this action would fall off and actual shortening might occur at night 
as a result of increased acidity and heightened transpiration. These 
reversible changes in form also take place in young joints, and accompany 
7 MacDougal, D. T., Carnegie Inst. Wash. Year Book 14 (1915) 71. 
