EUPHORBIA VIROSA AND ALOE DIGHOTOMA 
49 
From this point the tall of all the internal temperature curves is very rapid. 
At 5.30 the temperature' of W is 19° C. lower than it was at the same hour 
on the previous day. It is to be noted that at 5.30 P and T are as much as 
4 0- 9 C. and W 3° C. below the shade temperature. These low records are no 
doubt due partly to the fact that the rain ceased soon after 4 o’clock, while the 
wind continued and raised the rate of evaporation from the surface. So great 
a fall below the shade temperature was not again observed ; but the course of 
the curves through the night was not determined. There is, however, little 
doubt that the minimum internal temperature is usually attained within the 
hour which immediately precedes sunrise. 
The marked feature of these records is the rapid response shewn by the 
internal temperatures of the Euphorbia to changes in the Black-Bulb and 
shade readings. The passing of a light cloud over the sun has an immediate 
influence upon the temperature of the air enclosed in the pith cavities and 
therefore upon that of the cells of the tissues surrounding them. The 
stem of the Aloe, which, compared with that of the Euphorbia, contains pro- 
portionately more water and less air, is, as one would expect, much less 
rapidly influenced by external temperature-changes. This is seen in the 
observations of December 18. 
This was an exceptionally cool day. Rain had fallen during the night. 
At 7 A.M. the sky was still clouded, but the clouds soon afterwards dispersed. 
By 11.30 clouds had again appeared, and it was raining at 12.45. I’he sun 
was not again unclouded until 2 o’clock. At 3.10 the sky again became 
overcast, and during the rest of the afternoon the sun made only occasional 
reappearances. At 6.38 the sun had set to all the plants under observation. 
The curve Iv (Euphorbia B) shews a close agreement with the BB and 
shade curves until 6 P.M., when it falls below the shade temperature and 
presumably remains below it until sunrise on the following morning. 
From 7 until 11.30 a.m. the P curve (Euphorbia C) follows Iv fairly closely, 
remaining consistently below it. Towards midday it crosses it and tor the 
rest of the day is far less dependent than is Iv upon the oscillations of BB. 
This is no doubt sufficiently explained by its situation on a steep hillside’ 
facing west. The external conditions affecting P are those which influence P 
(Aloe A) and W (Aloe B). These two curves are almost parallel after 7.30 A.M. 
They rise and fall more slowly than P \ they attain much lower maxima and 
December 18. 
Chart II (8.30 A.M. to 6.30 P.M.). 
Euphorbia B. Thermometer Iv. 
Aloe A. 
„ B. 
C. 
P. 
T. 
W. 
