EUPHORBIA VIROSA AND ALOE DIOHOTOMA 
59 
the atmosphere. The infliction of' a wound therefore lowers the internal 
pressure. Since the ramifications of the latex-system extend into the tissue- 
diaphragms of the pith, the pressure of the gases confined by these diaphragms 
will at once be affected by the withdrawal of latex. If the pith region con- 
tained a continuous column of gas, this expansion would be equal and practically 
instantaneous throughout the plant. Since, however, any change in pressure 
at one point must be transmitted through diaphragms of unequal thickness 
and different degrees of elasticity, and since the amounts of latex withdrawn 
from different diaphragms will certainly be unequal, its rate of transmission 
will be lowered and the response in different reservoirs will not be the same. 
There remains the question whether the gas-expansion caused by such an 
outflow of latex as was obtained in the various experiments would be sufficient 
to cause the reduction of temperature observed in the unwounded branches. 
With regard to this point, my colleague Dr J. C. Beattie has kindly made 
the following calculations for a column of dry air. 
In the case of an adiabatic change of volume let v be the volume, T the 
absolute temperature, 7 the ratio of the two specific heats, then the relation 
between v and T is given by 
Tip-' = constant. 
To find the volume to which an initial volume v at temperature 40 J C. 
will have to expand adiabatically, in order that the temperature may fall 4 C., 
let xVhe the new volume, then 
(xV)y ~ 1 313 
vv - 1 ~ 309 ‘ 
Whence for 7 = 1'4 (the value for air) we find x = T032, the volume must 
therefore increase approximately 3‘2 °/ ; in order to produce a fall of 1 viz. 
from 40° C. to 39° C. the increase must be slightly over 1 /. 
The equation connecting pressure p and temperature 1 for an adiabatic 
change is 
PL 2 
T y 
= constant, 
the new pressure xp for a fall of 4° C. from an original temperature of 40 C. 
is therefore given by 
(. xp)y - 1 _ 309? 
py~ l 313 y ’ 
whence x = 0’956 for 7 = 1'4, 
or the decrease in pressure will be approximately 4’4 / ; for a fall of a degiee 
from 40° to 39° C. the decrease in pressure will be approximately 1 1 /■ 
It appears then that a fall of temperature of 4 C. 1 of such a gas at 40 C . 
1 The greatest observed fall in an unwounded stem was 4 5C. (see Tal le I). 
5—2 
