60 
INTERNAL TEMPERATURES OF 
will be produced by an adiabatic increase in volume of approximately 3 - 2 / 
of the total volume at 40° C. Since the air is not dry but contains an 
unknown quantity of water-vapour, the expansion will be very slightly greater 
than 3'2 °/. In the absence of any, even approximate, estimate of the volume 
of gas concerned and of the latex exuding, it is impossible to be certain 
whether a wound would make it possible for a 3-4 °/ increase in volume to 
occur. All that can be said is that the outflow of latex is so great that 
there seems to be no difficulty in regarding a wound 5 squire inches in extent 
(Exp. 4) as sufficient to cause the expansion necessary to reduce the tempera- 
ture by the amount observed. 
The effects of expansion in the neighbourhood of the wound must be 
almost instantaneous. This will not be the case farther away, partly because 
the impulse will be delayed by the diaphragms, and also because, as long as 
the latex flows, pressure in more distant parts of the branch from which it is 
being drawn will continue to fall. Therefore the expansion and consequent 
fall in temperature go on till the outflow of latex ceases ; its effects on the 
temperature of distant parts of the plant will be apparent therefore for some 
minutes ; at the wound itself they are quickly masked by the cooling effects 
of evaporation. Confirmation of this is probably seen in Exp. 6. Here the 
only wound inflicted was made by removing the top of the stem. The effects 
of evaporation are excluded and the exudation of latex was small and of very 
short duration. The change in internal pressure was therefore effected much 
more rapidly than in the other experiments. To be correlated with this are 
the facts that in this experiment the falls in temperature in the wounded 
and unwounded branches are completed in 2 minutes and 4 minutes 
respectively. 
If the hypothesis here adopted be correct, some interesting biological 
conclusions are indicated. Whatever may be the absolute internal pressure 
of the confined gases, it must vary by as much as ^ of an atmosphere from 
one part of the day to the other, and this during the vegetative season. The 
external form of the branch must be slightly different at the periods of low 
and high pressures and when the volume of the gas is large it is not impossible 
that this difference of form may be such as to exercise a controlling effect 
upon the rate of transpiration during the hotter part of the day. Oscillation 
between high and low intercellular pressures (amounting to as much as t ’q 
atmosphere) can hardly fail to influence the growth of the tissues, either in 
the rate or direction of cell-division or elongation. That this influence has 
not been such as to produce any marked effect upon the permanent form of 
the branch probably follows from the fact that other Euphorbias whose 
branches resemble these in outward form are without large air-containing 
intercellular spaces in the pith 1 . 
1 Worsdell, l.c. 
