50 
INTERNAL TEMPERATURES OF 
these much later in the day ; they are much less affected by sudden and brief 
changes in external conditions. 
4. Wounding Experiments. 
The experiments now to be described were carried out with a view to 
determining the effects of surface wounding upon the internal temperatures of 
the Euphorbia and the Aloe. In the Percy Sladen Memorial Expedition to 
the Orange River in 1910-11, it was found that such an effect is produced in 
the stem of Euphorbia Dregeana. It was, however, impossible at that time to 
continue the observations long enough to establish the results. As E. Dregeana 
was not met with in the Karasberg, the experiments were repeated on E. virosa. 
In this species the effects are far more marked than in E. Dregeana,. in 
consequence no doubt of the remarkable development of air-spaces in 
the pith. 
No. 1. December 13. 
Chart III. 
Plant. Euphorbia A . 
Thermometers. W, T, P in three separate branches. 
At 9.10 a.m. the superficial tissues were scraped off about 1 square inch on 
the leeward side of the stem in the furrow opposite to that in which the 
thermometer W was inserted ; the wounded area was so placed that its 
centre was as nearly as possible opposite to the bulb of the thermometer. 
Within 2 minutes the temperature fell 0°‘75C. ; it continued to fall for the 
succeeding 5 minutes; after remaining stationary for 3 minutes it then tell 
slightly, the total fall being 1°'5 C. 1 After a further 3 minutes’ rest, at 9.25, 
it began to rise again, more slowly than it fell, to the normal. 
With regard to this record it may be noted that the temperature ceases 
to fall as the exuded latex forms a pellicle on the surface. It may therefore 
be concluded that the fall of internal temperature is at least in part the direct 
effect of the evaporation of the fluids of the latex at the surface. But it 
cannot be wholly due to this cause for it is inconceivable that the effects of 
this evaporation could be felt in the pith across the intervening tissues of the 
epidermis, cortex, vascular cylinder and outer pith practically instantaneously ; 
nor is it to be explained by conduction through the glass of the thermometer 
since none of the evaporating latex is in contact with it. There is therefore 
another cause for the observed lowering of the internal temperature and 
one which acts more quickly than surface-evaporation. It is suggested that 
this cause is the expansion of the imprisoned gases of the pith consequent 
upon the exudation of latex. A confirmation of this hypothesis is found in 
1 See Table I. 
