62 
INTERNAL TEMPERATURES OF 
7. The internal temperature of’ the Aloe stem continues to rise for some 
hours after the meteorological maxima are past. Its fall does not usually 
commence until about sunset. This, however, in the two cases investigated, 
is partly due to the situation of the two plants — on a steep hillside, facing 
west. 
8. The maxima and daily ranges recorded are tabulated in Table II. 
9. When the Euphorbia is wounded in the neighbourhood of a thermo- 
meter, by scraping the epidermis or by a series of shallow incisions, deep 
enough to cause a flow of latex, the internal temperature falls suddenly. 
10. The fall commences within less than 2 minutes of the infliction of 
the wound. The steepness of the curve in these 2 minutes seems to indicate 
that the wounding and the initial drop in temperature are practically simul- 
taneous. 
11. The extent of the fall in temperature does not appear to be related 
to the length of period during which it continues, but it is in some degree 
dependent upon the area of the surface wounded. This is shewn in Table I. 
12. From §§10 and 11 it may be concluded that the fall in temperature 
in the wounded stem is due to at least two distinct causes, viz. 
a. A first cause which acts within 2 minutes of wounding, probably 
almost instantaneously. 
b. A second cause which comes into action later and continues to act 
for a longer time. This, almost certainly, is the cooling effect of 
the evaporation of exuded latex at the surface. The evidence for 
this conclusion is mainly 
a. The fall in temperature ceases very shortly after the cessation of 
evaporation — i.e. after the formation of a rubber-pellicle over 
the wound. 
/3. The extent of the fall in temperature is to some extent related 
to the area wounded — i.e. to the amount of exuded latex. 
13. The unwounded branches also usually shew either a distinct fall or 
an arrest of the normal rise of internal temperature, when another branch of 
the same plant is wounded. This fall or arrest commences within a very 
short interval of the wounding. The amount of the fall of temperature in the 
unwounded branch is usually less than in the one that is wounded (see 
Table I). 
14. The available evidence indicates that the cause of the fall of tem- 
perature in the unwounded branch is identical with the first of the two causes 
of the fall at the wound, viz. the expansion of the pith gases due to the with- 
drawal of latex. 
15. A similar lowering of the internal temperature of the stem of Aloe 
dichotoma follows the exposure of the secondary wood by a surface wound. 
This appears to be due entirely to surface-evaporation. 
