Volume I 
NOVEMBER, 1914 
No. 2 
OBSERVATIONS ON THE INTERNAL TEMPERA- 
TURES OF EUPHORBIA VIROSA AND ALOE 
DICHOTOMA 1 
By H. H. W. PEARSON 
1. Introduction. 
The observations recorded below were made in the course of the Percy 
Sladen Memorial Expedition to the Great Karasberg in Great Namaqualand, 
in the summer of 1912—13. The plants chosen for the purpose were growing 
on the lower western slopes of the Great Karasberg, near the military station 
Wasserfall Alt, at an elevation of 4200 ft. These slopes receive little rain ; 
their perennial vegetation consists mainly of Euphorbia virosa and another 
species of similar habit, Aloe dichotovia and the Capparidaceous tree, Meter ua 
arenicolct. At the time of our visit very little rain had yet fallen and, 
except in the sandy river beds, there were no annuals. The locality is one in 
which the summer temperatures are high. Strong winds are frequent — at 
times of daily occurrence ; they come, however, from the hot desert plains 
below or from the Kalahari ; in either case their direct cooling effect is small. 
The situation is open to the west and north-west, but to the east is closed by 
mountains which rise to more than 6000 ft. But, although it is in ordinary 
language a “ hot ” place, it is undoubtedly cooler than many localities in the 
desert, 3000 ft. lower down. If any of the desert plants possess a structure so 
favourable to the development of a high temperature as that found in 
Euphorbia virosa, they probably attain higher maxima than those recorded 
here. And further, although the habit and structure of this plant are such 
as apparently to necessitate an exceedingly low rate of transpiration, never- 
theless its massive stems are freely exposed to the air. Other things being 
equal, one would expect a dwarf succulent wedged in a rock-crevice, or between 
stones, to be liable to attain a higher internal temperature. And finally, the 
following observations were made in December, that is, in early summer. The 
mean temperatures in February and March are higher than in December. 
1 Percy Sladen Memorial Expeditions in South-West Africa (assisted by grants from the 
Royal Society and the South African Government), Report No. 21. 
A. B. H. 
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