EUPHORBIA VIROSA AND ALOE DIOHOTOMA 61 
The presence of gas at comparatively high pressure in the pith must 
increase the rigidity of the stem as a whole. This can hardly be without 
importance in a massive plant with little mechanical tissue, growing in 
unsheltered localities swept, with almost daily regularity, by winds which 
frequently assume the proportions of a gale ; and further, the rigidity increases 
as the temperature rises and will reach a maximum in the afternoon when 
the winds usually spring up. 
But it is improbable that so remarkable a structure, apparently of very 
exceptional occurrence in plants of this habit, has no other part to play in the 
economy of the plant. The composition of the gas is unknown ; it may be 
assumed to be air containing inconstant proportions of Oxygen and Carbon 
dioxide, and it is probably of importance in providing a reservoir with which 
free gaseous exchange can take place at times when the communication of the 
intercellular system with the external air is reduced or perhaps entirely 
cut off. 
6. Summary and Conclusions. 
1. The observations of internal temperatures were made on the stems of 
Euphorbia virosa and Aloe dichotoma, at 4200 ft. s.m., on the western flanks 
of the Great Karasberg Range in Great Namaqualand. 
2. In the Euphorbia the pith region occupies about two-thirds of the 
diameter of the fluted columnar stems ; and contains very numerous lysigenous 
“ air ’’-cavities of various sizes' (see Photograph 2); these spaces are separated 
by diaphragms of medullary tissue containing branches of the latex-system, 
never very thick, frequently very thin ; they occur through the whole length 
of the pith except at the extreme apex of the branch. The short main stem 
from which the branches arise has the same form when young as the branches 
which it bears later. 
3. The thickness of the Aloe stem is mainly made up of secondary wood, 
soft water-storing tissue, with small but numerous and uniformly distributed 
intercellular air-spaces 1 . 
4. The Euphorbia responds more quickly than the Aloe to changes in 
the external temperature-conditions, and attains higher maxima. 
5. In each case the internal temperature begins to rise very slowly a few 
minutes before the plant is touched by the rising sun ; when fully illuminated 
the temperature rises much more rapidly. 
6. The Euphorbia when growing in the open reaches its maximum 
temperature on unclouded days about 2 P.M. The time “ lag ’ was not 
determined but it is short, certainly less than 30 minutes, probably very 
much less. 
1 Worsdell, l . c . 
