70 
THE STEM-ANATOMY OF 
in view of their extreme conspicuousness in the stem which I examined, that 
his specimen was, for some strange reason, devoid of them ! 
The air-cavities of the secondary tissues being thus accounted for, I must 
next direct attention to those of the primary tissues. 
If the ground-tissue of the young stem, in a region not remote from the 
apex, be examined, it will be seen that the cells are much rounded off, leaving 
very numerous intercellular air-spaces between them, as seen in transverse 
section. The cells are of the typical water-storing type with large circular 
pits in their walls. As seen in Fig. 6, some of the air-spaces are of consider- 
able size, formed by a more complete separation of the walls of a number of 
cells. 
At a level more remote from the apex, the water-storing tissue is seen to 
be studded with much larger air-cavities, and evidently of quite a different 
type (Fig. 7). They have arisen by the lysigenous method, by the dissolution 
of the walls of groups of cells, and become progressively larger as the stem 
increases in age, until they form, in the central primary tissue of the stem, a 
network of air-spaces, very comparable, when viewed macroscopically, with 
that in the more external secondary tissue. 
It is probable that the larger among the schizogenous air-spaces of the 
younger tissues serve as the nucleus for the formation of the air-spaces of the 
older parts of the primary tissue, which is a point of difference from what 
obtains in the case of the Euphorbia above described. 
Thus we see that there are two distinct modes of origin for the air-cavities 
in the stem of this species of Aloe : (1) a schizogenous mode for those of the 
secondary tissue and for the earlier stages of those of the primary tissue ; and 
(2) a lysigenous mode for the later stages of the latter. 
In explanation of the presence of the air-cavities in this plant, all I can 
say is that we have to deal with a special kind of aerating tissue serving 
some necessary purpose in the economy of this species and of the others in 
which it occurs. 
I can discover no account in the literature of any such tissue in the 
stem of Aloe. On the other hand I have not as yet had the opportunity of 
examining the mature stem of any other species. 
Summary. 
1. The pith of the stem of the succulent Euphorbia virosa Willcl. from 
Namaqualand, as also of other species mentioned, is permeated by air-cavities 
of very large dimensions. 
2. These cavities have a lysigenous mode of origin. 
3. They probably serve as a special type of aerating channels. 
