69 
EUPHORBIA VI ROSA AND ALOE DIOHOTOMA 
As the piesence ot the laige amount of air m the pith may have a bearing 
on the assimilatory and respiratory functions of the plant, I may just refer to 
the distribution of the stomata. I he cortex is relatively narrow and contains 
green-cells in abundance ; the stomata are only slightly depressed, consist of 
the two guard-cells and conspicuous and very clear- looking subsidiary cells, 
and occur both in the furrows and on the surface of the spine-bearing ridges. 
Aloe dichotoma Linn. 
This plant grows in the same locality and habitat as the Euphorbia just 
described and is of arborescent habit 1 . 
Prof. Pearson asked me to determine the mode of origin of the peculiar 
large air-cavities which occur in the secondary tissue of the stem which, as in 
the case of those in the Euphorbia-stem, seemed a somewhat astounding 
feature in a thorough going succulent. 
The cambium occurs immediately within the very narrow cortex at the 
extreme periphery of the stem. The primary tissue of the central cylinder 
occupies an area at the solid centre of the stem of relatively very small 
diameter. The great bulk of the whole internal area of the stem is occupied 
by the secondary aerated tissue which is traversed at intervals by narrow 
zones of closely-fitting stone-cells which superficially resemble the successive 
vascular rings in a Dicotyledonous stem with extrafascicular cylinders. The 
vascular bundles and numerous sacs containing raphides-crystals, are scattered 
amongst the often somewhat radially-elongated cells of the secondary ground- 
tissue. The youngest portion of this tissue, situated outside the outermost 
zone of stone-cells, forms a fairly compact mass of cells, with small intercellular 
air-spaces at their corners. 
As to the mode of origin of the lacunae in the secondary ground-tissue, it 
was quite easy to determine that they arise schizogenously. The cells, once 
they are formed by the cambium, do not undergo division by means of radial 
walls. Hence, as the stem increases in girth owing to the growth of the 
cortical tissue, the tension set up in the tangential direction causes a partial 
splitting-apart of many of the rows of secondary ground-tissue cells situated 
between the zones of stone-cells (Fig. 8). The result is a system of wide 
meshes of air-cavities separated by strands, many cells thick, of narrow 
elongated cells, in which the vascular bundles and crystal-sacs are imbedded. 
Lindinger 2 gives a detailed account of the anatomy of an adult stem of this 
species which had been grown in the Hamburg Botanic Gardens. As he 
never once refers to the air-cavities of the secondary tissue, I must conclude, 
1 Pearson, l.c. Photograph 3. 
2 Lindinger: “ Ueber die Struktur von Aloe dichotoma” (Verb. Naturw. Ver. Hamburg , 
ser. 3, vol. xvi, 1909, pp. 96-8). 
