Dar bishire, — Observations on Mamillaria elongata. 405 
(c) Comparison with other plants . 
As already mentioned, the similarity in appearance of certain species 
of Mamillaria and certain species of Mesembryan themum is very striking. 
I have been able to examine the structure only of Mesembryanthemum 
stellatum , but there are quite a number of species which have the same 
external appearance. 
The conditions under which M. stellatum lives are probably very 
much the same as those under which our Mamillaria elongata flourishes. 
An examination of its structure is therefore of particular interest. 
Mesembryanthemum stellatum has fleshy leaves, which are roughly 
triangular in transverse section, but more or less cylindrical in longi- 
tudinal view. 
The leaves are here the chief assimilating organs, and in their function 
and structure they correspond exactly to the tubercles described for 
Mamillaria elongata. 
We can follow up the vascular bundles coming from the stem and 
see them branching and anastomosing freely in the leaf. They become 
closer and closer towards the tip of the leaf, where they end blindly, in 
a number of fairly large tracheids. Just underneath the apex of the leaf 
we get the largest mass of tracheidal tissue, the component cells of the 
latter being large reticulate storage-tracheids. 
The assimilating tissue forming the outer layers of the leaf-organ 
consists of cells arranged in regular rows, and represents roughly the same 
type of palisade-tissue as that met with in Mamillaria, It makes roughly 
the same angle with the epidermis as we found in Mamillaria elongata. 
The assimilating tissue of the leaf is traversed extensively by long 
but shallow air-spaces, which communicate with the outside air through 
the stomata. 
The leaves of M. stellatum are protected against the effects of the 
strong sunlight, to which the plant is exposed in its native localities, by 
two means. The whole leaf is covered by large cells, which grow out 
from the epidermis and expand so as to screen the epidermis and the 
underlying tissue very effectively from too strong light. 
These large cells possess a very thin lining of cytoplasm and a very 
large vacuole. The cell-wall is very thick and hard, and it seems to 
glisten in the sun, a sign that light is being reflected. In this way 
transpiration is reduced. 
Volkens refers to the large epidermal cells of Mesembryanthemum 
crystallinum as water-storers ( 33 , p. 123, PI. XIII, Figs. 4, 5). I would 
not however like to call them water-storing cells, although they may 
possibly retain water for a long time. Generally we find water stored 
inside the plant and away from its chief enemy the sun. Rather should 
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