Dar bishire . — Observations on Mamillaria elongata . 391 
It now remains to briefly describe the outer layer, which covers the 
whole spine and is a single plate of cells continuous with the epidermis 
of the tubercle. The cells are squarish and elongated in a direction 
parallel to the longitudinal axis of the spine. They are usually about 
100 ijl long and in external view about 25 /x broad. Their radial walls are 
3-4 /x thick, but are not quite even. 
At the lower end of the spine these cells are rather flattened radially, 
being then about 7 /x deep, of which 3-4 /x form the outer wall, and 2 /x 
the cell-cavity, which is filled with a brownish substance (PI. XXVI, 
Fig. 28, e). Higher up they appear quite uncrushed, being sometimes 
as much as 15 /x deep, 6 fx of which fall to the outer wall, and a similar 
number to the cavity. The inner wall is very thin (PI. XXVI, Fig. 29, e ). 
The outer wall of these cells has peculiar projections into the cavity, which 
give the wall a transversely striated appearance in surface view. In the 
upper half of the spine, and even lower down, the epidermal cells are 
remarkable on account of a peculiar knob, which grows out from the upper 
end of each cell, and is an outgrowth of the wall only (PI. XXV, Fig. 16). 
These knobs project as much as 25 /x, and in diameter measure about 12 y. 
The thick walls of the epidermal cells consist almost entirely of cellulose, 
a fine cuticle only covering them on the outside. 
Caspari briefly describes the structure of the spines in the Cactaceae 
generally. He finds, however, that the central portion of the spine con- 
sists of thick-walled sclerenchymatous cells, the outer portion of thin-walled 
cells (6, pp. 6, 7). This does certainly not agree with the observations 
on Mamillaria elongata just referred to above. 
It has already been mentioned that the corky tissues of the cushion 
pass rather abruptly into the hard fibrous cells of the spines. The central 
spine ends square and is not very firmly secured in the tissue of the 
cushion. It therefore is very easily broken off. This we very frequently 
find to be the case. The marginal spines are much more rigidly connected 
with the underlying tissues. On their lower or inner side — nearest the 
living tissues of the wart — the central hard fibrous cells of the spine are 
continued some distance into the cushion. They are smaller here than 
higher up and are very much crushed and contorted (PL XXV, Fig. 20). 
They are here in very close contact with the corky cells of the cushion, 
which at this point, also very much contorted, dip down slightly. This 
internal projection from the spines into the cork-cushion is found just 
above, but outside the margin of the vascular cup (PI. XXV, Fig. 15). 
For the sake of completeness I must mention here that I have 
occasionally found fungal hyphae between the hairs on the cushion and 
also on the spines. These hyphae even penetrate into the spines, and some 
sections reveal the fact that the central air-filled fibres contain numerous 
fungal hyphae. I have not attempted to make pure cultures of these 
