390 Dar bishire. — Observations on Mamillaria elongata . 
be a central or a marginal one, is seen to consist of a conical mass of thick- 
walled fibres which are always filled with air (PI. XXV, Fig. 15, 16, ij, h). 
The broad end of the cone somewhat abruptly passes into the corky layers 
of the cushion. It may become slightly narrower at its lower end. In 
a long central spine the mass of air-filled fibres would be about 700 to 
800 /x long from the base to the top, and about 250 /jl broad, the breadth of 
the spine being about 400 ju. For a shorter marginal spine the measure- 
ments would be about 700 ) a, 160/x, and 25° M respectively. The separate 
fibres are not in the least crushed, but they have rather angular walls, 
not rounded off at the corners (PI. XXVI, Fig. 38). Their length varies 
very much, but does not exceed 200 /x. They have tapering ends. In 
diameter they measure as much as 25 and of this only about 2 n all round 
must be taken as thickened wall. But, of course, smaller cavities are met 
with as the fibres taper off. The fibres form a compact mass at the lower 
end of the spine, but higher up they separate, and in transverse section 
appear to be quite separate (PI. XXVI, Fig. 30,^), though they are not 
actually separated in a longitudinal direction. The last tapering ends may 
be no more than 3-4 /x in diameter, their walls, to a large extent at least, 
consisting of cellulose. In a spine which has not been sectioned, but which 
has been mounted whole, the conical mass of fibres with their cavities filled 
with air looks very striking. It is impossible to remove the air except 
by exposing the cavity of each single fibre, so firmly is it held. 
These central fibres lie embedded in a mass of cortical fibres, which 
differ very much from the central ones (PI. XXVI, Figs. 28 and 29, t). 
They appear round in transverse section, but it is almost impossible to 
make out their length, so closely do they fit together and so much reduced 
are their cavities. But they appear to have long tapering ends, and one 
I measured was 200 /x long. This was one of the inner ones closely 
surrounding the central mass of air-containing fibres. The inner cortical 
ones appear fairly round in transverse section, whereas the outer ones 
are rather flattened and also have more contents than the inner ones. The 
outer rather irregular fibres preponderate in the lower part of the spine, but 
higher up they are almost entirely replaced by the regular round ones, which 
low down are at first only found sparingly. But they gradually make their 
appearance between the central fibres, and finally they make up the bulk 
of the tissue of the spine. The outer and lower ones are compressed 
in a radial direction. They are about 12-15 ju, in diameter, half of which 
may be wall-substance which seems to show no trace of any cellulose. 
The fibres higher up are generally isodiametrical and measure at the most 
about 20-25 with a minute cell-cavity of generally about 1 ^ diameter 
(PI. XXVI, Fig. 29). These fibres are more rounded off than the lower 
central air-containing fibres. The thick walls show very clearly concentric 
striation. 
