388 Dar bishire. — Observations on Mamillaria elongata. 
(PL XXVI, Figs. 13 and 14). These groups are joined together by the 
large tracheids. 
The larger tracheids, which are formed here centripetally at first and 
later on all round, are very large indeed and show very well developed 
spiral or reticulate thickenings, which are usually very close together 
(PI. XXV, Fig. 19). The tracheids may be as much as 60 /x across in 
a transverse direction, and 160 /x long. One particular tracheid measured 
200 by 40 /x, another 100 by 20 /x. Next to these large cells we find the 
small, narrow spiral tracheids of the old bundles, measuring 10 /x in diameter 
(PI. XXV, Fig. 19). 
The spiral thickenings of the larger spiral tracheids are very massive. 
Seen in surface view from the outside they are about 3 to 6 /x broad, their 
actual point of attachment being about \ or £ this measurement in each 
case. They project into the cell-cavity a distance of 3-6 /x. The central 
point of the attachment of any spiral thickening at any given height is very 
generally about 10 /x distant from the same point on the spiral above 
or below. The spirals do not in fact move further apart, they remain 
stationary, but the thickenings increase in size and so they appear to get 
nearer together. The form of these larger cells appears to be more or less 
barrel-shaped. The ends are slightly narrower than the middle. The 
difference, however, is less marked when the tracheids are long and narrow. 
The last tracheids at the top of the bundle-ending measure fairly regularly 
about 20-30 across and are 100 to 120 /x long (PI. XXV, Fig. 15). 
The fusion of the medullary bundles and the formation of the large 
spiral tracheids results in the production of a cup-like ending to the 
vascular system just underneath the top of the tubercle. The cup consists 
of the small groups of smaller tracheids united laterally by the larger ones. 
On the outside the tracheids are surrounded by large active paren- 
chymatous cells, about two or three of which intervene between the tracheids 
and the palisade-cells. The cup is filled with parenchymatous cells likewise, 
which again are large and active. The tissue inside the cup and 
immediately outside it appears lighter in section than the palisade-cells 
because its cells contain very few chloroplastids. No air-spaces are found 
inside the cup. 
The tissues mentioned so far as being found in the tubercles are 
not the only essential ones. The apex of the whole tubercle is occupied 
by a cushion of tissue in which are inserted a number of curiously complex 
spines, the lower ends of which are furthermore surrounded by a mass of 
hairs. There are on the average about twenty marginal spines in each set. 
In the centre is found a single spine, larger than the others. The marginal 
spines are 4 to 4-5 mm., the solitary central one 5 mm. long. The whole 
set of spines covers an area with a diameter of 7 to 8 mm. The distance 
from the centre of one set of spines to the centre of the nearest neigh- 
