Dar bishire. — Observations on Mamillaria elongata. 387 
bast-cells ( a ), being smaller, exceed the wood-cells in number. The com- 
ponents of the wood are tracheids, which in the smaller cases are spiral, but 
in the larger ones are spiral to reticulate. The tracheids of the cortical 
bundles do not show any such remarkable development as we shall meet 
with in the case of the central bundles. But we may here and there get 
a large tracheid developed centripetally from the inner end of the wood- 
portion of the bundle (c). But these large tracheids, which may be 16 ju in 
diameter, are large only as compared with the smaller ordinary tracheids 
of this bundle. These vary between 8 and 13 iu in their largest diameter. 
The last tracheids of a cortical bundle may be as much as 20 /u in 
diameter, the thickening spiral being about 5 ji deep (PI. XXV, Fig. 8). 
They are unaccompanied by any bast-tissue. 
The cortical bundles are closely surrounded by a number of large 
parenchymatous cells, which possess an internal cytoplasmic lining of 4-5 
thickness in which are embedded the very numerous chloroplaStids. 
Of great interest is the structure of the central mass of vascular 
bundles. 
If we follow out the course of the more centrally placed or medullary 
bundles, we see that they are quite separate from the outer ones. The 
cortical bundles run just inside the cell-rows of the palisade-tissue, 
the medullary bundles are found further inside. At first they form a 
disconnected ring in transverse view, although they are actually anastomosing 
freely (PI. XXV, Figs. 10, 11, 12 and 15). This refers to the lower end of 
the tubercle, as soon as the medullary and cortical bundles have become 
separate. The former appear gradually to move closer together, but as 
a matter of fact they are merely increasing in circumference at the expense 
of the surrounding ground-tissues. The diameter of the whole wart 
decreases, but the diameter of the bundle-ring may even increase slightly as 
we near the top of the tubercle. 
Each medullary bundle consists primarily of a number of spiral 
tracheids, which are fairly narrow in diameter and have already been 
described. External to this xylem is the bast, which in the beginning may 
be in extent nearly equal to the wood-portion (PI. XXV, Fig. 4). But 
gradually, as in the cortical bundles, large spiral tracheids are developed 
centripetally from the xylem of each bundle (PI. XXV, Figs. 4, 5, c). These 
spiral tracheids soon extend from bundle to bundle, and may even com- 
pletely surround all the wood of the original bundle (PL XXV, Prig. 6, c). 
But by this time the bast has practically disappeared. The completed 
ring of lignified cells consists finally then of groups of a few small spiral 
tracheids, which groups correspond in number to the original wood-bundles 
or branches of the latter (PI. XXVI, Prigs. 6 and 14). There may be as 
many as twelve such groups. Not unfrequently some of the bundles send 
branches into that part of the tissue enclosed by the cylinder of bundles 
