332 Knox: STEM OF IBERVILLEA SONORAE 
numbers of bundles are given by these authors for the same form, 
but in /berzillea, as Weiss (15) found to be the case, the number 
may vary not only in the same species and the same plant, but 
even in the same internode. In a well-developed branching stem 
with a length of 310.6 cm. there were at the base ten bundles, at 
the first fork twelve bundles in each branch, thirteen in the main 
branch above, and in its side shoots most frequently twelve, but 
sometimes thirteen or fourteen. Lotar (12), too, says that differences 
may occur, due to the temporary splitting of individual bundles. 
In the single growing tip which my material afforded there were 
eleven procambial strands, so that the number is not derived from 
a primitive procambial ten, but doubtless varies in the separation 
of the young meristems. The structure of the bundle is alto- 
gether normal. The well-formed hadrome, which is cut off cen- 
trifugally by the cambium which lies along its outer surface, shows 
the usual succession of ring, spiral, and pitted ducts, and the last 
are very large, and always surrounded by wood-parenchyma. The 
outer leptome contains large sieve-tubes with prominent sieve-plates 
and companion-cells ; there is also much leptome-parenchyma and 
the outer and inner leptome are alike in their constituent elements. 
The outer cambium region consists of several rows of brick-shaped 
cells, while the inner or medullary cambium (FIGURE 10) shows 
cells more polygonal in outline, and is more localized in its later 
divisions. 
The ground-tissue in /éerzillea is always -solid, consisting of 
large parenchyma-cells of which the walls are more and more 
conspicuously pitted as the stems grow older. The cells are 
ordinarily full of starch which occurs in large grains crowded to- 
gether so densely that the tissues of the bundles stand out in sharp 
contrast. Several rows of large cells intervene between the bundles 
and the stereome-ring. The latter (FIGURE 2, sf) is from two to 
three cells wide and consists of lignified fibrous cells marked with 
cross-shaped pits. It breaks up as soon as the stem begins to en- 
large, and the dilatation-changes progressively fill up the interstices. 
Without the stereome-ring is found a row of starch-containing 
cells (FIGURE 2, end) with the radial walls at right angles to the 
tangential, and slightly more oblong in cross-section than the 
adjacent parenchyma-cells, which Van Tieghem (13) cites as the 
