432 Scott and Hill.—Structure of Isoetes Hystrix. 
which cover in the cavities. Each of these bands bears three 
or four alternating longitudinal rows of stomata. 
Longitudinal sections show that the intercellular cavities 
are partitioned at intervals by transverse diaphragms, one cell 
thick in the middle, but thicker where they abut on the 
surrounding mesophyll, which constitutes the assimilatory 
tissue. The middle of the leaf is traversed by the vascular 
bundle, surrounded by a small-celled parenchyma. The 
structure of the vascular bundle is obviously collateral (Fig. 19). 
About the middle of the bundle is a well-marked, circular, 
intercellular space (Fig. 19 ,px), and on either side of this 
there is often a smaller space of similar form. On the inner, 
ventral side of the intercellular spaces are the tracheides, not 
more than about eight or nine in number, forming an irregular 
transverse band, and usually separated from one another by 
xylem-parenchyma. The thickenings of these tracheides are, 
as a rule, annular (Fig. 22). On the outer side is the phloem, 
somewhat crescentic in sectional form (Fig. 19, pk), with the 
horns of the crescent occupied by the smaller elements. In 
the mature bundle it appears to be the case, as stated by 
Kruch, that the sieve-tubes are limited to the lateral horns 
of the crescent, for none were found, at maturity, in the 
median position. The elements of the phloem, where fully 
developed, have thick, cellulose walls, so as to have the 
appearance of mechanical elements, though only sieve-tubes 
and phloem-parenchyma are represented. 
Both the central and lateral cavities of the bundle are 
surrounded by an endodermis, with suberized radial walls, 
showing the characteristic dark dots. This curious fact was 
first discovered by Russow (1872, p. 140), and is well illus¬ 
trated by Janczewski (1882, PI. IV, Fig. 8). Kruch (1890, 
p. 61) did not find the endodermal structure in his specimens; 
it is most distinctly present in ours, and the radial walls 
resist the action of sulphuric acid. Strasburger has already 
pointed out (1891, p. 464) that the endodermal differentiation 
varies with the conditions. Our observations show that it 
may be characteristically present in a purely terrestrial 
