90 Hill .— The Structure and Development of 
so that in quite old stems there is a ring of strongly lignified 
tissue, the cells of which often exhibit well-marked simple pits. 
The medulla of the young flowering stems has a large num¬ 
ber of air-spaces developed in it; these air-spaces are arranged 
in a very regular manner, and their appearance recalls that 
which is generally associated with aquatic plants. The older 
scapes are frequently hollow, this being due to the inequality 
of growth of the outer and the more central regions of the 
stem. 
THE LEAF. 
The leaves of Triglochin are acicular in shape, with a sheath¬ 
ing base, and are arranged in an equitant manner. In trans¬ 
verse section they are generally semicircular in outline. They 
may attain a length of 15-16 cm.; the breadth of course varies 
in the different regions of the leaf. The greater diameter of 
a half-cylindrical leaf was 3 mm., and the lesser i| mm., 
measured across the middle region. 
The structure of the leaves is illustrated by Figs. 7 and 8. 
Fig. 7 shows the general structure in a diagrammatic manner. 
Surrounding the two younger leaves the sheathing base of an 
older one is seen ; from this figure it is also apparent that air¬ 
spaces, which are schizogenous in origin, are largely developed, 
especially in the more central regions. Embedded in the 
ground-tissue are numerous collateral vascular bundles, the 
larger ones lying nearer to the median plane of the leaf (see 
Fig. 45, PI. VII). These bundles are very similar in structure 
to those already described for the peduncle, and hardly call 
for a detailed account here. 
On comparing the positions of the separate bundles, in the 
sheathing and upper regions respectively, of a leaf, it will be 
seen that as the bundles pass from the base towards the apex 
they alter very much as regards their relative positions ; hence 
it was desirable to trace out their course in some detail. 
Several bundles pass up from the rhizome to each leaf, 
e. g. all those represented in diagram A , Fig. 45, PI. VII, 
were found to run into the stem as separate leaf-traces. 
