442 Scott and HitL—Structure of Isoetes Hystrix. 
phloem. The central canal of the bundle, which, with the 
lateral canals, when present, is surrounded by a true endo- 
dermis, represents the primitive row of tracheides. Their 
transverse walls are persistent, forming perforated dia¬ 
phragms across the canal. 
2. In the base of the leaf, and in the leaf-trace, there 
are no xylem-canals, and the structure of the bundles is in 
some cases mesarch, a few tracheides being formed between 
the protoxylem and the phloem. 
3. The phloem contains true sieve-tubes with transverse 
sieve-plates, and lateral sieve-areas, on both of which callus is 
formed. 
4. The growth of the leaf, except at its first origin, is inter¬ 
calary. 
5. The ligule, which develops extremely early, secretes 
mucilage when young. The glossopodium is surrounded by 
a sheath of tracheides, which are most abundantly developed 
in the base of the labium. 
6. The labium and velum are derived from tissue above the 
sporangium, and not from sterilized sporogenous tissue. The 
velum shares in the intercalary growth of the leaf-base, and 
forms a complete pocket round the sporangium, only open by 
a narrow crevice at the base. 
7. The persistent spinose scales are formed by sclerosis of 
certain portions of the tissue of the leaf-bases, whether fertile 
or sterile, as described by Alex. Braun. 
III. The Root. 
1. The stele of the root has, in all parts, a monarch struc¬ 
ture, the differentiation of the xylem beginning with the 
development of a single tracheide, which lies directly opposite 
the protophloem. Neither at the base of the root nor else¬ 
where is there any indication of diarch structure. 
2. The apex of the root shows distinct histogenetic layers, 
as described by Bruchmann, Farmer, and Campbell. The 
initial groups give rise to the plerome, and to the inner and 
outer cortex. 
