468 
VASCULAR CRYPTOGAMS. 
prosenchymatous, or even sclerenchymatous, though they never become brown like 
those of the Ferns (Fig. 328). 
The axial fibro-vascular cylinder is sharply defined from the ground tissue by a well- 
developed bundle-sheath consisting of from one to three layers of cells. In the leaves 
of the heterophyllous species air-cavities exist, and they are found also in the stem 
of L. inundatum. In this species Hegelmaier found gum-canals in the stem and leaves 
(one in the mid-rib), which are formed by the gradual separation of cells from each 
other. The bordering cells project into the canal like varicose hairs. In L. annotlnum 
such canals occur only in the fertile branch. 
The Fibro-'vascular Bundles of the Lycopodieae are very characteristic. In the stem 
and in the root there is a large axial cylinder which has usually a circular outline. 
In this (Fig. 328) lie bands of xylem which are either isolated or united in various 
ways so as to form figures which would be symmetrically bisected by an axial longi- 
tudinal section. Transverse sections taken at different heights show the xylem arranged 
in different patterns, for the bands anastomose in their course. These bands of xylem 
consist, like those of Ferns, of tracheides which are pointed at the ends, which are 
wider in the centre than at the edges of the bundle, and which, when they are narrow, 
have round pits, when broad, the pits are fissure-like. Small spiral vessels (proto-xylem 
cells) are to be found at the edges of the bands of xylem. In creeping and obHque stems 
the concavity of the bands is always directed upwards. These bands are embedded in 
a mass of small-celled phloem, in which rows of wider cells lying between the xylem 
bundles occur. Although, according to Hegelmaier, these cells possess no sieve-plates, 
they may be regarded as the representatives of the sieve-tubes. 
The ' proto-phloem cells ' (bast-fibres) lie toward the exterior between the ends 
of the xylem bundles. The arrangement of the elements recalls that of the axial 
cylinder of roots. Within the bundle-sheath are several layers of rather large cells 
which invest the peripheral phloem; to these Hegelmaier has given the name of 
* phloem-sheath,' and probably they truly correspond to the layer occurring in Ferns 
to which the same name has been given. I maintain the view which I formerly ex- 
pressed that the axial cylinder of the stem of the Lycopodieae consists of several 
fibro-vascular bundles which have coalesced ; for Hegelmaier's argument against it, that 
the bands of xylem are not isolated throughout their whole extent, is by no means 
conclusive, and moreover the resemblance between the axial cylinder of the stem and 
that of the root supports my view. Each leaf contains a single thin fibro-vascular bundle 
of very simple structure, which runs very obliquely from the base of the leaf through 
the cortex, to become connected lower down with the margin of one of the xylem 
bundles of the stem. The leaves of Psilotum contain no fibro-vascular bundles. Ac- 
cording to Russow the xylem of the axial fibro-vascular cylinder of the stem forms 
an angular hollow cylinder at the projecting angles of which are groups of narrow spiral 
vessels. 
The axial cylinder is cauline : it can be followed in the procambial condition to just 
beneath the apex of the stem. The rows of spiral cells are first formed within the 
xylem bands, with which the similar elements in the bundles of the leaves become con- 
nected (Fig. 327) long before the development of the tracheides. 
Order II. Ligulat^\ 
I. The Sexual Generation (Oophore). Like the Rhizocarpege among the 
Filicineae, the Ligulatae, including the genera Selaginella and Isoetes, are distinguished 
Hofmeister, Vergleich. Unters. 1851. — (Germination, Development, and Fructification of the 
Higher Cryptogamia, Ray Soc.) — Hofmeister, Entwick. der Isoetes lacustris, in Abhand. d. Königl. 
Sachs, der Wiss. IV. 1855,— Nageli und Leitgeb, über Entstehung und Wachsthum der Wurzeln, in 
