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. annotinum 

 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 oblique 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 H. Ligulat^^ 



I. The Sexual Generation (Oophore). Like the Rhizocarpese among the 

 Filicineae^ the Ligulatae, including the genera Selaginella and Isoeles, are distinguished 



'-% 



^ Hofmeister, Vergleich. Unters. 1851. — (Germination, Development, and Fructification of the 

 Higher Cryptogamia, Ray Soc.) — Hofmeister, Entwick. der Isoetes lacustris, in Abhand. d. Konigl. 

 Sachs, der Wiss. IV. 1855. — Nageli und Leitgeb, liber Entstehung imd Wachsthum der Wurzeln, in 



