VASCULAR CRYPTOGAMS 19 



of the ' vascular ' bundles of Vascular Cryptogams, and prefers for them 

 the terms hadrome and leptome respectively. According to Van Tieghem, 

 the secondary tissues, like those of Flowering Plants, proceed normally 

 from two concentric generating layers an external one in the cortex, 

 forming bark outwardly and secondary cortex inwardly, and an inner one 

 in the central 'vascular 'cylinder intercalated in the liber and in the xylem 

 of the primary ' vascular ' bundles, producing secondary liber outwardly 

 and secondary xylem inwardly. The epiderm is in most cases abun- 

 dantly provided with trichomic appendages of various kinds. 



The size and form of the leaves are extremely various. In Lycopo- 

 diurn, Selaginella, and some other genera, they are very small, unseg- 

 mented, and lanceolate, not unlike those of mosses, and form a dense 

 imbricated clothing to the stem ; in Psilotum they are altogether rudi- 

 mentary ; in the Equisetaceae they are reduced to divisions or teeth of 

 a membranous sheath ; in Isoetes (Selaginellaceae), Pilularia (Rhizo- 

 carpeae), and Phylloglossum (Lycopodiaceae), they are long, narrow, and 

 awl- shaped. In some ferns the barren and fertile leaves differ from one 

 another in appearance, and especially in the degree of division of the 

 lamina. In Salvinia they are of two kinds, one floating on the surface 

 of the water and entire, the other submerged, very finely divided, and 

 performing the function of a root ; in Azolla (Rhizocarpeae) they are 

 floating and bilobed. In some genera of Filices and their allies the 

 leaves are quite entire ; in the Hymenophyllaceae they are very delicate, 

 consisting of only a single layer of cells, and in the smaller species 

 closely resemble those of the foliose Hepaticae ; while in most ferns they 

 are of considerable (in the tree-ferns of gigantic) size, with well-marked 

 petiole, rachis, and lamina, and distinguished by the great extent to 

 which the lamina is divided. In most cases (except the Hymenophyl- 

 laceae) they are abundantly provided with stomates. The tissue beneath 

 the epiderm consists of a parenchymatous mesophyll containing abun- 

 dance of chlorophyll, the portion of which adjacent to the upper 

 epiderm is frequently developed as palisade-parenchyme. This meso- 

 phyll is permeated by ' vascular ' bundles or veins, which branch off 

 from the cauline bundles, and are distinguished, in the majority of ferns, 

 by their dichotomous mode of branching, in contrast to the reticulate 

 anastomosing in Dicotyledons, and the parallel arrangement in most 

 Monocotyledons. Among Gymnosperms a similar arrangement is pre- 

 sented by Salisburia and Stangeria. The floral metamorphosis of 

 the leaves of Flowering Plants does not occur in Vascular Cryptogams, 

 nor their special agglomeration round the organs of reproduction as 

 in mosses. 



The mature sporange, theca, or spore-case, is usually a roundish 



c 2 



