i8 VASCULAR CRYPTOGAMS 



remains small ; the first stem bends upwards, and other leaves of a 

 more complicated structure appear on it. 



The roots of ^^ascular Cryptogams usually arise in acropetal succession 

 on the stem (in some ferns on the leaf-stalk), and branch either mono- 

 podially or dichotomously. There is never one preponderating root 

 continuous in a downward direction with the main stem corresponding 

 to the tap-root in Flowering Plants. They are distinguished from the 

 roots of Flowering Plants by the lateral roots springing, not from the 

 procambium, but from the innermost cortical layer of the mother-root. 

 They are abundantly covered with root-hairs, trichomic formations by 

 means of which the nutritive materials are absorbed from the soil. 

 They possess a true root-cap. Salvinia (Rhizocarpeae) and Psilotum 

 (Lycopodiacese) are altogether rootless, as also are a few Hymenophyl- 

 lacese (Filices), the function of roots being performed by underground 

 branches of the stem. 



The degree of development of the stem varies within very wide limits. 

 In the tree-ferns it is of erect habit, and attains great height and consider- 

 able thickness. In many herbaceous ferns the internodes of the erect 

 stem are altogether suppressed, while the underground portion forms an 

 elongated rhizome. In the existing Lycopodiacese, and in some Sela- 

 ginellaceae. the very elongated creeping stem is mainly above ground ; 

 in the rootless forms, like Psilotum, branches of the stem bending down 

 into the soil perform the function of roots. In some paludose species 

 belonging to the Rhizocarpeae the stem is almost entirely suppressed, 

 and in Salvinia the whole plant floats on the surface of the water. The 

 mode of branching is either monopodial or dichotomous ; the leaves do 

 not usually produce buds and branches in their axils, as in Flowering 

 Plants. 



In all the larger species the stem displays a distinct differentiation of 

 tissues into the three systems, fundamental, epidermal, and 'vascular' or 

 fascicular. The so-called vascular bundles are closed, like those of Mono- 

 cotyledons— that is, they contain no foimative cambium ; and they are 

 usually but not always concentric, the phloem portion surrounding the 

 xylem portion in the form of a phloem-sheath. Each bundle, or a group 

 of bundles, is again very frequently surrounded by a single layer of 

 strongly sclerenchymatous cells belonging to the fundamental tissue, the 

 vascular bundle-sheath, where it encloses a single, or plerome-sheath, 

 while it surrounds a group of bundles. The prevalent, though not the 

 exclusive form of thickening in the xylem is that of scalariform tra- 

 cheides ; true vessels formed from the coalescence of cells are rare ; in 

 the phloem sieve-ticbes are of common occurrence. Potonie holds that 

 there is no sharp differentiation between the xylem and phloem portions 



