26 PAL^EONTOLOGICAL BOTANY. 



FLORA OF THE PRIMARY OR PALAEOZOIC 



PERIOD. 



Reign of Ackogens. 



In the present day, acrogenous i^lants are represented by 

 cellular and vascular Cryptogams. In considering fossil 

 plants our attention is specially directed to the latter. In 

 the recent Floras, vascular acrogens are represented by such 

 plants as Ferns, Lycopods, and Equisetums. Some of them 

 have an arborescent habit, but the greater number are shrubby 

 and herbaceous. Many of them have creeping rhizomes, 

 which are either subterranean, or run along the surface of the 

 ground. One of these arborescent forms is seen in Tree-ferns 

 (Fig. 9). Another form with a rhizome is seen in Fig. 10. The 

 trunks of ferns are marked by scars, which indicate the parts 

 where the bases of the fronds were attached, and where the 

 vascular tissue passes out from the interior (Fig. 11, a and 6). 

 A transverse section of the stem (Fig. 12) shows a continuous 

 cylinder of scalariform vessels (Fig. 13), enclosing a large 

 mass of cellular tissue frequently penetrated by small scala- 

 riform bundles. The cylinder is pierced by meshes, from 

 the inner sides of which rise the vascular bundles going to the 

 leaves, while some of the free bundles of the axis pass through 

 the mesh, carrying Avith them a portion of the cellular tissue 

 into the petiole. The fructification consists of spore-cases 

 (sporangia), often with an elastic ring round them, containing 

 spores in their interior (Fig. 14). 



Among Acrogens of the j)resent day there are also plants 

 belonging to the natural order Lycopodiacea? or Club-mosses 

 (Fig. 15), having creeping stems, which give rise to leafy 

 branches. The leaves are small, sessile, and moss-like, and 

 the fructification consists of two kinds of cellular bodies. 



