222 



THIRD GROUP. VASCULAR CRYPTOGAMS. 



in Aspidium Filix-mas according to Schacht rounded stalked glands occur in the fun- 

 damental tissue of the stem ; Sachs has found them also in the green parenchyma of 

 the leaves and on the stalks of sporangia of the same plant (Fig. 164 C, d}. 



It is only in the Hymenophyllaceae that the lamina of the leaf is formed, as in 

 the Mosses, of a single layer of cells ; in all other Ferns it is of several layers, and be- 

 tween the upper and lower epidermis lies the mesophyll, a spongy parenchyma rich in 

 chlorophyll and traversed by the vascular bundles which form the venation of the leaf. 

 The course of the veins is very various ; sometimes they run from below upwards and 

 sideways spreading like a fan and branching dichotomously with acute angles, without 

 anastomosing and without forming a stouter mid-rib (Fig. 168) ; more commonly the 

 undivided lamina or a lacinia of the lobed, divided or pinnate leaf is traversed by an 

 evident though only slightly projecting median vein, from which finer veinlets run 

 branching monopodially or dichotomously to the lateral margins ; sometimes these 

 slenderer veins anastomose, as in the leaves of most Dicotyledons, and divide the surface 

 into areolae of characteristic appearance. 



The vascular bundles of the stems of Ferns, Osmunda excepted, are concentric, that 



FIG. 171. Polypodium vulgare. Transverse section through a weak vascular bundle of the creeping stem (rhizome) ; 

 j the phloem (the sieve-tube structure not clear), sp narrow spiral tracheides of the xylem, the larger part of the elements 

 of the bundle being scalariform tracheides ; it the endodermis (bundle-sheath) evidently formed by tangential division 

 from the same cell-layer as the layer of parenchyma which adjoins it on the inside ; on the outside of u parenchyma with 

 pitted cell-walls, the pits being shown in the figure only in a few places. From De Bary, Vergl. Anat. 



is, the vascular portion, the xylem, is in the centre, and the sieve-tube portion, the 

 phloem, lies round it (Fig. 171) ; in Osmunda they are collateral, that is, the phloem 

 lies beside the xylem and usually somewhat in front of it towards the circumference of 

 the stem. Collateral bundles are found also in the leaves of many Ferns. 



In the xylem of the bundle we rarely find true vessels with perforated walls ; 

 it usually consists chiefly of long broad scalariform tracheides with bordered pits. 

 True vessels occur in Pteris aquilina with septa perforated in a scalariform manner 

 (Fig. 170). At certain points between or more rarely outside these scalariform 

 tracheides are found a few narrow spiral and narrow scalariform tracheides, the primary 

 formations of the xylem (protoxylem), the starting points of the development of the 

 broad tracheides (Fig. 171). Sometimes tracheides occur in the xylem of the vascular 

 bundle ; sometimes narrow thin-walled cells, which contain starch in winter, are found 



