224 



THIRD GROUP. VASCULAR CRYPTOGAMS. 



bends into the first leaf; from the point where it turns off another bundle begins and 

 bends into the second leaf, and this proceeding is repeated indefinitely. An axile 

 bundle is found also in a number of Ferns with slender stems, as in the Hymenophyl- 

 laceae, Gleichenia, Lygodtu/n, species of Schizaea and others, and also in the hetero- 

 sporous Filices Salvinia and Azolla. In the Osmundaceae (Figs. 174-176) the 

 vascular bundles are arranged so as to form a cylinder, and this is inclosed by a 

 sclerosed cortical tissue through which the bundles run obliquely outwards into the 

 leaves, one into each leaf ; those which pass into the first leaves of the young plant 

 unite into a single axile bundle in which there is no medullary tissue, and this gradually 

 opens out into the ring of bundles surrounding a pith. A similar process takes place in 

 most Ferns ; the original axile bundle of the young plant becomes a cylinder which 

 incloses the pith and is itself inclosed by a parenchymatous rind ; there is a gap, the 

 foliar gap, in the tube opposite the point of insertion of every leaf, and from its margin 

 the bundles pass into the leaf; everywhere else the cylinder is closed, or its con- 



KF 



^t 



FIG 174. 



fa 



JO 



FIG. 175. 



FIG. 176. 



FIGS. 174 176. Osmanda regalis. FIG. 174. Transverse section of a strong stem, about twice the natural size ; 

 i lowest leaf-trace-bundle with a root-bundle going off from it through the rind. FIG. 175. Sketch of the vascular bundle- 

 cylinder of the former fig. more highly magnified ; i lowest leaf trace-bundle cut through exactly where it enters the cylinder 

 with one of the two root-bundles which are attached to it at this point ; 113 the leaf-trace-bundies of the thirteen suc- 

 cessive leaves seen in the transverse section (10 being united abnormally with 2). FIG. 176. Diagrammatic representation 

 of the course of the vascular bundles in the stem when the cylinder is projected in one plane and with a phyllotaxis of-^. 

 The foliar bundles are numbered at their point of exit according to their genetic succession, each with two root-bundles 

 indicated by two short transverse strokes ; 2 and 10 show the irregularity indicated in the transverse section. From De 

 Bary, Vergl. Anat. 



tinuity is broken by perforations giving it a net-like character. Separate bundles are 

 sometimes found in the pith and in the cortical tissue along with this simple cylinder, 

 or several concentric cylinders (rings) may be formed. 



The simplest case is where the originally axile bundle opens out as the stem 

 strengthens into a cylinder, which is usually closed all round except at the insertion of 

 the leaves, and there the parenchyma of the pith communicates with the cortical tissue 

 through the foliar gap, from the margin of which one or more bundles pass into the leaf; 

 examples are to be seen in the species of Marstlia, in Pilularia globulifera, and others. 



Most Ferns with ascending or erect rhizomes or stems differ from the type just 

 described in having the foliar gaps large and the bands of the vascular bundle-cylinder 



