J.54 



VA SC ULA R CR YP TOGA MS. 



the apex, when the vessels are not yet in existence. Adventitious lateral rootlets 

 (arising behind those already formed) do not occur. The mother-cell of a lateral 

 rootlet first of all forms its three-sided pyramidal apical cell by three oblique divi- 

 sions ; the first la)er of the cap being then formed from it. When two primary 

 vascular bundles arise in a lateral rootlet, they lie right and left in reference to the 

 primary root. The cortex of the primary root is simply penetrated, no root-sheath 

 being formed. 



The fibro-vascular bundles are always formed singly and in the axis of the 

 root, even in very slender filiform stems, as in those of Hymenophyllacese, and in 

 the young plants of larger species. When the stems of the latter and their leaf-stalks 



Fig. 26^.— Pteris aquilinn ; A transverse section of 

 the stem, r its brown sheath (the layer of sclerenchyma 

 beneath the epidermis), / the soft colourless paren- 

 chyma of the fundamental tissue ; ig inner fibro- 

 vascular bundles ; ag upper broad primary string 

 of the outer bundle ; B the separated upper fibro- 

 vascular bundle of the stem st and of its branches st' 

 and st", b bundles of the leaf-stalk, ii u outline of the 

 stem (natural size). 



Fig. 264.— a quarter of the transverse section of a fibro-vascular 

 bundle from the stem of Pteris aquilina, with the adjacent parenchyma 

 P containing starch, sg the bundle-sheath, b the layer of bast, sp the 

 large sieve-tubes, g g the large vessels thickened in a scalariform manner, 

 5 a spiral vessel surrounded by cells containing starch (X300). 



become thicker with increase of growth, a network of anastomosing bundles is 

 formed in place of the central bundle, presenting, in typical cases, a wide-meshed 

 hollow cylinder, by which the fundamental tissue of the stem is separated into an 

 outer cortical layer and an inner medullary portion (Fig. 262, A and E). Not un- 

 frequently, however, isolated scattered bundles also arise in addition ; thus in Pteris 

 aquilina two strong broad cauline bundles are formed within the medullary portion 

 (Fig. 263, A, I'g), and in Tree-ferns a number of filiform bundles are scattered 

 through it which enter into the leaf- stalk through the meshes of the primary bundle ^ 



' For a more special description see Mettenius on Angiopteris, in Abhandlungen der konigl. 

 Sachs. Gesellsch. der Wiss. 1864, vol. VI. 



