XXI] 



ANATOMY 



135 



which is general for living Osmundaceae; for it corresponds with singular 

 fidelity of detail to that seen in the Permian fossil Thamnopteris Schlechten- 

 dalii (Eichwald), and in other fossil Osmundaceae. It is thus a very 

 ancient type of shoot-construction (Kidston and Gwynne-Vaughan, I.e. iii, 

 p. 656). 



Turning to the vascular tissue of living Osmundaceae, and especially to 

 that of the central stele, it is found to be constantly and completely circum- 

 scribed by endodermis: even when a leaf-trace passes off, the endodermal 

 sheath does not open. The only exception to its completeness is where 

 a "ramular gap" is formed, and even then an opening is only of occasional 

 occurrence, as in O. cinnaniomea (Vol. I, p. 134). Otherwise the endodermal 

 investment of the vascular system is complete in the living Osmundaceae. 



The description of the vascular system of the shoot may start from the 

 leaf-blade. The numerous peripheral forked veins fuse on passing downwards 

 from pinna and pinnule to the leaf-stalk, forming a single half-cylindrical 

 strand, its concave channel beino- directed inwards. This strand consists of 



Fig. 426. A, diagram showing the anangemeiit of the xylein-strands in the axis of 

 Osnmndaregalis. (After Zenetti.) The phloem and endodermis are omitted. B, C, 

 transverse sections of the stele of Todea Barbara, with leaf-trace (after Seward and 

 Ford), showing a greater continuity of the xylem (shaded) than in Osiniinda ; j= phloem, 

 j/= sclerotic tissue. 



a continuous band of metaxylem, with numerous protoxylem-groups at its 

 concave limit, which alternate in position with groups of mucilage-sacs. 

 Peripherally is a mantle of phloem, and surrounding the whole is a con- 

 tinuous endodermis. As it passes to the base of the petiole the strand 

 contracts, and the protoxylems unite into a single one in a median position, 

 while in transverse section the whole strand assumes a deep U-shaped outline 

 (Fig. 426, A, B). It is here that the vascular supply to the roots is given off 

 laterally from the leaf-trace (Fig. 426, A, III). The strand thus contracted 

 then enters the stele, and takes its place in a ring of similar traces surrounding 

 the central pith. As it does so its endodermis becomes continuous with that 

 which completely surrounds the stelar system (Fig. 426, B, C). The stele, 

 as seen in transverse section, is composed of several layers of parenchyma 



