314 FILICALES 



[CH. 



work or a cylinder with the wall pierced by large meshes. The 

 manner of evolution of the dictyostele has been ably dealt with 

 by Gwynne-Vaughan^ and other authors. In a few ferns, e.g. 

 Matonia pectinata^, a transverse section of the stem (fig. 237, B) 

 reveals the presence of two or in some cases three concentric 

 solenosteles with a solid protostele in the centre: this polycylic 

 type may be regarded as the expression of the fact that in 

 response to the need for an adequate water-supply to the large 

 fronds, ferns have increased the conducting channels by a method 

 other than by the mere increase of the diameter of a single stele. 

 Fig. 237, A, shows the vascular tissue of a petiole of Matonia in 

 transverse section. 



The two genera of Osmundaceae, Todea and Osmunda, are 

 peculiar among recent ferns in having a vascular cylinder com- 

 posed of separate strands of xylem varying considerably in 

 shape and size, from U-shaped strands with the concavity 

 facing the centre of the stem and with the protoxylem in the 

 hollow of the U, to oval or more or less circular strands with 

 a mesarch protoxylem or without any protoxylem elements 

 (fig. 221, A, B). These different forms are the expression of the 

 change in contour or in structure which the parts of the lattice- 

 work undergo at different levels in the stem^ Beyond this 

 ring of xylem bundles is a continuous sheath of phloem of 

 characteristic structure. A transverse section of a stem of 

 Osmunda regalis may show 15 or more xylem strands; in 0. 

 Glaytoniana there may be as many as 40. In Todea barbara 

 (fig. 221, B) the leaf-gaps are shorter, and in consequence of the 

 less amount of overlapping the xylem cylinder becomes an 

 almost continuous tube. The recent researches of Kidston and 

 Gwynne-Vaughan* have resulted in the discovery of fossil 

 Osmundaceous stems with a complete xylem ring, the stele 

 being of the medullated protostele type; in another extinct 

 member of the family the stele consists of a solid xylem core. 

 The Osmundaceous type of stele is complicated in 0. cinnamomea 

 (fig. 221, A) by the occurrence of local internal phloem and by 



1 Gwynne-Vaughan (03). = geward (99=) ; Wigglesworth (02). 



3 Seward and Ford (03) ; Jeffrey (03) ; Paull (01). 

 * Kidston and Gwynne-Vaughan (07); (08); (09). 



