Pteridophj^ten. 461 



lous in all the Pteridophyta except perhaps the Lycopodiales and 

 therefore, the simpler types of leaves miist in all cases be consi- 

 dered as reduced. In the Lycopodiales , however, it is admitted that 

 the subordinate members of the successive dichotomie may have 

 been simple and unbranched ab initio. 



The Botryopteridaceae are first described because the simplest 

 type of vascular system is found in ihis order; a protostele with a 

 solid cylindrical Strand of xylem without parenchyma surrounded 

 by a continuous zone of phloem. In dealing with this order an inge- 

 nious Suggestion is made that the key to the Zygopteris type of 

 petiolar Strand is to be found in the cruciate tetraxylic xylem of 

 the petiole of Stauropteris Oldhamia. It is only necessary for the cen- 

 tral part of the xylem to become solid and flathened in a horizontal 

 plane in order to obtain the cross-bar of the )— ( of Zj'^o^/'ms while the 

 four rays would develope into the arms of the )— ( . The following 

 very interesting suggestions are also made. If a Zygopteris frond 

 were to lose its two abaxial rows of pinnae with the abortion of the 

 corresponding wings of the vascual Strand the characteristic C-shaped 

 Strand of the modern Ferns would be obtained. On the other band 

 if the adaxial pinnae and wings were to disappear an inversely 

 oriented C-shaped Strand would result with the concavity facing 

 abaxially, as in Tubicaidis. 



In the Hymenophyllaceae the author regards the type of vascular 

 System exemplitied by Tvichornanes renifonne as Standing nearer 

 than that of any other fern to the primitive hypothetical type of 

 Filicinean Organization. The stele contains a more or less interrupted 

 ring of xylem elements surrounding parenchyma mixed with scat- 

 tered tracheides some of which form the endarch protoxylem. The 

 similarity in structure between the leaf-trace and the stem stele is 

 taken as indicating the primitive identity of the two structures. 



In the Gleicheniaceae the open C-shaped type of leaf-trace is 

 regarded as being the more primitive, the other forms being derived 

 from it by reduction. The vascular System in the stem of Platysoma 

 appears to have been reduced from a solenostele such as still occurs 

 in Gleichenia pectinata. 



The different types of vascular system exhibited by the Lind- 

 sayeae are used by the author to illustrate the several stages in the 

 evolution of the solenostele from a protostele and it is then shewn 

 how a further advance might give rise to the different types 

 of radial and dorsiventral dictyostele found in the Polypodiaceae. 

 This linally leads up to the still more complex polycyclic arrange- 

 ments with two or more internal accessory Strands or with concen- 

 tric zones of the same as found in- the Cyatheaceae, Marattiaceae, 

 Psaronius and in certain Polypodiaceae. The origin of this medullary 

 vascular tissue is followed in detail, in particular of Matonia pecti- 

 nata, Pteris incisa and aquilina and in other cases of particular 

 interest. It is also shewn that the internal vascular Strands of the 

 Marattiaceae are of the same nature and origin as those of the 

 Leptosporangiate Ferns. 



In describing the Osmundaceae the alternative theories of 

 the origin of their typical stele (a) by progression from a proto- 

 stele or (b) by reduction from a dictyostele are critically discussed. 

 The author decides that the balance of the evidence is on the 

 side of the former. On the other band it is thought probable that 

 the Ophioglossales represent a reduction series. 



The concluding lectures are devoted to general considerations 



