2l8 



BOTANICAL GAZETTE 



[SEPTEMBER 



bundles ; the whole is thus merely a variant on the solenostele, in 

 which latter the leaf-insertions are not crowded ; both conditions 



J 



iph 



ostelic, or tubular type of stele. Van Tieghem^ was therefore 

 in error in regarding the dialystelic condition as derived merely 



ph 



r)—-X 



Fig. 4. 



dition. 



from branching of the primitive 

 solid stele. Dialystely is very com- 

 mon in ferns, occurring in Maratti- 

 pU^ aceae, A?ieimia Phyllitidis (Schizae- 



aceae) , Cyatheaceae, Dicksoniae, 

 and in almost all Polypodiaceae, the 

 most advanced of the fern series. 

 The vascular cylinders of Osmtmda 

 T., .. 1 . ,- reFalis and (9. cinjiMHomea probably 



i he dialystelic con- ^ ^ 



represent, as Jeffrey ^ has recently 

 suggested, reduced ionns, in a greater 

 and a less degree respectively, of the dialystelic condition. It is 

 also found in several of the Cycadofilices, the group inter- 

 mediate between ferns and cycads, such as the Medulloseae and 

 Cladoxylon, all fossil types. 



If we now take these last named polystelic plants as our 

 starting point, we may trace from them the origin and evolution 

 of the vascular tissue of the gymnosperms. The modification 

 which eventually produced this latter consisted, as the writer, 

 along with Potonie.s holds, in the gradual reduction of the con- 

 centric type of bundle or stele to form the first and simplest 

 collateral tyT^Q. lx\?iiorm\\\ie Medullosa anglica Xh^rc is a single 

 ring of concentric steles. The theory which the writer regards 

 as best explaining the facts is this : in order that it might give 

 rise to the next succeeding type, as he regards it, represented 

 by such forms as the fossil Cycadofilices, Lyginodcndron Old- 



"JEFFREY, E. C, Trans. Brit. Assoc. Adv. Sci. 869. 1897; Mem. Bost. Soc. Nat. 

 Hist. V. 5: 160. 



3 Van TiEGHEM, Ph., Sur la Polyst^lie. Ann. Sci, Nat. Bot. VII. 3: —• ^^^^' 

 Traite de Botanique, p. 1370. 1892. 



♦Jeffrey, E, C, The structure and development of the stem in pteridophytes 

 and gymnosperms. Phil. Trans. Roy. Soc. June 1902. 



SPOTONIE, Lehrbuch Pfianzenpalaeontologie, iSgg. 



1' 



