THE STRUCTURE AND AFFINITIES OF DIPLOLABIS ROMERI (SOLMS). 731 



in comparison with the horizontal bar the higher we go, until a constant stage is reached. 

 This constant stage we may regard as the normal Diplolabis petiole-trace. Thus we 

 cannot regard the Zygopteris primaria trace as derived from that of Diplolabis, but we 

 must consider the Diplolabis trace as having been evolved through a Zygopteris 

 primaria stage. It is also clear that the Zygopteris primaria stage must come some- 

 where between the Clepsydropsis stage with the arms non-existent, and the Diplolabis 

 stage with the arms dominant. 



As far as we can judge from the sequence shown in the petiole-trace of Diplolabis 

 rbmeri, the earliest zygopterid petiole-trace must have been elliptical, with two sunk 

 protoxylem groups— one at each end. A petiole with such a trace is, of course, entirely 

 hypothetical ; but, if the early stages of the petiole-trace of Diplolabis are of any 

 phylogenetic value, such a type must certainly have existed at some previous time. 

 This type we may call the Protoclepsydropsis petiole. The stem of this type would 

 have been solid, as in Diplolabis, but probably all the tracheides would be of one type. 

 The traces which would depart from such a stem must of necessity have done so in a 

 protostelic manner. If the petioles were given off at infrequent intervals, the general 

 shape of the xylem would be circular in transverse section, i.e. similar in outline to that 

 of D. rbmeri. But if, on the other hand, the internodes were short, then a stellate 

 form would be impressed on the xylem, since more than one trace would be shown 

 connected with the stem in any transverse section. M. Paul Bertrand has derived 

 the Zygopteridese from a different type, also hypothetical — the Eoclepsydropsis — with 

 a stellate stem-xylem. This stem, however, has a pith, and the islands of parenchyma 

 are shown decurrent into the pith of the stem.* The stem of Diplolabis throws some 

 further light on this question, and the Protoclepsydropsis, as defined above, seems to be 

 more in accordance with the known facts than the Eoclepsydropsis. Further, the 

 Protoclepsydropsis comes much nearer to the type from which the Osmundaceae appear 

 to have been derived, but this will be considered shortly. The relationships of the 

 Zygopterideae immediately connected with Diplolabis may be expressed in the table of 

 text-fig. 4. 



From the hypothetical type, Protoclepsydropsis, the other zygopterid stems have 

 been derived by the formation of a pith. As has been lately pointed out, the increase 

 in diameter of the stem-xylem probably causes the central tracheides to become func- 

 tionless as far as water conduction is concerned, though they probably serve as storage 

 tissue. With this change of function the shortening of the tracheides may be correlated. 

 D. rbmeri exhibits this type of stem-xylem. A still further extending of the idea 

 shows how the Ankyropteris corrugata type would be evolved from the Diplolabis. 

 The tracheides would never be differentiated as such, i.e. the cells would remain 

 parenchymatous. 



Our conceptions of palaeozoic fern genera are gradually becoming clearer, and the 

 work of Bertrand, Kidston and Gwynne-Vadghan, Arber, Tansley, and others have 



* Mudes sur lafronde des Zygopte'ridees, p. 261. 



