THE STRUCTURE AND AFFINITIES OF DIPLOLABIS ROMERI (SOLMS). 717 



petiole face towards those on the other, i.e. the petiole rachis is analogous to the stem of 

 living ferns, since the pinnae face the petiole in the same way that the fronds of modern 

 ferns face the stem. 



Each pinna-trace is at first bow-shaped, but the ends gradually turn inwards until 

 they almost meet, thus giving a C-shaped trace. An intermediate stage between the 

 bow-shaped and the C-shaped trace is shown in PL IV. fig. 47. In its subsequent 

 divisions the pinna-trace gives off small bundles from its free ends alternately on each 

 side. These, in turn, continue to divide in a similar manner. The ultimate branchings 



Fig. la. 



Fig. 26. 



Fig. la. — Plan of the pinnae on the rachis of Diplolabis romeri. The tendency to a dorsiventral arrangement is indicated. 



Fig. 26. — Elevation showing the vertical arrangement of the pinnae on the rachis. Pet. tr. = petiole-trace ; 



pin. tr. = pinna-trace ; aph. tr. = aphlehia-trace. 



of the pinnae, the foliage, and the sporangia of this interesting fern are unknown, 

 though Renault referred certain synangia to D. romeri on the grounds of association. 

 Mere association cannot, however, be taken as a criterion of any relationship between 

 plant members which are discovered detached. 



Turning now to follow this petiole into the stem, we find that at a lower level of 

 the petiole the parts corresponding to the vertical lines of the H bend outwards until 

 the angle made by each of those four arms and the horizontal bar of the H becomes 

 over 150° (PL II. fig. 27). 



Each of these arms has a protoxylem group near its apex on the inner side (PL II. 

 fig. 27, prx.). At this point the strands of a pair of reduced pinnae join together, and 



