XLV] CHEIROPLEURIA 205 



parallel among the living species of Diptcris, or the fossils attributed to the 

 family. 



In the sporophylls of Dipteris Lobbiana the sori form a single row on either 

 side of the mid-rib and, though they are variable in extent, they do not spread 

 beyond the individual mesh from the centre of which each arises (Fig. 571, 

 Vol. II). In D. qiiiiiqncfurcata they are not restricted to a single row, but still 

 each is limited to a single mesh (Fig. 572, Vol. II). It is here that Cheiropleiiria 

 shows an advance on them : for as the sori extend over the whole leaf-surface 

 they spread beyond the limits of the single mesh. A good index of what has 

 happened is found in the veins, which are extended into masses of storage 

 tracheides of the receptacle: these lie in a plane nearer to the lower surface 

 than the substantive venation. The receptacular tracheides are thus able to 



Fig. 715. « = part of a sporophyll of Cheiropleiiria, seen as a transparency showing the vascular 

 tissue. The steady lines are the venation ; the irregular patches are the storage-xylem of the soral 

 receptacles. At the points ( x ) these have crossed the veins at a lower plane than that in which 

 the veins lie. ( x 8.) ^ = part of a sporophyll of Platyceriuin angoleiise, showing the origin of the 

 receptacular xylem from the ends of the blind veins. The receptacles are more elongated than in 

 Cheiropleiiria, and pass to a lower level in the mesophyll, there extending frequently across the 

 course of the original venation, which is here represented by thin steady lines. ( x 4.) 



pass, in a lower plane, across the course of the latter, and so the sorus may 

 extend beyond the limits of the individual mesh (see the points marked (x) in 

 Fig. 715). Thus is initiated that "diplodesmic" state described in Vol. I, p.233. 

 The receptacular system is not developed largely in Cheiropleiiria, but it 

 becomes a marked feature in Platyceriuin and in Leptochiliis tricuspis. It may 

 be held as a concomitant of the Acrostichoid development of the sori seen in 

 many Dipteroid derivatives. In a transverse section of the fertile area of 

 Cheiropleiiria the two vascular systems can be readily distinguished (Fig. 716). 

 The fertile area is not defined as distinct sori ; it extends uniformly over the 

 lower surface. The numerous sporangia are of "mixed" origin, and they are 

 associated with simple hairs. The individual sporangium is larger and longer- 

 stalked than in Dipteris: the stalk is four-rowed as in that genus, and the 

 annulus forms a continuous oblique ring: but its induration is not continued 



