ZYGOPTERIS 317 



consist of smaller elements than the rest of the wood ; 

 "the spiral tracheides are placed on the inner edge of 

 the external bands. It was from the latter that the 

 strands were given off, which passed out into the 

 leaflets. In this species we find once more that the 

 scalariform type of tracheides prevailed. The petiole 

 bears characteristic spines, formed of sclerotic tissue. 1 



In connection with the petiole just described, a few 

 words may be said about Z. corrugata (Will.), 2 a species 

 in which all the vegetative organs are known. The 

 petiole (once called Rachiopteris insignis, but already 

 identified by Williamson) has the complex " Bibrac- 

 tensis" type of structure. The stele of the stem has 

 an almost cylindrical form, except where it is affected 

 by the emission of a leaf-trace, which is preceded by 

 the formation of a double lobe, quite comparable to a 

 bicornute arm of the stele in Z, Grayi. The system 

 of internal tracheides is the same as in that plant, but 

 in Z. corrugata there is no external protoxylem ; its 

 absence is correlated with the fact that the protoxylem- 

 groups of the leaf-trace are immersed, as continues to 

 be the case when the petiolar strand assumes its 

 ultimate structure (cf. Z. bibractensis). The leaf-trace, 



1 It must be remembered that, in the case of these isolated petioles, the 

 affinity with the more complete specimens of Zygopteris is not strictly 

 proved, for we have only the anatomical characters of the petiole to judge 

 by. These characters, however, are so peculiar that we run no risk of 

 serious error in taking them as a provisional guide to affinity. In any 

 case the question is not a very grave one, for we know enough of the 

 petioles actually connected with the stem in other species, to be certain as 

 to the main points in their structure. 



2 The Rachiopteris corrugata of Williamson, " Organisation of Fossil 

 Plants of Coal-measures," Part viii. Phil. Trans. Roy. Soc. vol. 167, 

 1876, p. 213, Figs. 1-24. The plant has proved to be quite distinct from 

 Tubicaulis, with which I was once inclined to connect it. 



