3 i8 STUDIES IN FOSSIL BOTANY 



on first leaving the stele, has a curved transverse 

 section, concave outwards, as in Tubicaulis, and the 

 H-like form is only very gradually assumed. The 

 " Aphlebiae," which in this species are found chiefly in 

 the petiole, have already been noticed (Fig. 1 1 7). 

 The young leaves bear, in addition, a copious growth 

 of ramenta. 



The great peculiarity of Z. corrugata, as compared 

 with the other three species in which the stem is known, 

 consists in its mode of branching, which is not axillary, 

 but rather of the nature of a dichotomy, the stem 

 forking into two nearly equal branches without obvious 

 relation to the leaf- insertion. This fact raises the 

 question whether, as has been suggested, the apparent 

 axillary branching of other species and of recent 

 Hymenophyllaceae may not be a modified dichotomy, 

 in which case the " undivided leaf-trace " would really 

 be the stele of the smaller branch, and the " subtending " 

 leaf would belong to this branch and not to the main 

 axis. The data are insufficient to settle the question, 

 and for the present it seems, better to keep up the 

 distinction between the two kinds of branching. 



The adventitious roots of Z. corrugata are often 

 well preserved ; they are of relatively large size, usually 

 with a diarch stele and a lacunar cortex. 



The elements of the large-celled, peripheral wood 

 in the stem of Z. corrugata occasionally show some 

 tendency to a radial arrangement. In a fossil, pro- 

 visionally placed in a new genus, Botrychioxylon, which 

 in the structure of the internal xylem, the leaf-traces, 

 and the mode of branching shows clear affinity with a 

 Zygopteris of the corrugata type, the peripheral wood 



