6o 
A. G. Tansley. 
typical fern frond 1 (horizontal plane, see Fig. 11). In Z. Lacattii, at 
least, one of the two branch bundles on one side of the rachis 
becomes much less important than the other, 2 while in this form 
and in Zygopteris duplex both branches divide again close to the 
base in the same plane (Figs. 12, 19). 3 
pieces of the characteristic H-shaped strand have each an external band of 
small tracheids joined to the large tracheids of the side piece at each end, and 
separated from them by an internal band of thin-walled parenchyma. The 
crushed tissue outside these external xylem-bands probably represents phloem 
and pericycle. Above and below the central band of xylem, large thin-walled 
elements probably representing sieve tubes of the metaphloem can be seen 
broken away from the central band. Outside these are narrow elements 
(pericycle and possibly protophloem), and outside these again a few of 
the large ground tissue cells have been indicated. Univ. Coll. Collection, 
K 33. X about 35. 
In Z. Bibractensis and Z. covvugata (Figs. 8 and 9) the 
structure of the petiolar strand is even more complex. The cross¬ 
bar of the H is curved and there are two additional outer curved 
bands of smaller tracheids following the outline of the main 
“ sidepieces ” and connected with them only at the ends. The 
1 The plane of the pair of branches is actually oblique to the 
vertical plane, but may be considered theoretically as per¬ 
pendicular to it. 
2 Williamson, ’74. Plate LVIII., Fig. 48. 
Williamson, ’74. Plates LV., LV1., Fig. 35 
3 
