63 
The Botryopteridece. 
All these “ tetraxylic ” types of petiolar strand present the 
striking character that in addition to the normal (vertical) plane of 
symmetry passing through the centre of the petiole and including 
the axis of the stem, they also tend to show bilateral symmetry 
about the (horizontal) plane perpendicular to this, i.e., about the 
plane of the frond in Zygopteris. This is directly related to the fact 
Fig. 13. Zygopteris duplex. Strand of rachis (st. r.), giving of branch- 
traces (st. a., st. b.). The xylem and the limit between inner and outer cortex 
are outlined. The protoxylem is indicated in the concavities of the furrows 
and of the traces. The figures should be turned through a right angle to 
correspond with Figs. 8—11 and 14. Univ. Coll. Collection K 17, 18. x 7. 
that the frond in these types has pairs of branches lying in a plane 
perpendicular to the horizontal or plane of the frond, in what we 
may have called the transverse plane. Fig. 19, which represents a 
series of transverse sections through the rachis of Z. Lacnttii is 
another illustration of this fact. In A the double-anchor-shaped 
strand of the rachis is seen having recently given off 
branches both above and below (in the figure) each of 
which divides into two in the transverse plane. The two 
strands so formed pass through the cortex (B) and enter an 
appendage or branch of the frond (C) which itself correspondingly 
branches in the same plane, each branch receiving a bundle. One 
of these however soon becomes less important than the other. 1 In 
Zygopteris duplex, as we have seen ( essentially the same type of 
branching occurs (Figs. 12, 13.) Now this type of branching is funda¬ 
mentally different from anything that is known in modern ferns, in 
1 Williamson, ’74. Plate LVIII., Fig. 48. 
