62 
A. G. Tansley. 
of the two xylem groups on one side of the axis (Figs. 16 and 17.) 
In the smaller branches the four xylem groups are reduced to three 
or two and eventually to one, the smallest monarch twigs ending in 
sporangia 1 (Fig. 18). 
Fig. 11. Diagram illustrating the planes of symmetry of the frond of 
Zygopteris and allied types. The vertical plane is the plane including the axis 
of the stem, and is the normal plane of symmetry of the leaves of all the 
vascular plants. The horizontal plane is the plane in which the normal leaf 
is expanded, and which is also a plane of symmetry in Zygopteris. The 
transverse planes (the plane of the paper in this diagram is one) are those in 
which each successive pair of branches theoretically lies. 
Fig. 12. Zygopteris (“ Rachiopteris") duplex. Diagram of transverse 
section of rachis showing branching, st. r., strand of rachis, st. a., st. h., 
strands of the pair of primary branches, st. b.,' strand of a branch of one of 
these. After Williamson. 
1 Scott, ’05. Dr. Scott has kindly allowed the publication of 
Figs. 15—18, which are taken from beautiful specimens that 
he was good enough to lend me. 
