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A. G. Tansley. 
groups according to the arrangement of the leaves : the distichous, 
the tetrastichous (decussate) and the polystichous (spiral). Perhaps 
the distichous arrangement—unknown in modern tree-ferns—is the 
primitive leaf-arrangement. From our present point of view a 
general description of the anatomy of the genus will suffice. 
The external vascular ring (Fig. 70) consists of a number of 
peripheral strands, each U-shaped in cross-section, with the 
concavity of the U facing inwards, and separated one from another 
by radial bands of sclerenchyma. Every alternate strand of this 
peripheral series is a leaf-trace ( l.t ., Fig. 70), while the intermediate 
Fig. 70. Diagrammatic transverse section of vascular system of a poly¬ 
stichous Psaronius, showing five dictyostelic concentrically arranged cylinders. 
c.st., cauline meristele; l.t., leaf-trace. From Rudolph after Zcillcr. 
strands (steles peripheriqiies of Zeiller) are cauline (c.st., Fig. 70). 
The internal cylinders consist of series of curved band-shaped 
strands, the edge of each strand being more or less approximated 
to that of its neighbour. In passing up the stem a series of 
anastomoses occur between the successive cylinders in a radial line 
corresponding with a leaf-orthostichy, and this ends by the passing 
out of a strand from the penultimate cylinder. This strand fills the 
gap caused by the departure of the leaf-trace immediately below, 
receives contributions from the “peripheral stele” on each side, and 
itself becomes the leaf-trace supplying the next leaf of the ortho- 
stichy. The exact method of anastomosis between the different 
strands varies in different types. The gap-filling strand sometimes 
comes into actual connexion with the adjacent “peripheral steles,” 
and sometimes receives off-shoots from them, but the general 
method is always the same in those cases in which the details have 
