Polycycly. 227 
been followed. It will be seen that the sequence of events is exactly 
that which obtains in Saccoloma adiantoides. A strand from the 
second cylinder moves out to fill the leaf-gap in the outer one, and 
the gap so made is filled by a branch from the next, and soon. The 
main difference is that the strand which fills the gap in the outer 
ring at once becomes a clearly defined and separate leaf-trace, 
instead of merely forming part of a continuous cylinder. The 
agreement extends to the formation of the ends of the leaf-trace 
curve by strands arising from the sides of the leaf-gap, i.e. in 
Psaronius from the adjacent “peripheral steles.” 
Thus the vascular system of' Psaronius agrees perfectly with 
the polycyclic solenostelic types, and though we have, of course, no 
evidence as to its devolopment, we may conjecture that the internal 
systems of this type had an origin similar to that which obtains in 
the Dennstadtia-Mntonia series. While, as has already been said, 
it is extremely unlikely that the Psaronieae have any direct 
connexion with the Cyatheaceac or with any polycyclic lepto- 
sporangiate form, there is something to be said for the view that 
the Cyatheaceous type, with its blindly ending medullary strands 
connected with the leaf-trace, is derived by reduction from a more 
highly developed polycyclic type, somewhat like Psaronius , but in 
which the internal strands, or some of them, had been drawn into 
contributing to the leaf-trace. 
Marattiacece. Five genera belonging to this family are known 
to exist at present and of these we now have a fairly complete 
knowledge. The leaves in all cases are relatively large, with big 
leaf-bases. The young plants of all the genera, so far as they are 
known, have elongated stems. In Kaulfussia the adult plant has a 
creeping dorsiventral rhizome bearing a single row of leaves on its 
upper surface. Datum has an elongated, but vertical or obliquely 
directed, rhizome with radial structure. In Archangiopteris the 
rhizome is radially organised and comparatively slender, while in 
Marattia and Angiopteris the adult plant has a short thick rhizome, 
with huge leaf-bases covering almost the whole of its surface. 
So far as the adult plants are concerned the simplest type of 
vascular structure is found m Archangiopteris (Gwynne-Vaughan ’05). 
The vascular system of the stem is dictyostelic (Fig. 71), consisting 
of two to four small strands which anastomose irregularly, and 
usually a small internal accessory strand (i.s.) in addition. The 
internal strand is free for the greater part of its length, but 
approaches the cylinder and fuses with those meristeles which are 
