[Illustration: Fig. 91.--Transverse Section of Stem of _Psaronius_
_v_, Numerous irregularly-shaped steles; _s_, irregular patches of sclerenchyma; _l_, leaf trace going out as a horseshoe-shaped stele; _c_, zone of cortex with numerous adventitious roots _r_ running through it; _sc_, sclerized cortical zone of roots; _w_, vascular strand of roots.]
Before leaving the palæozoic ferns, mention should be made of the very numerous leaf impressions which seem to show true fern characters, though they have not been connected with material showing their internal structure. Among them it is rare to get impressions with the _sori_ or sporangia, but such are known and are in themselves enough to prove the contention that true ferns existed in the Palæozoic epoch. For it might be mentioned as a scientific curiosity, that after the discovery that so many of the leaf impressions which had always been supposed to be ferns, really belonged to the seed-bearing Pteridosperms, there was a period of panic among some botanists, who brought forward the startling idea that there were _no_ ferns at all in the Palæozoic periods, and that modern ferns were degenerated seed-bearing plants!
[Illustration: Fig. 92.--Impression of Palæozoic Fern, showing _sori_ on the pinnules. (Photo.)]
These two big groups from the Palæozoic include practically all the ferns that then flourished. They have been spoken of (together with a few other types of which little is known) as the _Primofilices_, a name which emphasizes their primitive characters. As can be seen by the complex organization of the genera, however, they themselves had advanced far beyond their really primitive ancestors. There is clear indication that the Botryopterideæ were in a period of change, what might almost be termed a condition of flux, and that from their central types various families separated and specialized. Behind the Botryopterideæ, however, we have no specimens to show us the connection between them and the simpler groups from which they must have sprung. From a detailed comparative study of plant anatomy we can deduce some of the essential characters of such ancestral plants, but here the realm of fossil botany ceases, to give place to theoretical speculation. As a fact, there is a deep abyss between the ferns and the other families of the Pteridophytes, which is not yet bridged firmly enough for any but specialists, used to the hazardous footing on such structures, to attempt to cross it. Until the buttresses and pillars of the bridge are built of the strong stone of fossil structures we must beware of setting out on what would prove a perilous journey.