SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 617 
The classification of the plants so far described from the Rhyme peat-bed is 
therefore as follows : — 
PTERID OPHYT A. 
Class. PSILOPHYTALES. 
Family, rhyniacea;. 
Genera. Rhynia (2 species). 
Hornea (l species). 
Comparative Discussion. 
The Vascular Cryptogams preserved in the Rhynie peat-bed are the most ancient 
plants of which the internal structure and external morphology are adequately 
known. They facilitate our understanding of those plants of early Devonian age 
which are only known as impressions, while these assist in forming a conception of 
the form and habit of the plants of this remarkable flora. The consideration of this 
flora as a whole is beyond the scope of the present paper. In this discussion we are 
concerned only with the simpler types of the Psilophytales which have been placed 
above in the Family Rhyniacese. 
The great interest of these plants ( Rhynia , Hornea ) is that they acquaint us 
with a type of construction fundamentally more primitive, not only than that 
of existing land plants but of most plants composing the Upper Devonian and 
Carboniferous floras. 
Between the Lower and Middle Old Red Sandstone and the Upper Old Red 
Sandstone or Devonian, as represented by the flora of the Kiltorkan beds, there is 
a most remarkable difference in the type of plant life. It is doubtful if a single 
species which occurs in the Lower and Middle Old Red Sandstone of Scotland is 
present in the Upper Old Red as represented by the Kiltorkan beds. In fact, the 
botanical affinity of. the plants of the latter is with the Lower Carboniferous flora 
rather than with the Middle and Lower Old Red Sandstone plants. 
The type of plant exhibited by Hornea and Rhynia is the simplest known in 
undoubted Pteridophyta. We know nothing of the sexual generation, the existence 
of which there is no reason to doubt, and of the early stages of the sporophyte. The 
full-grown sporophyte is differentiated into rhizome, stems, and sporangia. It has 
no leaves or roots. The rhizome, which was probably subterranean, was intimately 
connected to the substratum by long non-septate . rhizoids. In Hornea it was a 
tuberous parenchymatous structure from which the stems arose. In Rhynia it had 
a central vascular strand, and is like a peculiar region of the stem. The cylindrical 
stems branched dichotomously, diminishing in diameter. They had a simple 
vascular system, the stele consisting of a cylindrical strand of xylem surrounded 
by a zone of phloem. Stomata occurred in the epidermis of Rhynia, and there 
