SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 841 
uncertainty must remain as to the reproductive region of Asteroxylon, the evidence 
of association is in favour of the sporangia having been borne on leafless branch- 
systems. In this respect also Asteroxylon would agree with Psilophyton. 
The general organisation of the sporophyte in Asteroxylon and Psilophyton gives 
us a distinct type of plant-body which contrasts with the Fern, Equisetum, and 
Lycopod types, but may be more closely compared with the plant-body in the 
existing Psilotacese. The Lycopod type is distinguished from the type of Asteroxylon 
in its vegetative construction by possessing true roots ; the range of diversity in the 
roots and root-bearing organs of the Lycopodiales must, however, be noted in this 
connection. 
The great interest of the organisation of Asteroxylon and Psilophyton , though 
both these plants are in their respective ways more or less specialised, is the 
suggestion it affords of a synthetic type, combining external and anatomical features 
found in distinct groups of the Vascular Cryptogams. The morphological, and to a 
less extent the anatomical, characters of the Psilotacese can be interpreted in the 
light of what we know of Asteroxylon and Psilophyton. The external morphology of 
the shoot, and especially the anatomy of Lycopodium , find parallels in Asteroxylon. 
The comparison between the characters of Asteroxylon and those found in plants of 
the Fern type is less apparent, but comes out especially when the Zygopteridese are 
taken into consideration. The geological age of Asteroxylon and Psilophyton adds 
importance to the synthetic type they exhibit. The definition of the various types 
of Pteridophyta, that might be regarded as possibly derived from such a synthetic 
type, is afforded partly by vegetative form and structure, but largely by the relative 
positions occupied by the sporangia. The diversity attained in this respect by the 
Fern and Lycopod types, for example, can be most readily conceived if a branch- 
system bearing sporangia on some of its ultimate subdivisions is taken as the point 
of departure. Such a structure appears to have held for the fertile regions of the 
sporophytes of Asteroxylon and Psilophyton , while the vegetative shoots bore more 
or less crowded small leaves. 
The other three plants ( Rhynia Gwynne-Vaughani, Rliynia major , Hornea 
Lignieri) found at Rhynie along with Asteroxylon exhibit still greater simplicity 
of organisation in being not only rootless but leafless. They have been placed 
together in a very natural family called the Rhyniacese which, along with the 
Asteroxylacese, to which Asteroxylon and probably Psilophyton belong, represent 
the distinct class of the Pteridophyta which has been named the Psilophytales after 
its first clearly recognised member. The simple Rhyniacese afford a surprising 
addition to our knowledge of the plant-forms realised in the Vascular Cryptogams. 
With merely differences of detail the plant-body in Rhynia and Hornea consisted 
of (a) a rhizomatous, subterranean portion, bearing absorbent hairs, without leaves, 
and with nothing suggestive of the organisation of a root, ( b ) a dichotomously 
branched, aerial system of cylindrical axes without leaves, and (c) large sporangia 
