SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 843 
the case of some other Early Devonian plants, that are, however, very imperfectly 
known, we may with fuller knowledge have to recognise such combinations of 
characters as would break down any sharp distinction between the Algae and the 
simplest Pteridophytes. This does not, however, appear to us to be the case for the 
Rhyniaceae, and still less for the more complex Asteroxylaceae. The combination of 
characters which the Rhynie plants possess (independent sporophytes, epidermis with 
cuticle and stomata, vascular system with characteristic tracheides, thick-walled 
sporangia with cuticularised spores) justify our recognising them as Vascular 
Cryptogams or Pteridophyta. The differences between them and other Pteridophyta 
are those of degree only, and are sufficiently expressed by placing them in a distinct 
class (Psilophytales) of this group. It can, however, be said that the members of 
this class are the Pteridophyta, which are most readily comparable with the Algae ; 
this holds most strongly at present for Hornea, in which stomata have not been 
demonstrated. 
Terms implying actual relationship by descent have been deliberately avoided 
in the comparisons which have been made above between the plants described in 
this series of papers and Algae, Bryophyta, and other Pteridophyta. The suggested 
comparisons are based on the observed facts of form and structure. In expressing 
the comparisons in terms of possible phyletic relationship we enter into a more 
speculative region. The characters in question are not such as to establish beyond 
question the course of evolution either as regards the divergent origin of the more 
specialised Pteridophyta from a common source, or, on the other hand, the direct 
derivation of the simplest Pteridophyta from any particular Algal group. 
It has been pointed out in the discussion in an earlier part of this series of papers 
(Part III, p. 673) that the characters of Asteroxylon and Psilophyton , as representing 
a synthetic type, are consistent with such a divergence of the great classes of 
Pteridophyta from a common type as had been suggested by a number of investi- 
gators. The Psilotacese would be of all existing plants the least altered from this ; 
the Lycopodiales, while specialised in various directions, would have preserved 
many features found in the archaic type ; the Fern type, if the large leaves are 
regarded as specialised branch-systems, would have been more profoundly modified. 
It is perhaps better to regard the point of divergence as represented by plants 
somewhat simpler than the Asteroxylaceae in that they did not possess definite 
small leaves ; this assumption is rendered a natural and legitimate one by the 
fact that in the Rhyniaceae we have Vascular Cryptogams which actually exhibit 
this simplicity. 
The Rhyniaceae lend themselves naturally, on the other hand, to comparison with 
Algae, as regards external form, the absence of definite roots, and especially in their 
large sporangia having arisen more or less evidently by the transformation of the 
tips of certain branches of the thalloid plant-body. These general resemblances, as 
V^as pointed out (Part II, p. 622), “ are consistent with the Rhyniaceae finding their 
