SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 845 
sporophyte by transformation of an Algal plant-body, but are equally consistent with 
such ancestral Algae having possessed an antithetic alternation of generations. 
They do not, however, support the detailed suggestions as to methods of advance 
made by either of the opposed theories, e.g. on the one hand the relegation of the 
differentiation of stem, leaves, and roots to the Algal ancestry, and on the other 
intercalation of the sporophyte by progressive sterilisation. If such detailed 
assumptions as these are put aside there is no longer the antagonism between the 
antithetic and homologous origin of a sporophyte that at one time seemed to hold. 
The simple sporophyte of Rhynia or Hornea was part of an antithetic life-history, 
while its organisation can be properly treated as homologous with the plant-body as 
realised in both the sexual and spore-bearing stages of many Algae. 
The important change in the working position of comparative morphology that 
has come about with the advance of our knowledge, both of Algae and of extinct 
Vascular Cryptogams, is that a common consideration of the differentiation of the 
plant-body in Pteridophyta, Bryophyta, and Algae is permissible and desirable. We 
may leave questions of phylogeny on one side, with the recognition that the 
characters of the Psilophy tales are such as on the one hand to bring these three 
groups of plants closer together, and on the other to enable us to form a clearer 
conception of a divergence towards the other classes of Pteridophyta and the land- 
plants generally. 
While the exact methods of advance, whether these implied actual genetic 
relationship or were parallel developments, must remain open questions, progress 
may be possible with such an extended comparative morphology. The bearing of 
the plants under consideration on some problems of general comparative morphology 
may, therefore, be referred to in conclusion. These remarks, which must necessarily 
be brief, may be divided into those on the origin of roots and the morphology of the 
protocorm, the origin of the leafy shoot and the relation between large-leaved and 
small-leaved types of this, the vascular system, and the nature and original position 
of the sporangia of Pteridophytes. 
{a) The Origin of Roots ; Morphology of the Protocorm. 
The absence of definite and characteristic roots is a common feature of all the 
Rhynie plants ; as a presumably primitive condition this is only retained by the 
Psilotacese among existing Vascular Cryptogams. In Asteroxylon , as in the 
Psilotaceae, the extensive rhizome system suggests the possibility of the derivation 
of roots by its further modification, and the occurrence of exogenous roots in some 
'Lycopodiales further narrows the gap. 
The subterranean region in the Rhyniacese was less extensive than in Asteroxylon 
even when, as in Rhynia , it consisted of cylindrical branched axes resembling the 
stems in general structure, but bearing rhizoids. It was especially interesting in the 
case of Hornea on account of its tuberous lobed nature, the lobes being often rather 
