SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 839 
descriptions of the plants in the three preceding papers of this series, comparisons 
have been made with existing and extinct plants, and some theoretical aspects of the 
facts have been indicated. Although this course involves a little repetition, some 
of the general bearings of these plants on the problem of the origin of the morpho- 
logical construction of land-plants must now be considered. 
As regards the origin of the type of alternation of generations and the balance of 
the sexual and spore-bearing stages in the life-history, the Rhynie plants give no 
information. The plants known to us were clearly the sporophyte generation. The 
development of the spores in tetrads similar to those of other Vascular Cryptogams 
points clearly to the existence of a sexual generation 'or gametophyte. Of the nature 
of this we have no evidence. The absence of evidence is so far in favour of the 
gametophyte having been much simpler and more delicate than the sporophyte, as 
is the case in all the Vascular Cryptogams the life-history of which is fully known. 
It is on the organisation of the sporophyte in the Vascular Cryptogams that most 
light is thrown by the Rhynie plants. Apart from questions of physiological 
morphology, with which we are not here concerned, this is a comparative and 
historical problem. The historical data are necessarily imperfect, and their lack has 
led to much speculation of doubtful value, although opposed speculations have served 
to define aspects of the problem. It is in the additions they make to the ascertained 
historical data that the great importance of the Rhynie plants consists. Without 
entering fully into speculations on the origin of the Pteridophyta and their character- 
istic organisation, it will be necessary to set the new facts in relation to them. 
Existing Vascular Cryptogams may be broadly said to present us with a few main 
types of organisation of the sporophyte. These are the Fern type, the Equisetum 
type, the Lycopod type, and the type of the Psilotacese. The general characteristics 
of these are familiar and need not be stated. It is only necessary to direct attention 
to the facts that all these types agree in having shoots composed of stem and leaves, 
and that in all, except the Psilotacese, the plant-body has definitely characterised 
roots. In the Psilotacese roots are wanting, the aerial stems, with their smaller or 
larger leaves, arising from a system of leafless rhizomes clothed with absorbent hairs. 
The extinct Pteridophyta of earlier vegetations, back to and including the 
Carboniferous and Upper Devonian periods, add, so far as we know, no essentially 
different types of organisation. They make us acquainted with varied and interesting 
forms of the Fern, Lycopod, and Equisetum types, and they add the wholly extinct 
type of the Sphenophyllales. The vegetative body of all these plants is, however, 
composed of root, stem, and leaf, and the comparison of the plants known from the 
Upper Devonian to the existing period does not throw light on the origin of this 
prevailing differentiation of the plant-body. 
Even before the discovery of the material described in this series of papers 
interest was naturally focussed on the plant-remains earlier than the Upper Devonian. 
These were always incomplete, a serious drawback in making comparisons, but on 
TRANS. ROY. SOC. EDIN., VOL. LII, PART IV (NO. 32). 130 
