622 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
Comparisons with Alg^e. 
Without implying direct derivation of the Pteridophytes from any more complex 
Algae or actual relationship with any known Algae, it is impossible to overlook the 
resemblance in habit between the plant of Rhynia and Hornea and some Algae. 
The comparison is closest with some Red Algae, which show a distinction of a rhizome- 
like basal region and cylindrical dichotomously branched axes and have their 
tetraspores developed within the more or less altered ends of some of the branches. 
As examples, Furcellaria fastigiata and Cordylecladia ( Gracilaria ) erecta may be 
mentioned. Such resemblances, though they may be superficial, indicate the need of 
caution in deciding whether impressions of early Devonian plants of this habit were 
Pteridophyta or Algae. Formerly a number of these plants, which we now know 
to be Vascular Cryptogams, were classed as Algae or Fucoids. We anticipate that 
others will prove really to be Algae, and that the comparison between them and 
the Pteridophyta of the same early geological age will be of peculiar interest. 
A more special question concerns the equivalent of the sporangium of the 
Rhyniaceae in the Algae. The tetrads of spores in certain Algae and the tetrads in 
the land plants appear to correspond strictly. In some Red Algae we find the spore- 
mother-cells (or tetrasporangia) confined to certain special branches, or to the tips of 
ordinary branches and there situated below the surface. Such special tetraspore- 
bearing branches are known as stichidia. It is one possible view of the sporangia 
of Pteridophyta to regard them as the equivalents of such stichidia. The general 
resemblance and the correspondence in regional anatomy which can be traced 
between the sporangium of Hornea and the less differentiated sporangia of Rhynia, 
on the one hand, and the tetraspore-containing branches of such Algae as Cordyle- 
cladia, Furcellaria, Gigartina, and some Rhodomelaceae on the other, supports 
this way of regarding the sporangia of these early Pteridophyta. 
We are here probably dealing with a homoplastic similarity which is an expression 
of general morphology rather than of relationship. The whole question is too wide 
to be appropriately treated in this place, but the preceding remarks will suffice to 
indicate the bearing of the new facts upon it. In our ignorance of the complete life- 
histories and the forms of the sexual and asexual generations, both of the ancient 
Algae and of the simplest Pteridophytes, any suggestion of relationship would be 
dangerous and is not implied in the above comparisons. The facts are, however, 
consistent with the Rhyniaceae finding their place near the beginning of a current of 
change from an Alga-like type of plant to the type of the simpler Vascular Cryptogams. 
Conclusion. 
If we review the comparisons we have been led to make, it will be evident that 
all the features of Rhynia and Hornea point to the Rhyniaceae illustrating an early 
