846 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
distinct, but in other specimens more or less confluent. A single, branched aerial 
axis sprang from each lobe. The features of this type of rhizome, though on a larger 
scale, have so much in common with the protocorm of the young embryonic plants 
of some species of Lycopodium that in the description it has been spoken of as the 
protocormous type of rhizome. 
This resemblance is of great interest on account of Treub’s view that the proto- 
corm of Lycopodium was the persistent relic of an organ, by which the sporophyte 
became physiologically related to the soil before roots had evolved. This compari- 
son has been briefly but sufficiently treated of in Part II (p. 620). It is only 
necessary to repeat that the underground parts of Hornea appear to be consistent 
with and to support Treub’s view. Just as in some species of Lycopodium a single 
leaf or protophyll springs from each lobular portion of the protocorm, so in Hornea 
a single aerial branch system occupies the corresponding position. This comparison 
would involve a correspondence of the protophylls or leaves of Lycopodium with 
the thalloid axes of Hornea. 
The rhizome of Rliynia , and the more extensive rhizome-systems in Asteroxylon 
and the existing Psilotacese, would then correspond morphologically to specialised 
extensions of the protocorm. This, as Hornea shows, would have been the basal 
region of the originally thalloid plant-body. 
On the other hand the subterranean rhizome-system of these rootless plants may 
be compared with the attaching region of certain Algse. The comparison is closer 
with those Algse in which a rhizome-system ramifies in sandy soil and gives off erect 
assimilating branch systems, than with the hold-fasts that attach many Algse to a 
rocky substratum.* 
Treub’s view of the protocorm involved the idea that the embryo of the early 
Pteridophyta was dependent on the prothallus and required to become physiologically 
independent. This is so in the ontogeny and may, as he assumed, have occurred in the 
phylogeny, though this is not a necessary assumption. There is nothing inconsistent 
with this in the morphology of Hornea. The repetition of the protocorm, though 
primarily an embryonic organ, in individuals isolated in vegetative reproduction is 
met with in Lycopodium, t If this view of the original nature of the protocorm and 
protocormous rhizome is entertained it should be noted that it affects, though it does 
not destroy, the comparison with the attaching bases of thalloid Algse. These 
various comparisons cannot be unravelled so long as we are completely ignorant of 
the history of the association between gametophyte and sporophyte in the ancestors 
of the Pteridophyta. 
From the simplicity of the rhizome-system in the Rhyniacese it would seem that 
we are justified in assuming the appearance of specialised roots to have taken place 
* Since we are dealing with general comparative morphology, and not with relationship, another more distant 
comparison may be made with the leafless rhizome-system of the gametophyte of some Liverworts {e.g. Calobryacete). 
f The repetition of the specialised tuberous protocorm of Phylloglossum may also he compared here. 
