SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 619 
Comparisons with Pteridophyta. 
In comparing the Rhyniacese with other Pteridophyta we are limited by the 
fact that we are dealing with the simplest types of Vascular Cryptogams known. 
All the other Vascular Cryptogams, including such archaic forms as Asteroxylon 
and Psilophyton, are more highly organised than these rootless and leafless plants, 
the body of which might as well be termed a cylindrical, branched, vascular thallus 
as a stem. Thus no detailed comparison with the organisation of the whole plant 
in the higher Pteridophyta is possible, and what we have to say on this question 
will find its proper place in dealing with Asteroxylon. 
We can, however, recognise correspondences with particular structures in some 
other Pteridophyta, and it is not without interest that some of these affect features 
which have always appeared aberrant in the plants possessing them. This applies 
especially to the rootless rhizome, which may be first briefly discussed. 
The only rootless Pteridophyta at present known that need be considered 
are the Psilotacese.* Both Psilotum and Tmesipteris have rhizomes embedded 
in the substratum and attached to this by non-septate rhizoids. It is not at 
present known whether a root is absent from the young sporophyte of Psilotum 
in its development from an embryo, but none is present in plants arising 
from the minute bulbils t formed on the rhizome. No root is present in the 
young plant of Tmesipteris \ borne on the prothallus. The resemblance between 
the rhizomes of Rliynia and of the Psilotacese is close, and in neither case is 
there anything suggesting that the rootless condition is other than primitive. 
Although more complicated in the aerial shoots, the Psilotacese appear to have 
retained the simplicity of the subterranean region characteristic of the most 
primitive Vascular Cryptogams. 
The rhizome of Hornea, on the other hand, appears to correspond most 
closely to that remarkable and much discussed region of the young plants of 
certain species of Lycopodium and of Phylloglossum known as the protocorm. 
As is well known since the investigations of Tretjb,§ the young plant of Lyco- 
podium cernuum does not at once initiate a shoot and root, but enlarges outside 
the prothallus (to which it is attached by a small foot) to form a tuberous 
parenchymatous body. This protocorm is attached to the soil by numerous 
rhizoids, and bears on its upper side a number of leaves in no apparent relation 
to a stem-apex ; these leaves are termed protophylls. Later a stem bearing 
spirally arranged leaves develops, and the first root forms at the base of this 
shoot. The corresponding stage in L. later ale and L. ramulosum, two New 
• Salvinia and some of the smallest Hymenophyllacese may be mentioned, but the rootless condition in them 
appears to be due to reduction. 
+ Solms, Annales du Jardin Bot. Buitenzorg, vol. iv, 1884, pp. 139-190. 
f Holloway, Trans. New Zealand Institute, vol. 1, 1917, pp. 1-44. 
§ Treub, Annales du Jardin Bot. Buitenzorg, vols. iv and viii. 
TRANS. ROY. SOC. EDIN., YOL. LII, PART III (NO. 24). 
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