672 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
that has emerged and been further developed of late years. On this view the plant 
of the Vascular Cryptogams is comparable with, and derived from, a thalloid plant- 
body such as we meet with in a number of the higher Algse. In its modern form it 
was first developed by Potonie, but it is especially in the form it assumed in the 
speculations of Lignier that it applies to Early Devonian plants. This investigator 
indeed made special use of Dawson’s reconstruction of Psilophyton in formulating 
his theory. 
It will avoid unnecessary repetition if we give a free summary of those salient 
features of the theoretical views of the late Professor Lignier, and of the discussion of 
them in relation to the Devonian flora by Dr Halle, which appear to us to bear on 
the consideration of Asteroxylon. The reader is referred to the full statements of 
the views of these investigators as given in the literature cited below.* 
The primitive type of plant-body in the Pteridophyta is supposed by Lignier to 
have consisted of erect, dichotomously branched, cylindrical cauloids bearing small 
flattened phylloids and with terminal sporangia. The later origin of roots is 
supposed to have taken place by the transformation of certain subterranean cauloids. 
Psilophyton is instanced as exhibiting most clearly this primitive type. Among 
existing plants the Psilotales are regarded as differing chiefly in the localisation 
of the sporangia on reduced branches. From such a primitive type of plant-body 
the Lycopods are derived by further specialisation, their leaves being regarded as 
phylloids. The leaves of the Filicales, on the other hand, are of a different origin, 
being derived by the specialisation of certain cauloidal branch-systems. In the 
more primitive forms ( Dimeripteris , Stauropteris) the bi-valvular sporangia were 
borne terminally on the ends of cauloids, as they were in Psilophyton. The flattened 
leaf-blades arose later by agglomeration of the cauloids into pinnules, and the 
sporangia, at first marginal, then became situated on the lower surface. 
Lignier’s view is susceptible of further simplification by Tansley’s suggestion f 
that the leaves of the Lycopodia] es may have been derived by “ foliar specialisation 
of short undivided branchlets of the thallus, instead of 'whole branch-systems as 
in the Filicinean type.” This does away with the sharp distinction of phylloids 
and cauloids. 
The general view provides us with a conception of a common origin of the 
divergent types of sporophyte represented by the Psilotales, Lycopodiales, and 
Filicales. + 
* Ligniek, M. 0., “ Equise tales et Sphenophyllales : Leur origine filicineene commune,” Bull. Soc. Linn. 
Normandie, ser. 5, vol. 7, 1903, p. 93. “Essai sur l’Evolution Morphologique du regne vegetal,” Gomptes de l’ Assoc. 
Frangaise pour VAvancement des Sciences, 1908, p. 580. “ Le Stauropteris Oldhamia Binney et les Coenopteridees a la 
lumiere de la theorie du meriphyte,” Bull. Soc. Bot. de France, t. 59, 1912, p. 1. 
Halle, T. G., “ Lower Devonian Plants from Roragen in Norway,” Kungl. Svenska Vetenskapsakad. Handlingar, 
Bd. 57, p. 1. 
y Lectures on the Evolution of the Filicinean Vascular System, p. 9. (Reprint, 1908.) 
| For the present purpose it is unnecessary to complicate the discussion by entering into the consideration of the 
Equisetales qnd Sphenophyllales. , 
