SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 847 
within the evolution of the vascular land-sporophyte and are not obliged, to account 
for them by the transformation of organs already present in complex ancestral Algae. 
We may, however, as above, trace comparisons with the corresponding region of the 
Algal plant-body. 
( b ) Nature of the Leafy Shoot ; Large-Leaved and Small-Leaved Shoots. 
The simplicity of organisation of the Rhyniacese establishes the fact that there 
were early Vascular Cryptogams in which the aerial parts consisted of a thalloid 
branch-system. This is in favour of the view that the differentiation of a shoot, 
with axis and leaves (though attained in the higher members of several groups of 
Algae) was independently elaborated in the early land-plants. It is of course possible 
that in some cases a more completely differentiated land-plant might have been 
directly derived from an Algal form with stem and leaves, but the existence of these 
simpler vascular plants renders it unnecessary to assume this without evidence. 
They bring the origin of the distinction of stem and leaf in a shoot within the 
evolution of the land sporophyte, although a corresponding differentiation is attained 
in various Algse and in the gametophyte of Bryophyta. Without entering into a 
full consideration of the problems of the relation of stem and leaf in the shoot, we 
have to note the comparisons which may be made with the Rhynie plants and how 
these bear on this much-discussed problem. 
In the first place it may be noted again how closely the thalloid plant-body of the 
leafless Rhyniacese can be compared with that of many Algse. The comparative 
problem of the origin of shoots with small or large leaves subordinated to a stem 
thus presents itself in both the Algae and the Vascular plants. Further insight as 
regards either group will bear on the problem in the other, quite apart from genetic 
relationship. 
The question of the comparison of a shoot in which the stem is clothed with 
small leaves with the leafless thalloid axis may be taken first, since it is presented 
to us directly by the co-existence of Asteroxylon with the Rhyniaceae. In Asteroxylon 
a gradual passage from the leafless rhizome into the shoot and the appearance at 
first of small scale-leaves without vascular bundles can be followed ; this is essentially 
similar to what is seen in Psilotum and Tmesipteris. The insertion, and in the case 
of the living forms the development, of these leaves appear lateral. Certain features 
of the shoots of the Psilotacese, especially of Tmesipteris, indicate, however, that this 
is not inconsistent with an interpretation of such small leaves on the lines of their 
being specialised branches subordinated to, and overtopped by, the further growth 
of the shoot. There is no clear evidence of this, however, and all that can be said 
in its favour at present is that the dichotomous branch system of the Rhyniacese 
provides us with the sort of starting-point from which small leaf-like branches 
originate in this way in certain Algse. 
Between the leafy shoots of Asteroxylon and the leafless, dichotomous thalloid 
TRANS. ROY. SOC. EDIN., VOL. LII, PART IV (NO. 32). 131 
