30 A. G. Tansley. 
which there is the strongest independent evidence, that the Equise- 
tales and Sphenophyllales are comparatively closely allied, we can 
hardly escape from the conclusion that their common ancestors had 
dichotomously branched leaves. The Ferns, as has been said, are 
almost certainly primitively megaphyllous, and their leaves, typically 
pinnately branched in the great majority of forms, nevertheless 
bear clear traces of derivation from a dichotomous type. Potonie 1 
has shown this in regard to a number of Palaeozoic fern-like fronds, 
many of which have now been assigned to Pteridosperms, but the 
evidence from undoubted Ferns is to my mind just as convincing. 2 
Thus there is very good reason for supposing that the common 
ancestors of these groups were megaphyllous, and that dichotomous 
branching was characteristic of their leaves. The fact that 
monopodial branching is very easily derivable from dichotomous 
makes the occurrence of the former in the Sphenophyllales and 
Equisetales quite consistent with supposing that dichotomy 
was prevalent in the stems also of these hypothetical ancestors. 
Only in the Lycopodiales (excluding Psilotaceae) do we find no 
suggestion of megaphylly and no trace whatever of dichotomy 
in the leaves. If they had a common origin with the other 
Pteridophytes it might seem that we must suppose the Lycopods 
to represent a further stage of reduction than the other strobiiate 
forms, and this is a perfectly possible view. The simple sporophyll 
with the sporangium borne on its ventral (upper) face might 
conceivably be the last term in a reduction-series which started 
with a complicated sporophyll like Cheirostrobus and passed through 
a stage such as that represented by the Psilotales, while the 
vascular system, as is well known, has a strong resemblance in 
Lepidodendron , Psilotum and Cheirostrobus. 3 On the other hand 
it must be admitted that the Lycopods show no evidence of being 
anything but simply organised primitively microphyllous forms, 
unless it be found in the mysterious ligule, and this dubious structure 
does not occur in the presumably more primitive homosporous 
types. 
1 Potonie. Die Bezichung zwischen dem echtgabeligen und dem 
fiederigen Wedel-Aufbau der Farnen. Bcr Deutsch. Bot. Ges., 
13, 1895, p. 244. 
2 I hope soon to publish a consideration of the evidence for 
regarding the fern-frond as derived from a dichotomously 
branched structure. 
* Lang’s “ Prothallus provisionally referred to Psilotum ” (Ann. 
of Bot., 18, 1904), if it should prove actually to belong to that 
plant, would furnish a strong piece of evidence in favour of the 
close relationship of Psilotum with the Lycopods. 
