34 
A. G. Tansley. 
and integration which can be clearly traced as operating within 
small groups in the thallus of the Algae and Liverworts and in the 
fronds of Ferns. 
The view which we take on this general question concerns the 
theoretical basis of this course of Lectures very closely. No 
student of the vascular morphology of Pteridophytes can have 
failed to be struck by the difference between the relation of the 
leaf-trace to the stem stele in the Ferns and that obtaining in the 
Lycopods. In the former the leaf-trace appears to “ take off ” part 
of the stele of the stem, in the latter the bases of the leaf-traces 
form on the exterior of the cylinder a confluent system which 
appears to be more or less separate from the metaxylem forming 
the interior. Some years ago, in discussing the significance of the 
relation of stem-stele to leaf-traces in the moss-stem, I was led, in 
conjunction with my wife, 1 to the view that the leaf-trace system in 
the Mosses was primitively distinct from the stem-stele, and 
that consequently the highly developed central cylinder of the 
Polytrichaceae was to be regarded as of dual origin. The analogy 
of this cylinder and its peripheral zone of narrow hydroids with the 
stele of the Lycopod (particularly of the Lepidodendroid) stem is 
very striking and led to the speculation that the latter might have 
a similar dual origin. 
The possibility of such a dual origin was recognised as 
depending on whether a definite vascular structure appeared in the 
axis before the appearance of appendicular organs with a vascular 
supply. If such appendicular organs are derived from branch 
systems or from branchlets of a primitive thallus specialised for 
assimilating functions, the appearance of an independent vascular 
system in the main axis is in the highest degree unlikely, and it 
would appear that the characteristic attachment of the leaf-traces 
in the microphyllous groups must be regarded as a secondary 
character dependent on the reduction or on the originally small size 
of the “ foliar branches ” (leaves) and the necessarily great relative 
increase in the size of the axis consequent on the crowding of these 
branches. 
We are in fact faced with alternatives from which there 
appears to be no escape. Either the leaves of Pteridophytes are 
all homologous, in which case the megaphyllous character and 
dichotomous nature of the leaves of most primitive forms leads to 
Tansley and Chick, Notes on the Conducting System of 
Bryophyta. Ann. of Bot. 15, 1901, pp. 32-36. 
1 
