30 
A. G. Tansley. 
sporophytic plant-body of the Vascular Plants, yet realise pretty 
closely in the one case the life-conditions of simple terrestrial 
microphyllous plants, and in the other those of simple terrestrial 
plants in which certain branches of a generalised thallus are 
tending to become specialised for assimilating functions—plants of 
a type which we have seen reason to believe may have been the 
ancestral type of the megaphyllous vascular forms. These converging 
lines of evidence are full warrant for the doctrine of the primitiveness 
of the simple central type of stele , 1 apart altogether from a priori 
considerations, into which we cannot enter here, but which appear 
to give ample ground for expecting what we actually find, that 
primitive conducting tissue tends to be developed in the centre of 
an axial plant-body. 
The subsequent evolution of the protostele depends very largely 
on the relation to it of the leaf-trace. The leaf-trace itself was also 
no doubt primitively a simple strand of conducting tissue, whose size 
would naturally depend upon the size of the leaf. If the leaf is 
primitively a branch of a dichotomous thallus the departure of the 
leaf-trace from the stele is primitively a forking of the stele into 
two equal or nearly equal branches; if it is a lateral member, 
originally of the same morphological category as the stem from 
which it arises, the branch of the stele may be expected to show 
an identical structure, but a lateral insertion ; and finally if the leaf 
is a relatively small appendage to the stem its trace will be a 
relatively small appendage to the cauline stele, connected directly 
only with the external tissue of the latter. Of course it is con¬ 
ceivable that any one of these conditions might arise in the course 
of evolution from either of the others by a gradual modification 
of the relation between axis and leaf. 
As we saw in the first lecture the available evidence does not 
enable us to decide the question, so far as the Lycopods are con¬ 
cerned, as to whether the last condition is primitive or has arisen 
from a state in which the leaves were more important in relation 
to the stem. But there is some reason to suppose that the 
Equisetales, Sphenophyllales and Psilotales, as we mainly know 
them, are reduced from forms with larger dichotomously branched 
leaves, though we have no sufficient information as to the relation 
between stele and leaf-trace in such forms. 
1 Brebner’s term haplostele is better than protostele when we 
wish to describe this structure without morphological impli¬ 
cation. In some cases no doubt a haplostele is not a proto¬ 
stele, but has arisen by reduction from a more complex type 
of structure. 
