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2 A. G. Tansley. 
as typical. A brief sketch of the subject from the point of view of 
these lectures must be attempted here. 
At the beginning of the course we saw reason to believe that 
the primitive Filicinean leaf was a specialised branch of a primitive 
thallus, a branch originally of the same morphological nature as the 
axis from which it arose. On such a hypothesis, the vascular strand 
of the branch would be primitively of the same nature as that of 
the main axis. Though modifications of the vascular system of 
the upper part of the foliar branch might be expected to occur at 
once in relation to the distribution of the assimilating surface, the 
insertion of the foliar vascular system upon that of the cauline axis 
ought to show signs of its original nature. As was shown in the 
second and third lectures, we do actually find, in the Botryopterideae 
and Hymenophyllaceas, the base of the leaf-trace exhibiting a 
structure which may be interpreted in this way. In the former 
family we find that it is nearly or quite iso-diametric, though its 
diameter is very much less than that of the stele, a mechanical 
necessity where the axis bears crowded leaves. In the Hymeno- 
phyllaceae the base of the leaf-trace (at least in many cases) is 
identical in structure with that of the cauline stele. The structure 
of the petiolar vascular system itself is very varied in the case of 
the Botryopterideae, whose members bore complex and varied 
fronds. In the Hymenophyllaceae, where the demands for water- 
conduction are much less, owing to the prevalent hygrophytic nature 
of the frond, the petiolar strand remains much simpler, and in 
certain types, which we may regard on other grounds as relatively 
primitive, it very largely retains stem-structure. 
The relations of the peculiar petiolar strands of Botryopterideas 
were sufficiently dealt with in the second lecture, so far as the data 
at present available permit. 
When we come to consider the series of modern Lepto- 
sporangiate Ferns, certain broad facts are at once obvious. As we 
pass from the protostelic to the solenostelic forms we find the leaf- 
trace taking on a corresponding expansion. Boodle has attributed 
the origin of the solenostele, with its increased diameter, to the 
necessity for accommodating the insertion of broader leaf-traces, 
thus implicitly recognising the fact, which seems to be indisputable, 
that the leaf-trace leads, and the stele follows, in the course of 
evolution. Let us consider, a little more in detail, the probable 
course of events. The increased diameter of the leaf-trace will 
necessarily tend, it would appear, to lead to a failure to develop 
