57 
The Botryopteridece. 
though we do not yet know if it contains spiral elements. The 
leaves are not very closely set and the diameter of the leaf- 
trace is not much less than that of the stele. The smallest elements 
of the leaf-trace are situated on its adaxial border and are prob¬ 
ably continuous with those in the centre of the stele, though they 
may apparently run down for some little distance below the insertion 
of the trace before fusion occurs. 1 Here we have an endarch 2 
structure ol the stele and leaf-trace. The protoxylem of the trace 
may be seen to segregate into three groups and these become the 
points of the cv-shaped petiolar strand characteristic of the genus 
(Fig. 5.) The branching of this strand is somewhat obscure, and 
Fig. 5. liotryopteris forensis. From Grand ’Croix. Outline of 
xylem of petiolar strand. Univ. Coll. Collection, K 4. x 13. 
we cannot at present pretend to explain the origin or significance 
of its peculiar form, though it is likely that its relation to the 
branching of the frond will turn out to be of the same general 
character as that obtaining in Zygopteris, which we must presently 
discuss. 
There are grounds for believing that the endarchy of the 
Botryopteridean stele is a primitive character. A comparative 
consideration of the evolution of the multicellular axis among 
Algae and Bryophytes leads to the conclusion that there is a 
fundamental tendency to develop elongated, relatively passive, 
tissue in the centre of the axis. This tissue is used for conduction 
where the construction of the plant requires it, and thus the 
primitive “ protostele ” originates. If this strand is sufficiently 
bulky and its elements do not develop simultaneously, the tendency 
for the more central elements to mature first will still hold, and 
thus, when there is differentiation between proto- and meta-xylem, 
1 Our information on these points is not at present at all full. 
Dr. Scott is, however, understood to be now working at 
the Botryopterideae, and no doubt he will soon clear up 
many doubtful points in their anatomy. 
2 The term “ endarch ” applied to a leaf-trace means that the 
protoxylem is situated on the adaxial border of the xylem- 
strand, which is conceived of as representing part of the stele 
of the stem. 
