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A. G. Tansley. 
give rise to endarch structure. A further expression of the same 
tendency is to be found in the early maturation of the pith in the 
meduilated stems of vascular plants generally. According to our 
basal hypothesis the primitive assimilating frond should also 
possess central protoxylem in its vascular strand. Apparently we 
do not get this hypothetically primitive condition realised in the 
Botryopteridese, which certainly possessed elaborate and highly 
evolved fronds. We shall see in the next lecture that the central 
protoxylem of the petiolar strand in certain Hymenophyllaceae 
may possibly be interpreted in this way. There is no doubt, 
however, that if endarchy is primitive in the protostelic and 
immediately derived types, it very rapidly gives place to mesarchy 
and exarchy in response to various demands, and it is not difficult 
to show how this may have occurred. On the other hand it it 
very difficult to conceive of exarchy as original and endarchy as 
derived in such forms as the Botryopterideae, and this consideration 
must be allowed due weight in formulating our general theory. 
In Zygopteris (Fig. 6) we have a more elaborate type of stele. 
Fig. 6. Zygopteris Grayi. Diagram of the stele of the stem, c.p., central 
parenchyma, with numerous narrow tracheids, which are indicated, mx., 
metaxylem : the narrow peripheral tracheids are also indicated. On the outer 
edfte of the xylem thin-walled tissue remains in places : this represents phloem 
(ph.) and pericyclic tissue. The numbers refer to leaf-traces. After Scott. 
It has a lobed instead of a circular outline, and the ends of the 
lobes are successively detached as leaf-traces. I he centre of the 
stele no longer consists of a solid mass of tracheids, but is filled 
with delicate parenchyma intermingled with narrow tracheids 
forming a connected system which extends into the arms of th 
