THE VASCULAR SKELETON 687 
itself as the individual shoot develops: so that the absence of it in the 
young embryo is only an apparent condition secondarily due to correlative 
reduction. 
A protostelic state will functionally serve only a limited vegetative 
system. Starting from relatively small beginnings, as that system enlarges— 
either by continued growth of the axis and multiplication of small leaves, 
or by increase in size of a more limited number of larger leaves—the 
size of the stele becomes proportionally increased : and this may be seen to 
be the case either in the individual life, or it may be illustrated by com- 
parison of different related species or genera. But there is a limit to the 
size which a solid protostele may attain with functional advantage, and as 
a matter of fact when large size is approached the protostelic character is 
sacrificed, and amplification begins, which may take several distinct forms. 
The simplest of these, as it is also the most general, is medullation. It 
is illustrated in many of the dendroid Lycopods. While certain of the 
early species of Lepidodendron have a solid protostele (LZ. rhodumuense), 
Lepidodendron selaginoides (Fig. 176, p. 336) has the centre of its stele 
composed of parenchyma and tracheides intermixed: others again, and 
especially later species, show a parenchymatous medulla (Z. Harcourtit, 
Fig. 174), derived by conversion of the central region of the wood into 
pith (Fig. 175). The result of a similar change is seen in S¥gz//aria, but 
with a further progression to the breaking up of the ring of xylem sur- 
rounding the pith into separate strands (p. 337). This condition is very 
nearly approached in Lepidostrobus Brownit (Fig. 175), and finds an 
interesting parallel also in the upper part of the shoot in Zmesipteris 
(Fig. 234): in the latter a sclerotic tissue takes the place of the pith in 
the lower regions of the axis, but is replaced by thin-walled tissue above. 
Such cases prepare the way for the view of the stelar structure adopted 
above for Zguzsetum (pp. 386-392); the condition there seen appears to 
be the result of carrying the medullation of the stele to an extreme. 
Turning to the larger-leaved forms, the condition seen in the Ophio- 
glossaceae (p. 459) may be referred in origin to a centroxylic protostele ; 
it appears in fact as a medullated monostele with opening of the xylem 
at departure of the leaf-traces. Lastly, the series of Osmundaceous fossils 
described by Kidston and Gwynne-Vaughan (p. 539) shows most convincingly 
how their vascular structure is also referable in first instance to the 
medullation of a protostele, with ultimate breaking of continuity of the 
xylem-ring. It is thus seen that in a number of Pteridophytes, and probably 
along quite distinct phyletic lines, a progression may be traced from a 
primitive protostele to a state of medullation, and in some cases even to 
the disintegration of the remaining xylem-ring into distinct strands. This 
progression may even be followed in the successive stages of the individual 
life, which are accordingly held as further evidence of the phyletic story. 
Another modification of the protostele, which probably has an importance 
in interrupting the continuity of an enlarging mass of xylem, is seen in 
