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 (L. rhodumnense), 

 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 (L. Harcourtii, 

 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 Sigillaria, 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 Brownii (Fig. 175), and finds an 

 interesting parallel also in the upper part of the shoot in Tmesipteris 

 (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 Equisetum (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 



