432 Boiver.—Studies in the Phytogeny of the Filicales. 
young stem, as seen in transverse section, that the first lignification of the 
xylem takes place ; prior to that event it is easy to make out the outlines 
of the dictyostele, and to recognize where the initiation of the foliar traces 
will occur ; these are first indicated by the appearance of a group of 
tracheides in a mesoxylic position. That the position is originally meso- 
xylic is clearly seen in Figs. 8 and 9, the former of which shows the very first 
lignification of two tracheides, with the young tracheides of the metaxylem 
still thin-walled, but easily recognized ; the latter represents an older state 
with metaxylem surrounding the smaller protoxylem-elements. The reason 
for being quite explicit on this point is that, on passing upwards from the point 
where the protoxylem is first recognizable, the mesoxylic position is soon 
lost; the indentation of the margin of the stele appears as above described 
on the side adjoining the pith, and the metaxylem shifts right and left, with 
the effect that the protoxylem appears as though it were endoxylic ; but 
that is not its original position : in the first instance it is mesoxylic. Oppo¬ 
site each indentation of the stelar tube the band of sclerenchyma which 
subtends each leaf-trace may early be recognized, first as a projection from 
the large central core, and later as a separate band, which passes outwards, 
taking a position in the angle of the leaf-trace; these points are readily 
observed in Fig. 7. The central protoxylem of the leaf is usually a single 
group (P. pycnophylla), though cases have been seen in which two groups are 
present from the first, separated by a median tract not yet lignified 
(. P . glaucci). About the same time a discontinuous fringe of protophloem 
makes its appearance on the abaxial side of the stelar tube. Passing 
upwards along the stelar tube the indentation opposite each leaf-trace 
deepens, and the protoxylem widens out ; sooner or later it divides into 
two strands, disposed right and left, a division which is usually completed 
before the margins of the V-shaped leaf-trace are abstricted from the 
stelar tube. These two protoxylem strands diverge further as the leaf-trace 
passes out, and run directly into the two abaxial angles of the W-shaped 
meristele of the petiole, as described above (Text-fig. 1). About the same 
level as this forking of the first protoxylem-group, or a little later, there 
appear two other protoxylem-groups at some distance right and left of it; 
they do not always appear simultaneously, and in point of origin they are 
quite distinct from the median protoxylems; they pursue their course 
upwards into the margins of the foliar strand. In the stelar tube itself 
meanwhile, apart from the protoxylem-groups thus noted, no definite proto¬ 
xylem is apparent, but the lignification which begins sporadically, and 
chiefly towards the exterior of the stelar tube, soon becomes general. 
(Compare Fig. 7 for the position of the protoxylems in the whole section.) 
In P. semicordcita the two protoxylems at the median region of the 
leaf-trace continue their separate course upwards into the leaf-base, and 
occupy the angles of the W-shaped strand ; the abstriction of the strand 
