ANATOMY 391 
as in other Equisetales, form the inner limit of the wood; but xylem of 
a considerable thickness, and consisting of typical tracheides, extends into 
the pith on the inner side of the canal, which is thus completely enclosed 
by the wood. Hence, starting from the spiral tracheides of the protoxylem, 
there was here a considerable development. of the xylem in a centripetal 
as well as a centrifugal direction. This appears to be the first case of 
centripetal wood observed in a Calamarian stem; it serves to furnish a 
new link between the Palaeozoic Equisetales and the Sphenophyllales, and 
through them also with the Lycopods. 
The question remains whether the young plant of Zguzsetum shows in 
its axis a structure indicative of a protostelic origin. Jeffrey! has traced 
the details for Z. Azemadle, and finds that the central cylinder of the first 
shoot makes its appearance as an unbroken tube of reticulated tracheides. 
There are no protoxylem elements, although the internal tracheides are 
formed first. The primitive axis, in fact, starts out with a similar organisa- 
tion to that which is subsequently found to recur in the nodes. These 
facts, though not in themselves conclusive, would tally well enough with 
an origin of the shoot from a protostelic ancestry. 
The facts and arguments contained in the preceding pages clearly 
indicate the line of comparison of the stelar state of the Equisetales 
with that of the other Pteridophytes.- The axis is monostelic, as in 
other primitive forms. It presents the appearance of a mere attenuated 
remnant of the probable archaic state of the protostele. Comparison 
makes it probable that in place of the solid xylem-core, which is seen 
in other phyla to be the primitive condition, the central part has become 
parenchymatous: in the early fossil, Calamztes pettycurensis, the change 
had advanced so far as to reduce the volume of the xylem, though a 
centripetal remnant still persisted, and serves to indicate the probability 
of a protostelic origin, comparable to that condition seen in some 
Lycopodiales and in the Sphenophyllales. In the ordinary Calamites, 
as well as in Eguisetum, the change has advanced so far that only minute 
remnants of the centripetal wood are to be recognised, and that recogni- 
tion would itself be uncertain were it not for the confirmation brought 
by the fossil from the Calciferous sandstone. But together the evidence 
appears conclusive, and the result is to place the Equisetales, which have 
so long been a structural problem, in line with other strobiloid forms: 
they, like the rest, have probably sprung from a protostelic ancestry. 
Physiologically the changes involved appear as a natural result of life in 
a semi-aquatic and muddy habitat, while the reduction of the leaves 
from effective assimilatory organs as they appear to have been in the 
early Calamites, to the protective sheaths of Zgwutsefum, would also 
harmonise with the anatomical change contemplated. : 
The leaves and the sterile bracts of the strobilus in the Equisetales 
are supplied with simple strands, which call for no special remark. But 
VZiéy pe 171. 
