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 Equisetiim shows in 

 its axis a structure indicative of a protostelic origin. Jeffrey a has traced 

 the details for E. hiemalc, 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, Calami tes petty cnrensis, 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 Eqiiisetnm, 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 Calawifes, to the protective sheaths of Equisetiim, 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 



1 /..,., p. 171. 



