I06 THE MORPHOLOGY OF PTERIDOPHYTES 



many bracts (according to an early description by Williamson 

 and Scott^^, but according to Hickling,*^ the same number). 

 Perhaps the most interesting feature shown by this species is 

 the course taken by the vascular bundle supplying the 

 sporangiophore. As illustrated in Fig. i6I, it travelled up in 

 the cortex of the cone axis to a point about midway between 

 the bracts, and then turned downwards, before entering the 

 stalk of the sporangiophore. To those morphologists who 

 regard vascular systems as highly conservative, this impUes 

 that Palaeostachya must have evolved from some ancestral 

 form in which the sporangiophore stood midway between 

 the bract whorls, as in Calamostachys, and that during the 

 'phyletic sUde' the vascular supply had lagged behind. 

 Palaeostachya Andrewsii showed the same feature, but in 

 P. decacnema the sporangiophore bundle took a direct 

 course. One concludes, therefore, that this last species is 

 more advanced than the others in this respect. In P. 

 Andrewsii, the numbers of sporangiophores and bracts in a 

 whorl were twelve and twenty-four respectively, while in 

 P. decacnema they were usually ten and twenty. 



The above brief review of the Calamitales brings out some 

 interesting evolutionary trends, which are paralleled very 

 closely in the Lepidodendrales. Thus, the production of 

 increasing amounts of secondary wood was accompanied, 

 in both groups, by a reduction of the primary wood, of 

 which the centripetal metaxylem was the first to go, being 

 replaced either by pith or by a central hollow. At the same 

 time, there was a trend in the fertile regions from a 'Selago 

 condition' to a compact cone, in which the sporangia were 

 protected by overlapping sporophylls in one group and by 

 bracts in the other. Then, having reached their zenith to- 

 gether, both groups became extinct at about the same time. 



Equisetales 



The only representatives of the Sphenopsida that are alive 

 today belong to the single genus Equisetum and, of this, only 



