276 THE ANATOMY OF WOODY PLANTS 



single large central tracheid in which the four masses of protoxylem 

 unite. In the smaller roots of Catamites the same general organi- 

 zation is found as presents itself in Equisetum. The radical organs 

 of this extinct group of the Equisetales are known as Astromyelon, 

 a name given before their connection with calamitean stems was 

 known. 



In the Equisetales as a whole, represented by the Sphenophyl- 

 laceae, Calamitaceae, and Equisetaceae, very marked features of 

 organization are present. In fact, the group shows characteristics 

 which may well be denominated unique. It is clear that the group 

 is of very ancient origin, since when it first comes into view it is 

 distinctly set off from the other large alliance of the Lycopsida, the 

 Lycopodiales, by the whorled character of its appendages, the 

 ridges and furrows of the stem, and the sporangiophoric manner of 

 reproduction. Scott has regarded these features as sufficiently 

 distinctive to warrant the establishment of a third great phylum 

 of vascular plants, the Sphenopsida. This group he considers as 

 on the whole more nearly allied to the Pteropsida than to the Lycop- 

 sida. In view of the absence of foliar gaps in the series under 

 discussion in the present chapter, particularly clear when both fossil 

 and living forms are brought into consideration, there does not 

 seem to be any adequate anatomical evidence to support the sepa- 

 ration of the Equisetales under the heading of Sphenopsida. The 

 reproductive characters of the equisetal series are likewise most 

 easily reconciled with an affinity to the Lycopsida in general and 

 to the Lycopodiales in particular. Scott regards the Psilotaceae as 

 more nearly related to the Sphenophyllaceae than to the lycopo- 

 dineous forms. The evidence in favor of this view does not seem, 

 however, to be of a compelling character. 



