The Present Position of Palaeozoic Botany. 157 



The anatomy of the stem in its young condition is closely similar 

 to that of a recent Equisetum and thus deviates widely from the 

 Sphenophyllaceous type. The usually fistular pith is surrounded by a 

 ring- of collateral bundles, each, as a rule, accompanied by its carinal 

 canal in which the disorganized remains of the spiral tracheides can 

 be detected. Thus the development of the wood was in these cases 

 wholly centrifugal. 



The wide gulf which thus exists between the Calamarian and 

 the Sphenophyllaceous type of stem-anatomy is, however, to some 

 extent bridged by a petrified Calamarian stem from the Lower 

 Carboniferous of Scotland (named provisionally Calamités pettyciirensis) 

 in which a certain amount of centripetal xylem is present, lying on 

 the medullary side of the carinal canals. It is possible that the 

 stem in question may prove to be referable to ArchaeocaJamites, though 

 the stems of that genus previously investigated have not been found 

 to contain centripetal Avood. 



The longitudinal course of the vascular bundles and their relation 

 to the leaves are broadly comparable to the conditions in Fquisefiim, 

 though more complex and variable in the Palaeozoic representatives 

 of the classs. The leaf-traces and leaves of successive verticils are 

 commonly alternate in most of the Calamariaceae, but in Archaeo- 

 mkmiitps they are superposed, an interesting point of agreement with 

 the Sphenophyllales. The absence of foliar gaps, on which Jeffrey 

 lays stress, may hold good in the case of Archaeocalamifes, but if I 

 rightly interpret the structure, they are present in most Calamariaceae 

 as well as in the recent genus. In fact there is no point in which 

 the Equisetales show so marked an advance on the Sphenophyllales 

 as in their vascular anatomy; in this respect they reach the level of 

 the simpler Gymnosperms or Dicotyledons — an interesting example 

 of parallel development. 



In all except the youngest twigs a zone of secondary wood and 

 bast, often of great thickness, has been formed by means of a normal 

 cambium, the cells of which, together with those of the phloem, can 

 be observed in favourable cases. In Calamités itself (the Arthropitys 

 of Goeppert) the secondary wood is of a simple structure comparable 

 to that of the less differentiated Coniferous woods, but usually with 

 more or less scalariform pitting on the tracheides. CaJamodendron, 

 from the Upper, and Arthrodendron from the Lower Coal-Measures are 

 characterized by the complex structure of the principal medullary 

 rays, which contain much fibrous tissue in addition to the usual ray- 

 parenchyma. 



The simple structure of the leaves need not detain us; the 

 adventitious roots (formerly known as AsfromyeJon) were borne directlj^ 

 on the stem, not. as in Equisetum, on special rhizophorous branches. 



