.4 TYPE OF PALAEOZOIC PLAXTS. 403 



frequently became disorganised, leading to the formation of 

 the infranodal canals, of which mention has already been 

 made in reference to the markings on the surface of a 

 medullary cast. 



Some few sections of stems show a periderm developed 

 in the inner region of the cortex ; the beginnings of such a 

 secondary tissue formation appear in the form of thin tan- 

 gential walls in certain cells of the primary cortex. 



Among structureless specimens of Catamites, it is a 

 matter of common occurrence for one end of a medullary 

 cast to taper somewhat rapidly to a point, at the same time 

 being slightly curved and showing a considerable decrease 

 in the length of the internodes. Such examples are casts of 

 the medullary cavity of branches, the pith of which was in 

 organic connection with the pith of an axis of higher order ; 

 the place of insertion of the branch being seen as a shallow 

 circular depression on the nodal line. In the Owens College 

 Museum, Manchester, there is a particularly good specimen, 

 showing three such tapered casts in contact with the nodes 

 of a stem. A tangential section through the wood of an 

 axis bearing branches, shows that the latter are given off 

 immediately above the nodes ; the basal portion of a branch 

 which is cut through on its way out from the parent stem, 

 presents the appearance of a calamitean axis with a paren- 

 chymatous pith, sometimes fistular, surrounded by a ring of 

 collateral bundles. At the actual base of <r branch, the 

 internodal canals are absent, but somewhat above the inner 

 extremity there is usually noticed a small gap in connection 

 with each xylem bundle. Tn these spaces there have been 

 found protoxylem elements, among the thin- walled cells 

 which generally occupy each lacuna, thus enabling us to 

 recognise in these spaces the homologues of the carinal 

 canals of ordinary stems. In certain cases the branches 

 appear to have become abortive ; this is rendered highly 

 probable by the peculiar structure occasionally seen in a 

 transversely cut axis, on its way through the parent stem. 

 Such abortive branches became subsequently enclosed by a 

 kind of callus wood, developed by the activity of a meristem 

 layer, formed across the region of the pith. The branches 



