The primary xylem of _Cordaites_ is in direct continuity with the secondary tracheids and does not form mesarch strands as in _Mesoxylon_. The pith is usually discoid. The pitting on the tracheids is a character of special importance: while it is true to say that as a rule the number of pits on the radial walls of a single tracheid is larger than in the Araucarineae, this is not always the case. In _Araucaria_ there are occasionally as many as five rows of alternate polygonal pits (fig. 691, A) and in some Palaeozoic Dadoxylons there are only one[659] or two rows. The very broad zone of transitional elements at the inner edge of the xylem-cylinder is a characteristic feature shared by the Araucarineae[660]; the spiral protoxylem-tracheids are succeeded by scalariform elements and these, by the gradual anastomosing of the transverse bars, pass into tracheids with multiseriate pitting. In this broad zone we probably have a primitive feature, an epitome in a single stem of the course of development of multiseriate from scalariform pitting. In some Palaeozoic species with wood of the pycnoxylic type and agreeing generally with typical _Cordaites_ the bordered pits are sometimes separate and circular, and opposite pits occasionally replace the usual alternate arrangement. Another feature on which stress has been laid is that in _Cordaites_ the pits occupy the whole breadth of the tracheal wall; but this, though frequently the case, is by no means a constant feature. In _Dadoxylon Newberryi_[661] the pits tend to form groups, leaving unpitted areas, as in the genus _Coenoxylon_[662]. In the stem of _Dadoxylon materiarum_ Daws. represented in fig. 475 the pits do not always cover the whole of the tracheid-walls: this stem is also instructive as an example of the different appearance presented by pitted tracheids according to the state of preservation. In some places an oblique pore is well shown while in others only the outer border of the pit is seen. Gothan[663] has described a specimen in which some of the pits are circular and occupy only the central area of the xylem elements: separate circular pits occur also in _D. Pedroi_ Zeill.[664] (fig. 476). Similar departures from the normal are illustrated by recent species of Araucarineae. The absence of a torus is another feature shared by _Dadoxylon_ and true Araucarian wood. Annual rings other than incomplete and spasmodically formed rows of narrower tracheids are not as a rule present, and in this respect also _Araucaria_ affords a close analogy. Thomson[665] has figured a transverse section of a root from English Coal Measures in which rings of growth are well defined; and other instances are recorded. In an Australian species named by Arber _D. australe_[666], there are well-marked rings of growth, and this is equally the case in some Indian wood[667] of Permo-Carboniferous age, more nearly allied to _Mesoxylon_ than to _Cordaites_, and in a _Dadoxylon_ of similar age from South Africa.