An interesting feature seen in transverse sections of the secondary wood is the occurrence of light bands concentric with the rings of growth which are broadest near the long axis of the stem (fig. 491, C). In their narrower parts these bands are clearly due to the partial destruction of the secondary tracheids, but in other places crushed parenchymatous tissue occurs which may be a traumatic phenomenon or possibly comparable with Nördlinger’s ‘medullary spots[788]’ formed by local hypertrophy of medullary tissue. Although the structure of the leaf-traces cannot be definitely determined, it would seem that each trace passed into the perimedullary region as a single bundle of relatively large size composed of spiral and scalariform tracheids narrower than the secondary elements. The traces during their outward course were accompanied by some parenchymatous tissue continuous with that in the pith, and the inner spiral tracheids of the trace were connected with isodiametric reticulate elements. The dimensions of the leaf-traces point to leaves of fairly large size. [Illustration: Fig. 491. A–F, _Antarcticoxylon Priestleyi._ A, B, transverse sections of the xylem; _a_, small tracheids simulating protoxylem; _x_, xylem at the edge of the pith. C, transverse section showing leaf-traces, _l_, _a_, _b_. D, leaf-trace with short tracheids, _t_, in the pith; _s_, sclerous cells. E, tangential section. F, pith in longitudinal section. G, _Pityosporites antarcticus_; see Vol. +iv.+ (All the sections are in the British Museum.)] In the structure of the secondary wood _Antarcticoxylon_ agrees on the whole with an Araucarian stem: the broad zone of xylem composed of spiral and scalariform tracheids at the edge of the pith is a feature common to _Mesoxylon_, _Cordaites_, and _Araucaria_. There is no evidence of the occurrence of double leaf-traces such as characterise certain Cordaitalean genera and some existing members of the Araucarineae. In the single nature of the leaf-traces the Antarctic stem resembles _Mesopitys Tchihatcheffi_ also in the presence of rings of growth[789], but in _Antarcticoxylon_ the preservation of the primary xylem is too imperfect to admit of any satisfactory comparison as regards this important tissue with other types. The precise age of the Beacon sandstone has not been determined, but the probability is that the upper beds from which the boulder containing _Antarcticoxylon_ was derived are not older than the Rhaetic period. The chief interest of this imperfectly preserved stem with undoubted Araucarian affinities is its occurrence in the rocks of Antarctica in association with other remains of comparatively large stems. CHAPTER XXXV. PALAEOZOIC GYMNOSPERMOUS SEEDS.