STRUCTURE AND AFFINITIES OF A LEPIDODENDROID STEM. 915* 



the sense of the typical cambium of recent Gymnosperms and Dicotyledons, or of such 

 Palaeozoic plants as Calamites, Sphenophyllum, Lyginodendron, and other genera ; but 

 in its place a broader band of meristematic elements, more similar in appearance to the 

 zone of meristem which is formed in the pericycle of certain Monocotyledons or to the 

 meristem zone of Isoetes and other recent plants. 



The leaf-trace bundles in their course through the meristematic band, cm, are 

 enclosed by an arc of the cambial tissue, m, which furnishes additions to the rows of 

 secondary tracheids, and internal to each trace we see some flattened rows of the 

 meristem zone of the stem (PL III. figs. 16 and 19). This relation of the leaf-bundles 

 to the cambial tissue, and the arching round them of a special leaf-trace meristem band, 

 may be compared with the behaviour of the petiole stele in Lygniodendron Oldhamium 

 (Binney) as figured by Williamson and Scott.* 



Beyond the meristem zone cm there are preserved portions of a partially dis- 

 organised tissue consisting in part of large sacs and smaller, more or less crushed cells 

 (sc) ; this zone is directly continuous internally with, the outermost cells of the band 

 cm, as shown in PI. II. fig. 9. This tissue, which may be termed the secretory zone, 

 contains numerous dark and irregular patches which most probably represent the 

 remains of secreted substances. The large circular sacs (sc, figs. 9, 15, and 16), which 

 present practically the same appearance in longitudinal as in transverse section, were no 

 doubt secretory in function. The areas bounded by the dark lines shown in fig. 9, sc, 

 PI. II., and fig. 16, sc, PL III., may enclose some small delicate cells ; it is probable 

 that these elements constitute glandular tissues, which gave rise to latex or other 

 products of secretion. External to this secretory zone the tissues have disappeared, and 

 the wide gap which intervenes between this region and the outer cortex is occupied by 

 volcanic ash. The large sac-like spaces in this zone often present the appearance of 

 having been tangentially stretched (figs. 9 and 16). 



No tissue has been found which can be described as typical phlcem, but to this point 

 reference is made in a later section of the paper. The so-called secretory zone occupies 

 the position of phloem tissue, and may represent it physiologically. The secretory zone 

 may probably be regarded as the outermost tissue of the stelar region, t 



vi. Outer cortex, chiefly phelloderm. — We now come to the shell of bark already 

 mentioned ; a piece of this cortical region is shown natural size in PL I. fig. 2. The 

 innermost portion consists of imperfectly preserved large parenchymatous cells of 

 polygonal form (a to b, fig. 2) ; the walls of the cells have been partially destroyed, 

 but the angles are more perfectly preserved, and small intercellular spaces occur between 

 the individual elements. At a in fig. 2 the remains of a leaf-trace occur, the primary 

 tracheids are seen in slightly oblique transverse view, and the more delicate secondary 

 elements are crushed and displaced. As we follow the parenchymatous tissue outwards, 



* Williamson and Scott (95), p. 712, pi. xxii. fig. C, 



t This view is supported by an examination of some unusually perfect specimens of Lepidophloios fuliginosus in the 

 Binney Collection. [Since this was written the sections referred to in the Binney Collection, Cambridge, have been 

 figured and described. Vide Seward (99).] 



VOL. XXXIX. PART IV. (NO. 34). 7 B 



