STRUCTURE AND AFFINITIES OF A LEPIDODENDROID STEM. 909 



medullary region of Fucus and other genera of large brown seaweeds, which arise as 

 outgrowths of short elements and insinuate themselves between neighbouring cells. 

 This form of tissue is also well known in the middle cortical region of various Lepido- 

 dendroid plants, and a similar parenchyma occurs in the cortex of some recent Lyco- 

 podiacese.* The photograph shown in PL I. fig. 4 affords distinct proof of active cell- 

 division taking place in both the smaller external cells and in the larger filamentous 

 cell-rows, f This phenomenon of growth in the pith is referred to in a later part of the 

 paper. It remains to call attention to the existence of numerous elements next the 

 primary xylem, which may be spoken of as short or isodiametric tracheids with fine 

 scalariform and reticulate bands of thickening ; these are shown in PI. IV. fig. 25. A 

 solitary short tracheid was observed at the inner edge of the pith-tissue, similar to those 

 of the peripheral region. These short tracheids are of the same character as the 

 familiar " barred-cells " of Williamson, scattered through the axial region of the central 

 cylinder of Lepidodendron vasculare (Binney).| 



ii. Primary xylem. — The limits of the annular band of primary xylem — x' — are 

 readily recognised in PI. I. fig. 1. This ring of wood consists of scalariform tracheids — 

 about 20 to 30 tracheids in breadth ; the narrowest tracheids form the outer limits of the 

 band, and the diameter of the elements increases towards the inner edge of the xylem 

 (PI. IV. fig. 24). The outer edge of the xylem ring or corona has an undulating 

 outline, prominent tapering teeth, which would appear as ridges in surface-view, and 

 shorter, broader projections alternating with bays or depressions as shown in fig. 24. 

 The two diagrams in fig. 34, PI. IV., illustrate the general appearance of the outline of 

 the primary wood. The character of the xylem surface appears to agree with that of 

 Lepidoj)hloios Harcourtii, as described by Bertrand.§ The narrowest and most external 

 tracheids, which constitute the protoxylem, have their thickening bands rather farther 

 apart than in the wider elements, and the scalariform type passes in some cases into a 

 spiral form of xylem element. There does not appear to be that well-marked distinction 

 between definite spirally thickened protoxylem elements and the larger scalariform 

 tracheids of the primary xylem, which we find in most recent Vascular Cryptogams. In 

 some ferns, however, the protoxylem tracheids are not clearly differentiated from the 

 rest of the xylem. The broad scalariform pits of the tracheids in both the primary and 



* Bower (93), p. 346. 



t The growth and division of pith-cells has been noticed in old coniferous stems. Of. Strasbdrger (88), p. 88. 



X This specific name has been adopted, following the example of Solms-Laubach, in preference to that of Lepidoden- 

 dron selaginoides used by Carruthers, Williamson, and others. In 1862 Binney described the internal structure of 

 certain stems, which he named Sigillaria vascularis and Lepidodendron vasculare ; these were afterwards shown to be 

 identical. The structure of the same species was also described by Carruthers [Carrdthers (69)], and identified by 

 him with L. selaginoides of Sternberg, a determination which Williamson, who extended Carruthers' account, doubt- 

 fully accepted. Sternberg's species is, however, included by Kidston as a synonym of L. Sternbergii, Brongn. [Kidston 

 (86), p. 151]. The simplest plan would seem to be to adopt Binney's name, as we are quite certain as to the characters 

 of the stem to which he gave the specific name of vascularis. Kidston has included Sigillaria vascularis, Binney, as a 

 synonym of Lepidodendron Harcourtii, Witham (loc cit., p. 169), but the difference in the anatomical characters is too 

 well marked to render their identity possible. 



§ Bertrand (91). 



