Knowledge of Lyginodendron . 73 
other hand, a product of a meristem internal to the centripetal 
wood. A tissue of precisely similar appearance occurs in 
a specimen of Lyginodendron Oldhamium which Williamson 
figured in his Memoir IV (1873), PL XXII, Fig. 4 1 2 . The 
dark patches of tissue internal to the wood consist of radially- 
disposed rows of parenchyma developed by a zone of cambium, 
and exactly corresponding in structure with the masses of 
tissue in the pith of Lyginodendron robustum. It is conceivable 
that the secondary parenchyma may be of a corky nature, 
and analogous to the periderm-tissue described in the pith of 
X tangeria paradoxa 2 and in some stems of Bennettites 3 . 
There remains to be considered the nature of the xylem- 
bands. Unfortunately the tissues between the outer edge of 
the centripetal xylem and the inner edge of the centrifugal 
xylem are very imperfectly preserved. In a few places, 
however, the two bands of wood are seen to be in direct 
continuity. The tracheids of the two zones of wood are 
frequently separated by a gap in the tissues ; and, as 
previously noticed, the bands of centrifugal wood when traced 
to their inner termination often pass into groups of tracheids, 
which do not show anything of the regular arrangement charac- 
teristic of secondary xylem. In Plate VI, Fig. 12, and in 
Figs. 9 and 16, the characteristic appearance of the imperfectly 
mineralized tissue between the two sets of xylem-elements is 
represented. In one or two places between the centripetal 
and centrifugal wood, traces have been observed of tracheids 
showing indications of spiral thickening ; it is possible that 
these may be the protoxylem-elements of the primary wood, 
but the preservation is hardly such as to justify any very 
decided statement. 
This brings us to a comparison with the structure of the 
xylem as described by Williamson and Scott in Lyginodendron 
Oldhamium (Binney). In that species the centrifugally- 
1 No. 1153. Cf.alsoni4, 1 1 18 and other specimens in the Williamson Collection. 
2 Solms-Laubach, Bot. Zeit. Jahrg. 48, p. 11. 
3 Capellini and Solms-Laubach. I Tronchi di Bennettitee (Mem. R. Accad. 
Sci. Inst. Bologna), Vol. ii, 1891, p. 48, Plate V, Figs. 2, 5. 
