926 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
tion is shown in a specimen previously figured,* in which a zone of radially arranged 
tissue, thin-walled at its inner edge, intervenes between the wood and the primary 
cortex. It is probable that this peridermal tissue arose from the division of cells 
belonging to the pericycle. At any rate, the whole of the primary cortex, lying 
outside this first-formed periderm, soon perished. Its remains, in various stages of 
disorganization, are found surrounding many of the more advanced stems (see, for 
example, Plate 83, fig. 40, where only a part of the primary cortex is represented), 
Soon, however, the original periderm was itself replaced by more internal layers, 
arising, as we have seen, from the secondary phloém-parenchyma. The oldest 
specimens had a regular scale-bark, formed by successive incomplete layers of 
periderm, cutting more and more deeply into the secondary cortical tissues (see 
photographs 22, and more especially 23, on Plate 75). At some places as many as 
five distinct peridermal masses can be traced, one outside the other. Each of these 
masses consists of thick-walled cells towards its external side, and of very regularly 
arranged thin-walled tissue, towards its inner margin. 
Before leaving Sphenophyllum plurifoliatum we will return for a moment to the 
subject of the cambium, in order to consider the question of its first origin. We very 
constantly find an irregular layer of comparatively thin-walled tissue, between the 
concave sides of the primary xylem-strand, and the first layer of interfascicular 
secondary wood (see especially Plate 75, photograph 20, and Plate 83, fig. 40). At 
the angles of the primary xylem on the other hand, the first secondary trachez 
appear to abut directly on those of the protoxylem (see also photograph 19 and 
fig. 43). We may infer then that the fascicular cambium (in the sense above 
explained) arose from the first parenchymatous layer immediately adjoining the 
protoxylem, while the enterfascicular cambium at its first origin was separated from 
the central wood by at least one layer of permanently parenchymatous cells. 
It is probable that the formation of secondary wood always began in the inter- 
fascicular region.t Sometimes it extended at once round the protoxylem-angles; in 
other cases the completion of the ring was delayed, so that we may even find three 
layers of interfascicular wood, while no trace of secondary tissue has as yet appeared 
opposite the groups of protoxylem. 
2. SPHENOPHYLLUM INSIGNE, NOBIS. 
This is the species originally described under the name of Asterophyllites insignis, 
and includes all the specimens of Sphenophyllum, showing structure, received from 
Burntisland. At the time when the fossil was first described, nothing was known of 
* C.N. 874, Wittramson, loc, cit., Part V., Plate 3, fig. 16. 
t See Wituramson, loc. cit., Part V., Plate 1, fig. 2. (C.N. 872.) 
¢ Witr1aMson, “ Organization,” Part V., 1878. 
