the Periderm of Fossil Lycopodiales. 313 
one may reasonably suppose to have reached full development, while dark 
substance seems too widely distributed over the sections to give reliable 
evidence of secreted material. The presence of cells showing signs of 
recent division is also unusual in secretory tissue, except in quite immature 
schizogenous glands . 1 
The appearance produced seems on the whole such as might be caused 
by the formation at intervals of layers or groups of cells with rather thinner 
walls (sometimes capable of division), which have in consequence been more 
liable to crushing or to degeneration ; somewhat as in the corks of many 
recent plants, where concentric zones, not always seasonal, are formed 
by the alternation of wide, thin-walled cells with layers of flatter cells with 
thicker walls, stone cells, &c . 2 
(10) Periderm Development in relation to other Tissues. 
The periderm appears early — before the secondary wood, where this is 
also developed — in all the species examined, except Lepidodendron brevi - 
folium and intermedium , Sigillaria scutellata , tesselata , and spinulosa , and 
Stigmariae, in which evidence on this point was lacking. Only two sections, 
one of which is of Lepidodendron selaginoides , show some few layers of 
secondary wood before periderm formation has extended round the stem 
(see Text-figs. 1 and 2). 
Correlation of the amount of periderm formed, and the amount of 
secondary wood developed, does not show any apparent connexion between 
the tissues. The periderm is nearly always as thick as the secondary wood, 
frequently much thicker , 3 in all the specimens examined which were com- 
plete enough to allow of determination ; but the periderm of Lepidodendron 
selaginoides , for example, is very much more strongly developed than that 
in a branch of Lepidodendron Hickii of about the same size, and which had 
no secondary wood. This serves to emphasize that these thick periderms 
had probably other functions than to compensate mechanically for the 
feeble development of the vascular system. 
In this connexion it is noticeable that in all the fossil Lycopodiales 
there is a great deficiency of the tissues which generally serve for carbo- 
hydrate storage. There may be no definite pith, as in Lepidodendron 
selaginoides , or it is comparatively small, owing to the central concentration 
of the vascular axis. There is no wood parenchyma, except in the 
anomalous secondary zone of the type found in Lepidodendron fuliginosum. 
The medullary rays, which are all secondary, are generally very narrow 
and often contain tracheides. 
1 In this connexion it might be mentioned that the 1 glands ’ in the primary cortex of Lepido- 
dendron fuliginosum consist almost entirely of delicate meristematic cells. 
2 de Bary ( 11 ), pp. 1 13-14, & c. 
3 As in Lepidodendron selaginoides, L . Wunschianum , Sigillaria scutellata , &c. 
