3i7 
the Periderm of Fossil Lycopodiales. 
were thrown off, leaving the outer edge of the phelloderm as the superficial 
tissue V the phellogen layer would disappear in the process, and it is not 
stated how further development of periderm, if any, continued. The pro- 
duction of a more internal phelloderm would be the obvious solution, but of 
this there is no mention in any of the old stems of the various species 2 
described as having a fissured bark. Nor has any evidence been obtained 
of the formation of a second periderm in these plants, except in one small 
and apparently abnormal specimen attributed to Lepidodendron fuliginosum 
(Text-fig. 26). Here periderm formation has started at four different levels. 
The bands are all narrow and discontinuous, and there is nothing to show 
that one is developed to replace the other. Scott has also mentioned the 
formation in places of a second periderm in Lepidodendron obovatum , 3 while 
in one or two species (. Lepidodendron selaginoides and fuliginosuni) a few 
layers may be added to the periderm by division of the cortical cells on the 
inner margin, but these only form a string of four or five cells (Text-fig. 27). 
In the massive corks of recent plants, when the stage of exfoliation has 
been reached, the cork layer is maintained at a uniform thickness by the 
internal renewal balancing loss by decortication, but there is no sign of this 
or any compensating arrangement in the periderm of the Lycopodiales 
should it also have been cast off during the life of the plant. 
In the absence of any provision for the consequences of exfoliation, and 
also of any physiological reason for the formation of bark and later decorti- 
cation in the development of a secondary cortex as opposed to cork, the 
evidence at present obtained may be held to support the remark of Scott 
that * when the leaves were shed their bases remained attached to the stem, 
forming the leaf-cushions, which were persistent, even on the larger trunks \ 4 
Summary. 
The periderm of the fossil Lycopod attained a great and conspicuous 
development. 
It is of the nature of secondary cortex, and is, for the greater part, 
morphologically phelloderm. 
The cells are prosenchymatous ; in certain species some become sub- 
divided into groups of smaller cells, the extreme development of this 
condition producing the type of tissue known as Dictyoxylon cortex. 
Concentric zones may be present in the periderm, due to differences in 
the nature of certain layers of cells. 
The periderm forms a complete and apparently continually increasing 
cylinder near the periphery of the organ, and seems to have served as the 
1 Seward (28), p. 1 18. 
2 Lepidodendron selaginoides , L. brevifolium , L. Wunschiamtm, Sigillaria. 
3 Scott (25), p. 318; figured Seward (28), p. 154. 
4 Scott (24), p, 128. 
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