308 
Kisch . — The Physiological Anatomy of 
(e) Mention may be made here of the presence in one section referred 
to, Lepidodendron brevifolium , of wedges of light-coloured cells, much 
extended tangentially, which start in the cortex on the outside of the peri- 
derm, and project some way into it. The cells are thin- walled and have 
often broken down ; the rest of the periderm is badly preserved, and 
unfortunately no tangential sections are available. Part of one of these 
wedges is shown (restored) in Text-fig. 22. Zalessky 1 mentions very 
similar wedges in the species he refers to Lepidodendron aculeatum , Sternb., 
and says that in tangential section the cells are ‘ divided by transverse, 
sometimes also by longitudinal, partitions into a row of cells 
Text-fig. 22. Lepidodenaron 
brevifolium. Diagrammatic restora- 
tion from a transverse section of a 
specimen which shows wedges of 
lighter-coloured extended cells on 
the outer border of the periderm. 
U. C. L. Coll., A 50. x 90. 
Text-fig. 21. Stigmaria sp. (type 2). 
Tangential section in the periderm showing 
the wide chambered cells in lenticular groups. 
Will. Coll., 703. x 30. 
In all the types described, heterogeneous periderms therefore arise by 
the further subdivision of certain of the cells. Whether these cells occur in 
definite groups or not, and though there may be vertical as well as trans- 
verse divisions, the original form of the mother-cell is retained, except in 
Dictyoxylon types ; in the latter the dividing cells have remained quite 
thin-walled and are capable of further growth, so that the original outline of 
the periderm cell is lost and the daughter-cells form a parenchymatous tissue, 
which becomes conspicuous in transverse as well as longitudinal sections. 
1 Zalessky ( 46 ), p. 294. 
