:oo 
Kisch. — The Physiological Anatomy of 
thus contributing to the irregular appearance. Sometimes almost the 
whole of the outer tissues appears to be in a meristematic condition. 
(4) Structure and Thickening of the Cell-wall. 
The outer primary cortex in the fossil Lycopodiales is typically thick- 
walled, and, in the species examined, the walls of the periderm cells are 
found to be of about the same thickness 1 (Text-fig. 8, &c.), though in some 
Sigillariae they are rather thicker, and in Stigmariae 
often considerably thinner. The frequent appearance 
of the periderm as a thickened band is generally due 
to the smaller size of the cells, sometimes also to 
brown coloration of the contents. 
The stages in the thickening up of a file of peri- 
derm cells of Lepidodendron selaginoides are shown in 
Text-fig. 12, from the thin-walled cells of the phellogen 
at ph, to the mature cells with walls of the same 
thickness as the primary cortex c adjacent on the 
inner side. (The very thick appearance of the radial 
walls is local, and is probably due to swelling of the 
membrane in mineralization.) 
The thickening is generally uniform over the 
surface of the periderm cell. Seward and Hill ob- 
served in L epidodendron Wunschianum that in some 
of the outer cells the lumen was almost obliterated, 
but this they considered might be due to swelling 
during petrifaction. 2 In some other species there is 
in places noticeable swelling of certain of the walls. 
This is shown in Text-fig. 13, A, in a Lepidophloios , 
where it occurs on the radial walls, while in certain speci- 
mens of Lepidodendron selaginoides (Text-fig. 13, B) 
the outer tangential walls have the same appearance. 
This thickening is quite irregular in occurrence, and 
seems due to local swelling of the membranes, and 
possibly splitting of the middle lamella and subsequent 
filling with intercellular substance. 
The walls of the periderm in general, as was 
noticed early, do not show any signs of pitting, not even 
of slit-like pits such as are found in the prosenchymatous tissues of recent 
plants. 3 Renault stated that the periderm of Stigmaria Brardi consisted of 
reticulated cells, 4 but these have since been referred to as if they were 
Text-fig. 12. Trans- 
verse section of a file of 
periderm cells of Lepi- 
dodendron selaginoides , 
showing the thickening of 
the walls, ph , phellogen ; 
c, primary cortex. U.C.L. 
Coll., A 012. x 410. 
1 The thin- walled cells of the meshes of Dictyoxylon types are not here considered. 
2 Seward and Hill (29), p. 917, PI. IV, Fig. 27. 
3 de Bary (11), p. 132, Haberlandt, &c. 4 Renault (21), p. 195, PI. XXXIX. 
