the Periderm of Fossil Lycopodiales . 309 
(8) Features of Mechanical Significance. 
The cells of the periderm are generally well preserved and retain their 
clear outline, even in specimens which have been compressed quite out of 
their original form. They do not seem to have been easily extensible, and 
are inclined to break rather than stretch. The thin-walled periderms of 
Stigmariae seem the least resistant to pressure, while many of the species 
are specially crushable along certain lines. 
The periderm of the stems in its structure and position is well adapted 
in every way to withstand physical strains, and its obvious mechanical 
usefulness has been pointed out by Renault, Scott, Seward, and others. 
‘Architecturally,’ to quote Seward, ‘ Lepidodendron owed its power of 
resistance to the bending force of the wind to its stout outer bark formed 
of thick-walled elements produced by the activity of a cylinder of cortical 
meristem. The vascular axis, of insignificant diameter in proportion to the 
size of the stem, must have played a subordinate part, from a mechanical 
point of view, as compared with the solid mass of wood of a Pine or 
an Oak.’ 1 
Although often irregular in its beginnings, the periderm gradually forms 
a ring of more or less uniform thickness round the organ. Sometimes it is 
deflected outwards where it is crossed by the leaf-traces, as in Lepidodendron 
Harcourtii and fuliginosutn , or it may be sinuous as in Sigillaria scutellata , 
& c., following the outline of the ribs and furrows on the stem. By its 
position it probably contributed considerably to the support of the large 
number of evergreen leaves, which were very closely crowded, whether up 
the stem or at the extremities of the older branches. In Lepidodendron 
selaginoides a few extra layers of periderm are produced under each leaf- 
base, while in dichotomizing branches, as noticed by Seward, 2 there is often 
a stronger development of periderm in the fork. There also appears to be 
more periderm formed under the furrows of ribbed Sigillariae, but this has 
not been determined with certainty, as the periderm is broken off internally. 
(9) Zonation. 
Nearly all the species examined show some sort of zonation of the 
periderm. In Dictyoxylon types, for example, the zoned appearance is 
said to be due merely to the meshes being formed at more or less regular 
intervals. 3 The zones are obviously of different kinds in various cases, and 
furnish another illustration of the difficulties of determining with certainty 
histological details in fossil-work. 
One may conveniently distinguish : 
(a) Colour zones. Here the whole periderm is divided into two or 
more bands, caused by differences in colour or in the preservation of the 
1 Seward (28), p. 95. 2 Loc. cit., p. 119, Fig. 152. 3 Renault (19), p. 252, 
