286 Kisch . — The Physiological Anatomy of 
Three papers remain to be considered. In a note on the cortex of 
Sigillaria mamillaris , Arber and Thomas showed that the periderm was 
very similar to that of S. sciitellatap while Zalessky in 1909 gave a detailed 
account of the periderm in specimens referred by him to Lepidodendron 
aculeatum 2 and Sigillaria Boblayi . 3 In the former there are in one specimen 
peculiar wedge-shaped groups of thinner-walled, lighter-coloured cells, and 
in both species the tissue is assumed to be phelloderm owing to the presence 
of tangentially extended cells, with the tangential thinner than the radial 
walls, at places on the outer border. 4 In a further paper published in 1911, 
Zalessky describes shortly the periderm of his Lepidodendron obovatum , 
which is probably identical with Z. Hickii , Watson. 5 
In 1910 appeared the second volume of Seward’s ‘Fossil Plants’, 6 in 
which there is a full account of the periderm in various species of 
Lepidodendron , Sigillaria , Stigmaria, and Bothrodendron. Special stress is 
laid upon the indication of the position of the phellogen by the line along 
which splitting tends to take place. As a result any secondary cortex 
which has split off with the leaf-bases (frequently found in Lepidophloios , 
&c.) is regarded morphologically as true periderm, though it is uncertain 
whether or no it agreed with the cork of recent plants. By this means 
Seward locates the phellogen in Z. vasculare (selaginoides), L. fuliginosum , 
Z. Harcourtii , and Lepidophloios , in all of which he finds the tissue is mainly 
thick-walled phelloderm. 
Following the same test for the position of the phellogen in Stigmaria, 
Seward considers that in young specimens all the periderm is on the outer 
side of the phellogen, but that it is permeable and not of the nature of cork ; 
that after a time phelloderm is formed on the inner side, and in old 
specimens constitutes the bulk of the secondary tissue. The zoning of the 
periderm of Z. selaginoides and Z. Wunschianum , and possibly of Stig- 
maria, is explained as due to the presence of secretory strands. 
Seward comments on the great mechanical importance of the periderm, 
and draws special attention to the effect of its development on the external 
appearance. The older specimens of Z. selaginoides , Z. Wunschianum , 
Z. brevifolium , and Sigillaria are all described as having thrown off their 
leaf-cushions and as possessing fissured bark. 
The amount of space devoted in this volume to the consideration 
of the periderm of the fossil Lycopods compared to that in other text-books 
may be taken as some measure of the increase of knowledge of the tissue in 
recent years. 
1 Arber and Thomas (2). 2 Not the same as Seward’s Z. aculeatum. 
3 The S. mamillaris of Arber and Thomas. 4 Zalessky (46). 
5 Zalessky (47). 6 Seward ( 28 ). 
