283 
the Periderm of Fossil Lycopodiales. 
Renault repeated his original observations on Dictoxylon cortex in his 
‘Structure comparee de quelques tiges/ &c. (1879) 1 under Lepidodendron 
rhodumnense , which is very similar to Sigillaria spinulosa , but he gave no 
explanation as to the origin or development of the assise subereuse. On the 
other hand, it was just this aspect of the subject which appealed most to 
Williamson, and he returned again to it in his Memoir X in reference to 
Lepidodendron Wunsckianum. ‘The bast tissues’, he writes, ‘ increase in 
thickness with the stem, . . . but no additional light is thrown on the 
physiological question, viz. whether the additions . . . are due to a plane 
of genetic activity on its external or its internal surface.’ 2 In Memoir XI 
he added that the ‘ prosenchymatous zone * of Lepidodendron selaginoides , 
which he refers to later for the first time as periderm , appears under the 
leaf-bases in the form of detached arcs, and that the meristematic layer was 
on the outside of the tissue, owing to ‘ regular continuity of its peripheral 
border \ 3 
Some years later, in his monograph on Stigmaria , Williamson gave 
what he always considered his most complete notice of the history of the 
development of the periderm. 4 In this he described the cuter layers of the 
prosenchymatous zone as being in a state of active division by both 
horizontal and vertical septa, and he explained the somewhat disturbed 
arrangement by concluding that the meristematic zone formed prosenchyma 
internally and parenchyma externally. 
In Solms-Laubach’s ‘ Einleitung in die Palaophytologie which was 
published in German in the same year as the Stigmaria monograph (1887), 
the terms phellogen, periderm, and phelloderm are for the first time freely 
used in describing the fossil Lycopods, but it is explained that periderm is 
not to be taken in its physiological sense, since the leaf-bases beyond it do 
not dry up. 5 Solms locates the phellogen near the outer margin in Lepido- 
dendron selaginoides , but, contrary to Williamson, on the inner margin in 
Stigmaria . 
In 1891 appeared Bertrand’s memoir on Lepidodendron HarcourtiiL‘ 
Here the periderm is described as arising from a diffuse continuous cambium 
on the inner boundary of the tissue, though there are indications that the 
most delicate elements were towards the centre. Bertrand refers to the 
periderm under the name of cork, but he points out that it had not yet 
caused exfoliation. 
Bertrand’s work was followed the next year by Hovelacque’s mono- 
graph on Lepidodendron selaginoides J Hovelacque denies that the secondary 
cortical tissue is a phelloderm, as had been stated by Williamson and 
1 Renault (19). 
3 Williamson (41), pp. 285-6. 
5 Solms-Laubach (30). 
7 Hovelacque (15). 
2 Williamson (40), p. 498. 
4 Williamson (43). 
6 Bertrand (3). 
