486 
Watson . — On the Structure and 
On p. 49, M. Renier says : 
‘Les cicatrices ulodendroi'des resultent evidemment de l’accroissement 
simultane en diametre de la tige et des rameaux. Ceux-ci acqueraient ainsi 
une base conique tout com me les rameaux des Catamites .’ 
The structure of calamitian stems is so extremely different from that of 
those of Lepidodendron that comparisons of the two always seem to be very 
unfortunate, and in the present case rest on a complete misconception of the 
mode of branching of a calamite. 
The branching calamitian specimens to which M. Renier obviously 
refers are those of which the originals of Figs. 3 and 2 of Scott’s ‘ Studies ’ 
are examples. 1 It is true they resemble superficially the D’Arcy Thompson 
specimen of Ulodendron . 2 Specimens of this type, however, are invariably 
only pith casts. Williamson and Scott have shown that mature calamitian 
branches show no contraction at their base. Dr. Scott has kindly pointed 
out to me that in very young primary specimens the attachment of branches 
was really small, but this contraction is obliterated during the secondary 
growth, and affords no parallel to M. Renier’s theory of the uiodendroid scar. 
Before discussing M. Renier’s specimens, it is necessary to obtain clear 
ideas of the meaning of the various surfaces which can be seen in impression 
material of lepidodendroid stems. As M. Renier correctly points out, there 
are at least three ‘ assises cellulaires ’ which may be preserved in impression 
material. 
The innermost of these is commonly the wood, but in a few cases is 
really the inner cortex, which is usually preserved in petrified stems of 
Lepidodendron vasculare and Bothrodendron mundum . This zone is not of 
much importance for our present purpose. 
The other two zones may give three distinct types of surface : 
(1) On the external surface, the leaf-bases. 
(2) The outer surface of the secondary cortex within the leaf-bases. 
(3) The inner surface of the outer cortex. 
It appears to be probable that in some species old trunks of Lepido- 
dendron shed their leaf-bases, leaving only the secondary cortex exposed. 
Such specimens have only two types of cortical surface. 
It is of interest to try and discover what thickness of coal film may be 
expected as the result of the conversion of any given thickness of outer cortex 
into coal. The average specific gravity of coal is about 1-25; that of dry 
cork 0*3. This means that, in being converted into coal, cork is reduced to 
about 0-24 of its volume if we assume, as is of course not the case, that no 
material is lost in such change. Making a slight allowance for this loss, we 
find that it is probable that a piece of coal occupies only one-fifth of the 
1 D. H. Scott, Studies in Fossil Botany, 2nd ed., vol. i, 1908, pp. 17 and 18. 
2 D. W. Thompson, Notes on Ulodendron and Halonia. Trans. Edin. Geol. Soc., vol. iii, 1880. 
