4§ 3 
Origin of the Ulodendroid Scar. 
the external surface of the scar really represents the internal surface of the 
conical base of the branch demanded by his theory. This is by far the most 
important argument advanced by him. 
3. M. Renier points out that, in the specimen on which the above 
observation was made, the leaf-trace sections on a cast of an admittedly 
subcortical surface of the stem are depressions ; the mould of the external 
surface of the scar also has its leaf-trace sections represented by impressions, 
and it is reasonable to assume that it is therefore of the same nature as the 
subcortical cast of the stem. 
4. There is usually a raised rim round a ulodendroid scar which can 
only be a part of the base of the branch. 
This is a statement — I believe a fair one— of the whole of the evidence 
brought forward in support of M. Renier’s theory. It will be noticed that 
none of it is direct ; in all cases it depends on the interpretation of certain 
observed facts. 
Discussion of M. Renier’s Theory. 
M. Renier’s theory is open to many objections from a purely general 
consideration of our knowledge of lepidodendroid stems ; some of these are 
pointed out below. 
The structure of the lepidodendroid stem, as it is now known in about 
twenty- five species of the genera Lepidodendron , Lepidophloios , Sigillaria, 
and Bothrodendron , is as follows : 
1. There is a stele of small size relatively to the diameter of the stem. 
2. Surrounding the stele is a broad zone of middle cortex, composed 
of very soft tissue, usually with intercellular spaces. This tissue decays 
very readily and is usually lacking in petrified specimens. 
3. Surrounding the middle cortex is the outer cortex, composed of 
hard cells which are proved to be very resistant to decay. This tissue is 
either uniform throughout or shows a gradual change of character along 
a radial line. 
In the middle of the outer cortex the secondary cortex arises from 
a cambium which is set up by the division of a cylinder of cells. The 
secondary cortex seems to resist decay better than any other tissue in the 
plant : in petrified material, both from the coal balls and such isolated 
specimens as the types of L. Wiinschiauum , it is always well preserved, 
and scraps of it are very common objects in coal-ball sections. 
4. Outside the secondary cortex is a thin layer of outer cortex 
carrying the leaf-bases. In really old specimens this seems to have been 
commonly shed. 
The problem of the £ growth and development ’ of lepidodendroid stems 
is a difficult one, and for our present purpose it is unnecessary to discuss 
the whole of it ; it is, however, necessary to discover what increase in the 
