49i 
Origin of the Ulodendroid Scar. 
a lepidodendroid shows that we are quite justified in supposing that such 
a process occurred in Ulodendron , where we have strong indirect evidence 
that such was the case. 
Description of an Abscission Layer in Structural Material. 
The two structural specimens in which I am able to demonstrate the 
presence of a cladopsis mechanism are both members of the genus Lepido - 
phloios , the first being that described by Williamson in his nineteenth 
memoir as ‘ Wild’s Ulodendron ’ ; though crushed flat, this specimen is very 
beautifully preserved, and although undoubtedly a Lepidophloios , it bore two 
opposite rows of lateral branches. 
The other specimen is an excellently preserved example of a Lepido- 
phloios agreeing closely with L. Harcourtii ; it comes from Shore and is 
represented by long series of transverse and longitudinal sections in my own 
collection. The evidence available is not sufficient to determine the arrange- 
ment of the lateral branches, which, however, are rather sparsely and 
apparently irregularly scattered, possibly somewhat as in Lindley and 
Hutton’s Halonia tortuosa . 
These specimens agree exactly in the structure of the abscission layer 
which cuts off their lateral branches, but this layer is somewhat younger in 
‘ Wild’s Ulodendron * than in the other case. 
In a transverse section of the specimen of L. Harcourtii , No. 203, part 
of which is shown in PI. XXXVIII, Fig. 2, a lateral branch, 14 mm. in 
diameter, is shown in accurate median longitudinal section. Of this branch 
only a short stump 5 mm. long is preserved, but this shows in excellent 
preservation the outer, middle, and inner cortices and the stele. In the 
section figured, the stele is shown in median section, passing inwards till it 
reaches the level of the periderm of the stem ; it then, as is shown by 
other sections, turns sharply downwards, and passes in gradually until it 
reaches the stele of the main stem. 
The leaf-base-bearing tissue of the stem is directly continuous with 
that of the branch, and from the periderm of the stem a little upstanding- 
rim of periderm is formed in the base of the branch ; this is quite normal 
and surrounds the branch in the usual position in the primary outer cortex. 
Inside the outer cortex the thick and well-preserved middle cortex 
occurs. It has the ordinary loose tissue full of air spaces, but is not 
composed of long hyphae-like filaments, as is that of Z. fuliginosum . 
Inside this are the inner cortex and stele, which show no unusual features. 
Such in brief is the structure of the branch. The evidence of other 
specimens, such as Weiss’s biserial Halonia , shows that in the young stem 
all these tissues of the branch are in direct connexion with the corresponding 
tissues of the stem. 
Ll 
