EXTERNAL MORPHOLOGY OF THE STEMS OF CALAMITES. 509 
5. Branch scars. These may or may not be present. Where they occur, 
they are situated just above the node. 
Three types of pith casts may be distinguished by the presence or 
absence of branch scars, and their distribution : 
(a) Eucalamites: branch scars occur on every node. 
(b) Calamitina : branch scars occur in periodic whorls, each branch- 
bearing node being separated from the next by a variable 
number of nodes without branch scars. 
(c) Stylocalamites: branch scars absent, or rare, or irregular in 
distribution. 
Tt has also been pointed out by one of us* that sub-medullary casts are 
frequent among Calamites. The surfaces of these casts do not represent the 
periphery of the pith, but a more external region within the secondary wood. 
These sub-medullary casts are characterised by very broad ribs corresponding 
to the medullary rays and by the absence of infranodal scars. Since they 
represent a region external to the pith, they naturally do not show the prints 
of the openings of the canals into that region. Such sub-medullary casts 
are specifically indeterminable. 
Specimens have also been figured by various authors which show the 
impression of the secondary wood, and which are neither medullary nor 
external casts. These need not detain us here. Such examples are easily 
recognised by the absence of nodes, though in Eucalamitean types one row of 
branches passing outwards clearly indicates the position of these regions. 
On the other hand, casts or impressions of the external features of the 
stems of Calamites show the following characters :— 
1. Nodes. It is imagined that these correspond in position to the nodes of 
the pith, though this fact has not yet been demonstrated. Petri- 
factions are almost silent on the subject of the external features of 
Calamite stems, since petrified Calamites have so far proved to be 
almost always decorticated, except in the case of very young twigs. 
2. Internodes, which may be smooth or longitudinally striated, or partly 
smooth and partly ridged. 
3. Leaf scars occurring on the node and representing the points of 
attachment of the leaves. These, however, are by no means always 
present. 
Branch scars, which may or may not be present, and which if they 
occur follow the same types of distribution as are met with in pith 
casts. (See p. 512.) 
5. Other features are root scars, frequently present on the nodes, and also 
in some cases scattered over the surface of the internodes. 
We now propose to consider these features more in detail. 
T 
* Arber (1918). 
