510 DR. NEWELL ARBER AND MR. F. W. LAWFIELD ON THE 
Nodes:—The nodes of the external surface of Calamites vary somewhat in 
prominence. In many species they appear to be of the nature of constrictions 
or shallow grooves, as for instance in the examples of C. britannicus and 
C. congenius and other types figured here (cf. Pl. 28. figs. 1 & 2). In other 
cases the nodes appear to be strongly salient, especially in the case of those 
nodes which bear branch sears, e. g. C. Sachsei (Pl. 24. fig. 16) and €. verti- 
cillatus (Pl. 24. fig. 18). 
Internodes.—The specimens figured on Pls. 28-25 are chosen primarily to 
exhibit the characters of the internodes in different types, and are not 
intended to represent the features of all the specimens of the external 
surfaces of Calamites known to us. t 
In studying the external features of Calamites one difficulty in particular 
is always present. It is often not easy to decide whether the features 
exhibited by a particular specimen represent a truly external surface or a 
somewhat decorticated example. It may in fact be found, when better and 
further examples of some of the types here described are available. that we 
have mistaken sub-cortical surfaces for the true external surfaces. Thus in 
the ease of some species in which, on the present evidence, we are inclined 
to think that the external surface was distinetly ribbed and not smooth, it 
may eventually prove, when better specimens have been obtained, that we 
have been mistaken in this respect. For the same reason we have perhaps 
maintained a larger namber of species than will eventually be admitted by 
other workers. With regard to the terminology, we propose, for the sake 
of convenience, to speak of the more internal surfaces as sub-cortical. It is 
quite clear from our knowledge of the petrifactions of Calamites that the 
features with which we are concerned here are connected with the marked 
activity of a cork cambium. In fact they are, whether truly external or 
sub-cortical, botanically of the nature of cork, cither periderm or, more 
probably, phelloderm. 
It must also have been the case in regard to these trees, especially the 
older examples, that the outer corky layers were frequently exfoliated : or 
else destroyed by decay before preservation. In many eases, however, we 
think that the true external corky layer is still present. 
In perhaps the majority of species of Calamophloios known to us the 
epidermal surface appears to be smooth, but it is nearly always possible to 
recognise by means of a lens a sub-cortical surface beneath, which is longi- 
tudinally striated—the stri being usually fine, continuous or discontinuous, 
This is the case in C. britannicus (Pl. 28. fig. 1), C. rugosus (fig. 3), C. 
Sachsei (Pl. 94. fig. 16), and C. undulatus (figs. 14, 15, and 17). This 
minute striation also oceurs in the case of the foreign species C. oAlsbachensis 
(Sterzel) * and. C. dictyoderma (Kidst. & Jongm.)t. In other types the 
* Sterzel (1907) p. 435, pl. 67. figs. 1, 1a-1le. 
T Kidston & Jongmans (1915) p. 50, pl. 50. figs. 1 & 2. 
