880 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
thickness.* Here the principal rays narrow out rapidly towards the exterior, the 
marginal series of the ray dying out altogether, while the more median rows become 
attenuated. These changes appear to be due to the vigorous tangential growth of 
the elements of secondary fascicular xylem; there is no formation of interfascicular 
tracheides, the ray being wholly parenchymatous throughout. 
The great majority of the specimens investigated belong to the type B, with inter- 
fascicular wood. In some cases (sub-type B 1) the principal rays come almost at 
once to a sudden end ; little or no secondary interfascicular parenchyma is formed, 
and the interfascicular wood immediately assumes, so far as the transverse section 
shows, the same structure as the fascicular wood, appearing to consist of radial 
strands of tracheides, with narrow secondary rays, usually ouly one cell in breadth, 
between them.t 
This case, however, though frequent, is exceptional. In the great majority of 
our English specimens the principal rays narrow gradually towards the exterior, the 
interpolation of interfascicular series of tracheides taking place step by step. This, 
the prevalent case (sub-type B 2, in the arrangement adopted), is the one which we 
have been able to study most in detail. It will therefore be best to base our descrip- 
tion on stems of this variety (see Plate 72, photographs 2 and 3, Plate 77, fig. 38f). 
With the exception of Calamoprtus, all our English specimens would probably 
fall under GéprERt’s genus Arthropitys, which is simply a synonym of Calamites, 
as we propose to limit that genus. The forms which we are about to consider may be 
taken as the type of Calamites in the above sense. Count Soums-LauBacn has 
pointed out the urgent need for further investigation of the structure of the wood in 
Calamariez.§ So far as the typically Calamitean structure is concerned we are now 
in a position to supply fairly complete information, the. preservation of many of the 
specimens from the English coal-fields being so perfect that the structure can be 
studied nearly as well as in a recent wood. 
The earliest stages of secondary growth scarcely need any further description. 
In fig. 1, on Plate 77, we have a transverse section of a stem, in which the very first 
tangential divisions have taken place in the interfascicular tissue, while the very 
definite radial seriation of the xylem of the bundles indicates that here also cambial 
activity has begun. This is in a twig about ‘7 millim. in diameter—the smallest 
yet observed. Another specimen, two sections of which are shown in photograph 1, 
on Plate 72, and in fig. 2, on Plate 77, though of larger size, shows scarcely a trace of 
any addition to the primary structure. In the bundles themselves there was pro- 
* Shown in some new sections not yet incorporated in the collection. 
t See Wittiamson, “ Organization,” Part I., Plate 24, fig. 15. Wass, ‘ Steinkohlen-Calamarien,’ 
Part IL, p. 10, fig. 3. Shown in C.N. 15, 16, 17, one section in 118*, &. Good longitudinal sections of 
this form are still required. 
¢ See also WinitiaMson, “ Organization,” Part L., figs. 9, 14, 16, 17, 26. 
§ ‘Fossil Botany,’ English translation, p. 298. 
