ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 865 
5. The above general characters are common to all the stems observed. The 
specimens, however, present great differences among themselves, which are due (1) to 
the order of the branch in question, or to the part of the branch from which the 
specimen is derived; (2) to age; and (3) to specific distinctions. The differences 
under the first two heads can be readily distinguished from one another with proper 
care. It is, however, we believe, as a rule, impossible to distinguish between 
branches of different orders belonging to the same species and those of different 
species, though in special cases, as in the form originally described as Calamopitus, 
the specific distinction is evident.* The same difficulty would exist in the case of 
Equisetum if we had to depend on isolated fragments of branches for our anatomical 
knowledge. 
6, Variations in structure, due to the part of the branch from which the section is 
cut, are as follows :— 
a. In the size of the pith and consequently of the primary central cylinder (see 
Plate 73, photographs 7, 8, and 9); 
b. In the number of the vascular bundles ; 
ce. In the presence or absence of the canals accompanying the bundles. 
a. The size of the pith, in most if not in all cases, is at a minimum at the base of 
the branch, and increases rapidly upwards, until its approximate maximum is 
attained, after which it remains nearly constant. 
b. In like manner the number of vascular bundles is at a minimum at the base of 
the branch, and increases in the ascending direction in successive internodes, until a 
maximum is reached, when it remains approximately constant. 
c. The canals accompanying the bundles are absent from the base of the branch, 
but are otherwise almost always present in the internodes. 
The evidence on which these conclusions are founded will be fully given below. 
7. Variations known to be due to the order of the branch are: (a) in the size of 
the pitht and primary cylinder, and (6) in the number of the vascular bundles. It is 
probable that the solid or fistular character of the pith may also vary with the order 
of the branch, as well as with age, as is the case in many species of Equisetum. 
8. Differences in the relative width of the bundles and primary medullary rays, in 
the structure of the rays, in the size of the canals, and in many other points, may 
either be of specific value, or be due to the order of the branch, or to mere individual 
variations. 
9. Differences due to age consist (a) in the degree of preservation of the pith. The 
presence of a broad zone of persistent pith within the ring of bundles (see Plate 72, 
* Wiuamson, “Organization of the Fossil Plants of the Coal-Measures,” Part I., ‘Phil. Trans.,’ 
1871, p. 488; ‘Memoirs of the Literary and Philesophical Society of Manchester,’ ser. 3, vol. 4, 1869. 
+ These variations are enormous in degree. The diameter of the pith in the specimen figured in 
Plate 77, fig. 1, is at most 0°3 millim. Bronentart describes his Calamites gigas, a medullary cast, as 
““diametro pedem subequante.” The ratio is about as 1 : 1000. 
58 2 
