ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 691 
structure. The pithless specimens, however, really differ conspicuously from the 
ordinary type of Astromyelon. Now that we know that Astromyelon, in its typical 
medullate form, is nothing but the root of Calamites, it becomes desirable to recon- 
sider the evidence for the identification with it, of the minute pithless specimens. We 
believe we are able to prove that here also the identification is correct, and that the 
smallest specimens, without medulla, are, in fact, the finer branches, or rootlets, which 
were borne on the larger medullate organs. We shall, therefore, employ the word 
“rootlet” throughout the description, though, as we shall find, there is no sharp 
distinction between these finer branches and the larger medullate specimens which we 
term simply “ roots.” 
The structure of the rootlets will first be described, after which we will discuss the 
evidence as to their nature.* 
In some of them the middle of the central cylinder is entirely occupied by the 
primary xylem, and there is no pith whatever. Neither does there seem to be 
parenchyma of any kind among the primary trachez (see figs. 5, 7, and 8; also 
fig. 1 in “ Organization,” Part XII.). In other cases, a very few elements, with some- 
what thinner walls, can be distinguished at the centre (see, for example, fig. 4). In such 
cases it is often impossible to make out for certain whether these elements are really 
parenchymatous cells or merely thin-walled tracheze. 1n other specimens, however, 
there is no doubt of the existence of a very small pith.t In the remarkable branched 
specimen, figured in a previous memoir,{ the rootlets seen in longitudinal section 
show no pith, while the branch, which is cut transversely, has a little thin-walled 
tissue within the primary wood.§ 
In all these rootlets the primary wood, whether actually reaching the centre of the 
stele or not, is perfectly continuous, so that the number of strands of which it is 
composed is indicated by the protoxylem-angles alone. The latter are often quite 
distinct, as, for example, in Plate 16, fig. 4, where the rootlet was evidently tetrarch. 
So also, no doubt, was the rootlet shown in Plate 15, photograph 5.| 
In the typical rootlets, with little or no pith, we have never found more than four 
* The rootlets are illustrated in “ Organization,” Part XII, figs. 1,5, and 7 ; and in photograph 5 and 
figs. 4, 5, 7, and 8 in the present paper. 
+ See, for example, the rootlet figured in “ Organization,” Part XV., Plate 3, fig. 238, which is almost 
certainly of the same nature. This rootlet occurs inside the stem of a Lepidodendron selaginotdes, in the 
hollow zone left by the decay of the middle cortex. It is very probable that the rootlet had made it# 
way, by its own growth, into the decaying stem, as so constantly happened in the case of the rootlets of 
Stigmaria. 
tf “Organization,” Part XII., Plate 29, fig. 7. 
§ Another very clear case of a minute pith (not figured) is C.N. 1348. Here the tetrarch rootlet has 
a perfectly distinct thin-walled medulla, consisting of three or four cells only, as seen in transverse 
section. 
|| See also Wiitiamson, “ Organization,” Part XII., Plate 30, fig. 5, which shows another tetrarch 
rootlet. 
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