ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 889 
the nature of internal periderm. In other parts the cortex consists of prosenchy- 
matous cells. 
A specimen of a comparatively small stem, which we have recently examined, 
throws great light on the origin of the periderm from the primary cortical tissues 
(see Plate 79, fig. 18). The transverse section shows that the cortex is well-preserved 
throughout, though in most places somewhat separated from the wood by the 
intrusion of rootlets of Stigmaria. Many of the large parenchymatous cells of the 
cortex are divided up by tangential septa into short radial series. The divisions are 
not limited strictly to any one zone of the cortical tissue, but occur chiefly in its inner 
portion, through which a fairly regular band of dividing cells can be traced. We 
think that there can be no doubt that we have here to do with an early stage of the 
formation of periderm. It is certain that a secondary tissue of some kind is being 
formed by the division of cortical cells. We know, from the older specimen, that 
internal periderm was formed abundantly in the cortex; we can scarcely be wrong in 
correlating” the two facts. 
It may be worth while to state expressly that the specimen from which fig. 18 is 
drawn has the whole thickness of the wood perfectly preserved, and is a typical 
Calamites, such as that shown in Plate 72, photograph 3. 
THe DIAPHRAGMS. 
Before leaving the subject of secondary changes in the stem of Calamites, we wish 
to call attention to the formation of periderm on the diaphragms. These persistent 
plates of parenchyma, which separate the internodal cavities of Calamites from one 
another, have often been described.* When well preserved it is always evident that 
they are several cells in thickness. The inner layers of cells of the diaphragm often 
have thicker walls than those towards the upper and lower surfaces. We have often 
observed that the thin-walled superficial cells are divided up by cell walls parallel to 
the surface. In some specimens these divisions have taken place so freely that a 
layer of regular periderm-like tissue coats the diaphragm on either side (see 
Plate 79, fig. 19). It is most probable that this secondary tissue really represents a 
layer of internal cork, which isolated the persistent diaphragm from the disorganizing 
tissue, by the destruction of which the medulla became fistular. 
Analogies are not wanting, among recent plants, for the formation of periderm in 
the pith. It occurs, for example, in certain anomalous Campanulacez and Gentianez, 
and in Aconitum.t In Calamites, however, so far as we have observed, the medullary 
periderm seems to be limited to the surface of the diaphragms, and not to extend to 
the peripheral layer of persistent pith which surrounds the medullary cavity. 
* See Wixtiamson, “ Organization,” Parts I. and IX. 
+ See Jost, ‘ Bot. Zeitung,’ 1890, pp. 443 and 491; Hératt, “‘ Recherches sur l’Anatomie comparée de 
la tige des Dicotylédones,” ‘ Ann, Sci. Nat., Bot.,’ series 7, vol. 2, 1885. 
a xX 2 
