694 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
the rootlets (see photographs 4 and 5, figs. 7 and 8). The specimens shown in 
photograph 5 and in fig. 8 throw great light on the origin of these intralacunar 
cells. It is evident that they arose as hernia-like outgrowths from the cells of 
the trabecule. An early stage of this process is shown in fig. 8; in photograph 5 
the outgrowths have become larger and more numerous, while in the specimens 
represented in photograph 4 and in fig. 7 they have increased to such an extent as 
to block up some of the lacune. The process is evidently of the nature of thylosis, 
which is known to occur in the intercellular spaces of Equisetum.* 
Wherever the outermost tissues of the cortex are preserved, whether in a rootlet, 
or in one of the larger roots, we find the surface protected by a special thick-walled 
layer of cells. The outer wall of this layer in some cases attains a great thickness, 
which may exceed the diameter of the cell-cavity. The thickened membrane often 
shows a distinctly laminated structure (see photograph 5, and figs. 5, 7 and 8).t 
We propose to call this outer, protective layer of cells, the epidermoidal layer ; it 
has the structure of a thick-walled epidermis, and appears to correspond in all respects 
with the epidermoidal layer{ described by OLIVIER in many recent roots. It is 
probable that the epidermoidal layer in the roots of Calamites was not superficial in 
origin, for in some cases the remains of a layer of cells exterior to it can be traced. 
Most probably the actual absorptive epidermis of the young root was cast off, and the 
protective function was assumed by the cortical layer next below (the exodermis$), just 
as is the case in so many roots of recent plants, 
In one of the larger roots the epidermoidal layer is evidently more than one cell 
thick (see fig. 11). It is possible that we here have the commencement of periderm- 
formation, such as was observed by RENAULT in one of his specimens. || 
From the description already given, it is evident that the rootlets and the larger 
roots show a complete agreement in structure, except for the absence of pith and of 
primary rays in the former. In order, however, to prove decisively that the rootlets 
were really the minor branches of those roots which, as we know, belonged to Cala- 
mites, further evidence of two kinds is necessary. (1.) The most conclusive proof 
would be given, if we could find the small pithless rootlets borne as branches upon 
the typical medullate roots of Calamites. (2.) The point could also be proved by 
showing that an unbroken series of intermediate forms exists, connecting the smallest 
rootlets with the undoubted Calamitean roots. 
The evidence available is chiefly of the latter kind, but we have some specimens 
which appear to show direct continuity between rootlet and root. It will therefore 
* SrrasBurcer, “ Histologische Beitrige,” vol. 3, p. 437. 
t In “ Organization,” Part XII, fig. 5 (Plate 30), shows the layer in question at e, and it is also 
conspicuous in the rootlet figured in Part XV., Plate 3, fig. 23. 
ft Otsvisr, “ Appareil tégumentaire des Racines,” ‘ Ann. des Sci. Nat., Bot.,’ sér. 6, vol. 11, 1881. 
§ Cf. Straspurcer, ‘ Das botanische Practicum,’ 2nd ed., 1887, p. 181. 
|| Renautr, “ Genre Astromyelon,” ‘ Ann, des Sci. Géologiques,’ vol. 17, 1885, Plate 7, fig. 2. 
