402 HICK AND CASH I FLORA OF LOWER COAL MEASURES. 
ing of whose walls is not quite uniform, but is chiefly of the barred 
and dotted types. These vessels seem to be intermixed with a few- 
elongated cells, of a more fibrous character, with plain walls ; but 
our preparations are not quite decisive on this point. The pith and 
bundles combined form a central cylindrical axis to the stem, which 
has a diameter of *056 el an inch. 
3. The Cambium layer. — External to the bundles, and sepa- 
rated from them by a somewhat sharp line of demarcation, is a 
thin stratum of delicate cells which again recalls the structure of 
young dicotyledonous stems and branchlets. It forms a continuous 
ring, enclosing the whole of the bundles ; and though from the 
delicacy of the tissue it has not been equally well preserved through- 
out, it remains in sufficient perfection to show its close similarity, 
if not identity, with the cambium layer of the stems referred to. 
4. The Cortex. — This part of the stem is the one that gives it 
its distinctive character, and that exhibits a structure which, so 
far as we know, has not been previously met with in fossil plants. 
A.S will be seen by comparing the total diameter of the stem with 
that of the central axis, the cortical layer is of considerable thick- 
ness in proportion to the other tissues. For the purposes of descrip- 
tion, it may be said to consist of an inner, a median, and an outer 
portion, which, though intimately connected with one another, and 
forming parts of the same tissue, are nevertheless in marked con- 
trast. 
The inner portion is composed of cells which are slightly elong- 
ated in a tangential direction, and have some likeness to those met 
with in the cortical parenchyma of dicotyledons. Internally it 
abuts on the Cambium layer, but it appears to be wholly destitute 
of fibrous elements, and of anything which can be regarded as 
the horaologue of the bark of ordinary phanerogams. 
The median portion, seen in transverse section, consists of a 
number of radiating filaments, composed of cells, united end to end, 
which connect the inner with the outer portions of the cortex. 
