HICK AND CASH : FLORA OF LOWER COAL MEASURES. 401 
surface. It consists of a central pith, surrounded by a number of 
slightly wedge-shaped masses of tissue, which are either of a fibro- 
vascular or of a wholly-vascular character. Outside these masses 
of tissue or bundles, as we may term them, is a cambium ring, 
followed by a comparatively thick cortex. This is composed, in a 
great measure, of radiating plates composed of cells, with large inter- 
vening longitudinal air cavities, similar to those met with in recent 
aquatc plants. The cellular plates do not anastomose, so that the air 
cavities are continuous from the central axis to the peripheral 
portion of the stem, which is composed of a few layers of somewhat 
rounded cells. The dimensions of the stem — full particulars of 
which are given at the end of the paper — are comparatively small, 
the whole diameter probably not exceeding 2 3 0 of an inch. 
2. Special description. 1 . The Pith. — The pith is composed 
of cellular tissue, which in all essential particulars is similar to that 
met with in recent herbaceous stems during the growing season, and 
in the youngest shoots of woody perennials ; that is to say, the cells 
are of a rounded or polyhedral form, with a few very small inter- 
cellular spaces, and with thin walls. As is not uncommon in such 
stems, the cells at the periphery of the pith have slightly thicker 
walls than those which occupy the centre. The size of the cells is 
by no means uniform, but varies from -0028 inch to '0056 in 
diameter. The longitudinal section shows very tew of the pith 
cells, but from such as are present it would seem that they were 
somewhat elongated in that direction. 
2. The wedge-shaped masses of tissue or bundles. — These are 
arranged side by side in a circle which surrounds the pith, and bear 
a remarkable resemblance to the xylem portions of the fibro- vascular 
bundles of dicotyledons during the first season of growth. They 
are composed of elements, which in the transverse section appear 
as small, thick-walled, generally square cells, arranged four or five 
deep in radiating lines. The breadth of the zone formed by the 
bundles is about -007 of an inch. In the longitudinal section 
the bundles are seen to be made up of elongated vessels, the mark- 
