HICK AND CASH : AFFINITIES OF LEPIDODENDRON. 
327 
layers of cells elongated in the longitudinal direction, but as in the 
case of Lycupodium this is probably derived from the cortex. 
The stem outside the vascular bundle is made up of a thick 
cortex, in which we can again distinguish inner parenchyma and 
hypoderma as in Lijropndium. The h3^poderma is a thick- walled 
prosenchymatous tissue, destitute of intercellular spaces, which 
gxadually merges into that of the inner parenchyma. The latter is a 
thin-w^alled tissue, destitute likewise of intercellular spaces, save in 
the immediate neighbourhood of the vascular bundle. Here there 
occurs a large lacuna or air-cavity, in which the bundle lies, and 
across which a number of cortical cells run in threads or chains called 
trabecular. As the trabecuke cross the air-cavity more or less trans- 
versely, the cells which compose them are more or less radially dis- 
posed, a point to which we shall return. 
The air-cavity is not equally w^ell-developed in all species of 
JSelagiiiella, and in some more than a single vascular bundle are 
normally present. 
Comparison of the stem tissues of Lepidodendron avith those 
OF Lycopodium and Selaginella. 
We now come to the delicate and ditiicult task of comparing the 
stems of the three genera Avhose anatomy and histology have been 
described so far as seemed material to our purpose. Taking the 
widest and most general conclusions first the following positions may 
be assumed as hardly to be contested : — 
1. Secondary growth in thickness being absent from Lyco- 
podium and Selaginella, only the primary tissues of Lepidodendron, 
should in the first instance be brought into comparison with those of 
the other genera. 
2. In all the three genera the surface of contact between the vas- 
cular and the fundamental tissues may be held to be separate homo- 
logous structures. That is to say, the primary vascular bundles of 
Lepidodendron, as a whole, are homologous with the axile vascular 
cylinder of Lycoijodium, as a whole, and with the vascular system of 
Selaginella as a whole. Similarly, the primary cortex of Lepidoden- 
dron is, as a ivhole, homologous with the entire cortex of Lycopodium 
and that of Selaginella. The species of Selaginella which have more 
