340 ON LEPIDODENDRON AND CALAMITES. 
drawing of Lepidodendron Harcourtii, in the * Archives du Muséum,’ 
vol. i. plate 31. Its absence in fossils is owing to its extremely deli- 
cate structure. The cells of the middle zone have thicker walls, and 
they have consequently frequently resisted decomposition before 
fossilization made them permanent. In the outer zones the cells are 
very much lengthened, and have a smaller diameter. They nearly re- 
semble true;vascular tissue; but the progress of lengthening may 
easily be traced from the interior outwards, and no distinction can be 
drawn between the true cells, and the long and slender ones of the 
outer circumference. The cell-walls of all the three zones are without 
markings of any kind. 
These three cellular zones are traversed -by the vascular bundles 
which rise from the outside of the interior woody cylinder, if they do 
not actually pass through it, and pass to the leaves and branches. 
These bundles separate from the woody cylinder a long way below the 
point where they pass off into the leaf. At first their direction is al- 
most parallel with the cylinder, slightly inclining outwards ; they then 
incline more outwards, and as they approach the circumference of the 
stem, they resume their nearly ascending direction for some distance, 
until they finally pass out to the leaves which they support. Each 
bundle consists of scalariform vessels, very much finer than those of 
the woody cylinder, surrounded by elongated cells like those of the 
outer zone, and probably still further enclosed by a delicate parenchy- 
ma, which disappeared before it could be fossilized. The only evidence 
I have of the existence of this cellular tissue is, that the bundles never 
fill the cavities in the parenchyma of the stem through which they 
pass. The bundles terminate in the points seen on the areoles of the 
stem, which are the scars of the leaves. 
The woody cylinder is of different thicknesses in different stems, 
and appears to have increased with the growth of the tree. There is, 
however, no indication of interruption in the growth or of seasonal 
layers. Yet it cannot be conceived that the whole vascular cylinder 
arose and was developed at the same time. Ii is very probable that 
the zone of slender, and consequently rarely preserved cellular tissue 
which surrounded the woody cylinder, was analogous in its functions 
to the cambium layer of phanerogamous stems, like the similar layers 
in recent Lycopodiacee, described by Spring in his * Monographie de 
la Famille des Lycopodiacées " (page 294). 
