ON LEPIDODENDRON AND CALAMITES. 343 
fossil ; but specimens of Lepidostrobus, attached to branches of Lepi- 
dodendron, have been described by Dr. Paterson, Brongniart, and others ; 
and I have noticed a fine specimen in the Museum of the Edinburgh 
Botanic Gardens, and others exist in the collections at the British 
Museum and elsewhere. 
In tracing the affinities of Lepidodendron, we have the safest guide 
in the organs of fructification, and fortunately these have been satisfac- 
torily determined. The sporiferous strobilus shows that it is a true 
eryptogam ; and in general appearance and arrangement of parts, the 
strobilus can scarcely be distinguished from that of some living Lyco- 
podia, except in the great difference of size; this affinity is strength- 
ened by the character of the leaves, and the structure of the stem. 
But the possession of both kinds of spores in the same sporangium 
exhibits stronger affinity to Rhizocarpee than to Lycopodiacee. 
The structure of the arboreal stem of Lepidodendron is much more 
complex than that of any known eryptogam. The central axis of 
irregularly-arranged vascular tissue in Lycopodium is suited to the low 
stature of the plants of that genus; but in the giant Lepidodendron 
there is a complexity, which approaches the structure of some dicoty- 
ledonous stems. The general arrangement of the tissues, resembling 
what exists in some Cycadee and -Cactacee, has caused this fossil plant 
to be referred sometimes to the one, and sometimes to the other of 
these Orders; but the resemblance is only one of analogy, and not of 
affinity. The presence of scalariform tissue, of which the woody por- 
tion is entirely composed, and the absence of medullary rays, would, 
even if the fruit were unknown, be sufficient to establish the crypto- 
gamic nature of the plant. A comparison with the Cycadean stem 
may help us, by the resemblances and differences which will appear, 
better to understand the stem of Lepidodendron. The Cycads have 
all a large medulla, composed of large-sized parenchyma; in some 
genera traversed by numerous vascular bundles, as in Encephalartus, 
and in others entirely cellular, as in Qycas and Zamia. This is sur- 
rounded by a single woody cylinder, or several, everywhere penetrated 
with medullary rays. Beyond this there is a considerable thickness of 
parenchyma, composed, in their inner portion, of cells whose length 
exceeds only slightly their breadth; these gradually lengthen, until 
they assume an appearance very like the external portion of Lepido- 
dendron. This cortical parenchyma is traversed by the vascular 
