ON LEPIDODENDRON AND CALAMITES. 341 
If we separate the different structures we have described in the axis 
into two series, the one series axial, and the other epidermal, we shall 
have the axis composed of scalariform utricles, the woody cylinder and 
the vascular bundles passing to the leaves belonging to the first series, 
and the two external zones of the vascular tissue to the second. The 
inner zone of cellular tissue, like the cambium layer, was most probably 
common to both series, the cells of the outer circumference being de- 
veloped into the parenchyma of the epidermal series, while the vessels 
of the woody axis were produced from the cells of the inner series. 
Stigmarioid roots have been determined to belong to Lepidodendron 
as well as to Sigillaria, and their whole structure supports this deter- 
mination. I have satisfied myself that there is nothing that can be 
truly called a medullary ray in the woody cylinder of Stigmaria, but 
into the proof of this I will not now stay to enter. The base of the 
trunk was divided into a few principal roots, and these again divided 
dichotomously, but the ultimate divisions were never much attenuated. 
Throughout their whole course, and from every portion of their cireum- 
ference, they gave off rootlets of considerable length, which, with the 
exception of a slender vascular bundle, were entirely composed of deli- 
cate hexagonal cells. They were articulated to flagon-shaped bodies 
sunk in cavities, arranged in a quincuncial manner over the stem. 
The internal structure of. the Stigmaria root corresponds to that of the 
trunk of Zepidodendron. . The axis was composed of fusiform barred 
cells, and this was surrounded by a woody cylinder, which was cer- 
tainly penetrated by the vascular bundles that supported the rootlets. 
Beyond the woody cylinder came a great thickness of cellular tissue, 
almost always destroyed, but probably agreeing in its structure with 
the three zones of the stem. 
In speculating upon the conditions under which the forests of Lepi- 
dodendron flourished, it is most important to observe whatever is 
peculiar in those organs by which the plants were connected with the 
physical conditions around them. Geologists have too much over- 
looked such considerations in their deductions as to the physical phe- 
nomena of a period from the plants and animals that then existed. 
They have often taken for granted that the known conditions of the 
living species of a genus are true also of the fossil members of the same 
In the want of other evidence, such an assumption may be 
cantata’ employed ; but unless its true value be accurately estimated, 
