382 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
Lepidodendron. The morphology of Stigmaria has been much dis- 
‘puted; so far as the main axis is concerned the best analogy, though 
a somewhat remote one, appears to be with the rhizophores of Selagi- 
nella; the rootlets, which have a totally different structure, agree so 
nearly with the roots of some recent Lycopods (/soétes and Selagi- 
nella) that there seems little doubt as to their homologies, though 
their peculiar arrangement has led some authors to interpret them 
as modified leaves. 
The Lepidodendrese are a well-characterized group, as to the 
affinities of which there can be no doubt, even apart from the evi- 
dence of fructification. The primary anatomy is of simple Lyco- 
podiaceous type, comparable to that of the aerial stem of Selaginella 
spinosa or a large stem of Psilotum—the higher anatomical organi- 
zation is chiefly expressed in the general occurrence of secondary 
growth. Except for the very different arrangement of the foliar 
traces there isa certain resemblance 
between the stelar structure of a 
Lepidodendron and that of Chei- 
rostrobus among the Sphenophyl- 
lales. Otherwise there is little in 
the vegetative characters which 
throws any new lght on the affini- 
ties of the class. 
The fructifications of the Lepido- 
dendrez are grouped under several 
generic names. In Lepidostrobus, 
Fic. 4.—Lepidostrobus. Diagram of het. the most extensive and oldest es- 
erosporous cone, in radial section. az, tablished of these genera, the or- 
axis; sph, sporophylls; sm, sporangia, : 0 6 nat 
seated singly on the upper surface of ganization 1S essentially that of a 
each sporophyll ; 1g, ligules. The micro- Lycopodiaceous cone. (Fig. 4.) 
sporangia, in upper part of cone, con- 26 : ; 
tain numerous microspores, while the The AXIS, resembling a vegetative 
megasporangia below are shown con- twig in structure, bears numerous 
taining four megaspores each. : 
spirally arranged sporophylls, each 
of which has a single large sporangium on its upper surface, at- 
tached almost throughout its whole length. The sporophyll has 
an upturned lamina, between which and the end of the sporangium 
a ligule is situated, showing that the whole of the long horizontal 
pedicel on which the sporangium is seated corresponds to the base 
of the vegetative leaf. The sporangium, often of very large dimen- 
- sions compared with that of a recent Lycopod, commonly has a 
palisade-like outer wall. It is almost certain that all Lepidostrobi 
were heterosporous, the microsporangia and megasporangia being 
sometimes produced on separate cones, sometimes on different parts 
of the same cone, as in recent Selaginelle. The microspores are 
very small, while the megaspores are of relatively great size, often 
