Stigmarian Rootlets . 5 7 1 
rootlet, and I regard both this conducting strand and the 
tracheids of the outer cortex as a highly specialized adaptation 
of the rootlet, and not as the survival or transformation of 
transfusion-cells of a leaf-like organ. Except in their position, 
the rootlets and leaves of the Lepidodendraceae seem to be 
as different from one another and quite as highly specialized 
as those of Isoetes. In this latter plant, too, the rootlets 
have a fairly regular arrangement on the basal part of the 
stem, which may readily be compared with the dilated base of 
Pleuromeia. It is quite possible also to conceive the latter to 
be morphologically equivalent to the ‘ rhizome * or 1 rhizo- 
phore * of Stigmaria , and possibly of both, as a special develop- 
ment of the protocorm of the ancestral sporophyte. Such 
a protocorm would not have borne any leaves at its base, but 
its rootlets may have had the same position as the leaves on 
the stem. 
