284 THE AMERICAN NATURALIST. (VoL: XXXIX. 
The peculiar, jointed, hollow stem, with the characteristic 
arrangement of the bundles, except for their alternation in 
succeeding internodes, is as marked in the earliest Equisetales 
as it is in the existing genus, and cannot readily be derived from 
either the type of the ferns or lycopods, both of which are 
clearly recognizable in the formations where they first occur. 
So far as we can judge from the geological record, the central 
solid stele of Sphenophyllum is a more recent development than 
the separated vascular strands of the typical Equisetales, as 
exemplified by Archzeocalamites. 
CONCLUSIONS. 
As we have endeavored to show, the gametophyte of Equise- 
tum resembles that of the eusporangiate ferns rather than Lyco- 
podium, both in its dorsiventral character, and method of growth, 
and especially in the large multiciliate spermatozoids. We be- 
lieve that these resemblances indicate a real, although extremely 
remote, relationship with the lower ferns. 
The early divisions of the embryo are not unlike those in the 
fern embryo, but the early preponderance of the shoot, and the 
subordination of the leaves, very soon produce an extremely dif- 
ferent type of sporophyte, and it is highly probable that the 
peculiarities of the sporophyte were established at a very remote 
period, and-are not modifications of another type. The dichoto- 
mously divided leaves of the older Equisetales and Spheno- 
phyllales are somewhat reminiscent of those of some ferns, 
although it is not at all likely that there is any direct genetic 
relationship between these. 
The most probable conclusion to be reached, in view of the 
evidence at hand, would seem to be that the two series, the 
ferns and Equisetacez, are descended from a common stock, in 
which the gametophyte was not unlike that of the existing spe- 
cies of Equisetum and the lower eusporangiate ferns. It must 
be assumed, however, that the peculiarities of the sporophytes 
became established at so remote a period that one cannot say 
that either was derived from the other — ;. e., from the same 
ancestral stock represented by the gametophyte as it now exists 
