178 STRUCTURE AND LIFE HISTORIES 
ized, sometimes being entirely so; while in other cases 
sporangia occur on the foliage-leaf. As in the replace- 
ment of sporophylls by sterile leaves in the ostrich fern, 
Onoclea struthiopteris (paragraph 148), these abnormalities 
indicate the close relationship between leaves and spore- 
bearing organs, and clearly show that the latter may be 
completely transformed, by sterilization, into foliage- 
leaves. 
In Ophioglossum the foliage-leaf and spore-bearing spike 
are both unbranched, the latter suggesting an adder's 
tongue, whence the name, Ophioglossum. In both Ophio- 
glossum and Botrychium the sporangia originate from a group 
of epidermal and sub-epidermal cells, and are consequently 
imbedded in the surrounding tissue. Their walls are 
more than one cell in thickness, the annulus is lacking, 
and they open by a slit. Ferns of this type are called 
eusporangiate. Their prothallia are usually fleshy and 
subterranean, bear the antheridia and archegonia on the 
dorsal instead of on the ventral surface, and are perennial, 
often living on after the sporophyte has died. In general 
the sporophyte possesses less sterile tissue in proportion 
to the fertile tissue than is the case with the leptosporan- 
giate forms. These characters mark the group as more 
primitive than the leptosporangiate ferns, and they are 
much less numerous, only about 100 species being known 
from the entire world, while of the leptosporangiate ferns 
between 3,000 and 4,000 species have been described. 
