1382 EVOLUTION OF PLANTS 
ferns may have an erect stem ten or fifteen metres in 
height, with leaves five or six metres long. These gi- 
gantic sporophytes offer a strong contrast to the insig- 
nificant sporophyte of the mosses, and corresponding to 
this is the late appearance of the sporogenous tissue, 
which may not be formed until after many years. This 
extreme subordination of the sporogenous tissue is a 
wide departure from the condition existing in such low 
liverworts as Riccia, where practically the whole sporo- 
phyte is composed of sporogenous cells. 
In all of the Pteridophytes the sporogenous tissue is 
restricted to certain definite areas, these being confined 
to more or less distinct organs, sporangia. The latter 
are possibly foreshadowed by the imperfect segregation 
of the sporogenous tissue in the Anthocerotacee, the 
highest of the liverworts. Among the ferns, the forms 
which approach nearest the condition existing in the 
Anthocerotaceew are the species of Ophioglossum or 
adder-tongues, where the limits of the sporangia are 
scarcely indicated at all upon the surface (Fig. 34, 
A, B, C). In these ferns the sporogenous tissue occurs 
in masses of considerable size, but is not clearly sepa- 
rated from the surrounding tissue. The archesporial 
cells are separated from the epidermis of the leaf 
(sporophyll) by several layers of cells, and the spores 
finally escape through a cleft which opens at the sur- 
face of the sporophyll. The archesporium is at first 
of sub-epidermal origin, as in Anthoceros, the latter 
being in this particular more like the ferns than like 
the typical mosses, where the sporogenous cells are 
originally derived from the central part of the sporo- 
phyte. Even in Anthoceros, however, the separate 
