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1900 | SPOROPHYLLS AND SPORANGIA OF ISOETES 325 
be in the root-tip, there is consequently no true epidermis, and 
the part which performs the functions of an epidermis is the 
outermost layer of the periblem derivatives. Thus, while sper- 
matophytes have three embryonic tissue regions, the plerome, 
the periblem, and the dermatogen, pteridophytes, with the possible 
exception of the root-tip, have only the two first-named regions. 
The archesporium of all spermatophytes is hypodermal. In 
no case is it known to be derived from the superficial cells. The 
epidermis is from the beginning distinct from the inner cells of 
the sporangium. Though there may be periclinal divisions in 
the superficial cells, as in gymnosperms, many Ranunculacee, 
etc., these occur only after the differentiation of the archespor- 
ium, and the cells so added merely increase the thickness of the 
wall or apex of the sporangium, but never become part of the 
sporogenous complex. The true epidermis, set apart at a very 
early period from the inner tissue of the embryo, is incapable of 
producing spore-forming cells. That rdle is played by special 
cells of the outer layer of the periblem. 
It is otherwise with the ‘ hypodermal archesporium” of 
pteridophytes. This is cut off by periclinal divisions from a 
superficial cell in the case of ferns and Equisetum, and from a 
group of such cells in the case of the Lycopodiales and Isoetes. 
But if the absence of a true epidermis in pteridophytes and 
the homology of periblem with periblem in all vascular plants 
are conceded, then the hypodermal cells of spermatophytes are 
comparable, not to the hypodermal cells of pteridophytes, but 
to the superficial cells. As we have seen, the sporogenous mass 
in pteridophytes can always be traced to superficial cells, but in 
spermatophytes to hypodermal cells and no further; that 1s, " 
all cases the function of giving rise to spore-producing cells 1s 
localized in certain cells of the outer layer of the periblem. 
These facts serve to show the inconsistency of undertaking 
to limit the archesporium to a hypodermal position in all cases. 
The cell or group of cells whether superficial or hypodermal, to 
which in a last analysis all the sporogenous portion of a sporan- 
gium can be traced, ought to be called the archesporium. The 
