1900] SPOROPHYLIS AND SPORANGIA OF ISOETES 329 
as a whole, but quite as fully to the embryonic stages of its dif- 
ferent organs, such as leaf, root, sporangium, and the like. 
In conformity with these principles it is proper, in the deter- 
mination of natural affinities, to place great emphasis upon the 
reproductive parts, for such parts are found to show very great 
constancy in their form and occurrence. The sporangia espe- 
cially, and the form and arrangement of the sporophylls, have 
long been recognized as of the highest importance. Thus, the 
classification of the Filicales is largely based on sporangial 
characters; and the position of the Salviniacee and Marsiliacez, 
which was formerly as unsettled as that of Isoetes, was estab- 
lished beyond doubt as soon as the development of the sporan- 
gia was fully understood. 
It is chiefly on the basis of the superficial resemblances of 
the sporangia of Isoetes with those of Lycopodium and Selagi- 
nella that it has been so long associated with them. If we 
enumerate the chief differences between the sporangia of Lyco- 
podiales and of ferns, we shall see that in every particular Isoetes 
agrees with the former. While the Filicales bear numerous 
sporangia on the dorsal surface of the leaf, Isoetes and the 
Lycopodiales, with the exception of the Psilotacee, the exact 
relation of whose sporangia to the leaf is still in dispute, bear 
but one Sporangium to a sporophyll, and that on the ventral 
Surface at the base. Such exceptional forms as the Ophioglos- 
Sacez and Marsiliacee do not help us in this inquiry. Though 
it may be true that the whole sporangiophore of the Ophioglos- 
Sacee is, as several morphologists have suggested, the homo- 
logue of the single sporangium of Lycopodium or Isoetes, the 
*uggestion is so hypothetical in itself as to give no support to 
any view based upon it. It is only in the position of the sporan- 
Sium that these families approach Isoetes; in other sporangial 
characters, such as number and development, they are like other 
ferns, 
The relative age of the leaves, when the sporangial rudi- 
ments first make their appearance, is of considerable significance. 
In the Filicales, with the exception of the heterosporous forms, 
