1900] SPOROPHYLLS AND SPORANGIA OF ISOETES 331 
Certain other features of the sporangium of Isoetes find 
duplication only among members of the Lycopodiales. In all 
the higher leptosporangiate ferns there is an elaborate mechan- 
ism for the bursting of the sporangium and the scattering of the 
spores. This device, consisting of a row of peculiarly thickened 
cells (the annulus), and a group of cells which form an easy 
place of rupture (the stomium), is very rudimentary in the 
lower leptosporangiates (Osmundacez), and in the Ophioglos- 
sacee and Marattiacee, but it is not altogether absent. There 
is at least a predetermined line along which dehiscence shall 
take place. The elaboration of this dehiscence apparatus is one 
of the chief peculiarities of the higher leptosporangiates. When 
we turn to the Lycopodiales and Isoetes, however, we find posi- 
tively no contrivance for dehiscence, and no vestige of an 
annulus or stomium. The sporangium wall is simple, and 
bursts by desiccation in Lycopodium and Selaginella, and by 
decay in Isoetes; and neither method can be regarded as a 
Specialization. | 
Another analogy has been brought to light by Bower’s dis- 
covery in Lepidostrobus of certain radiating strands or processes 
in the Sporangium which are regarded by him as very probably 
of the nature of trabeculae. Since the relationship of Lepidostro- 
bus to Lycopodium can hardly be doubted, there is here a point 
of contact with this group of plants in a feature in which other- 
wise Isoetes stands alone. 
Again, Selaginella and Isoetes agree very nearly in the man- 
ner of selection of the megaspore mother cells. The unselected 
Mother cells do not divide at all, and all the spores resulting 
from the division of the fertile ones as a rule reach maturity. 
In heterosporous ferns all the mother cells divide into spores, of 
which but one becomes a megaspore. The contrast may be 
*xpressed in the statement that the megasporangium is differ- 
entiated in Isoetes and Selaginella defore the tetrad division, but 
in heterosporous ferns not until afer that division. 
The persistence of the tapetum in Lycopodiales and Isoetes 
'S a character to which no great importance is to be attached, for 
