Scott and Hill—Structure of Isoetes Hystrix 447 
anything in Ferns. Isoetes is in fact, in this respect, the 
nearest living representative of the Palaeozoic Lepidodendreae 
(Bower, 1894); the form and large size of the sporangium, its 
attachment along the whole of its base to the median line of 
the ventral foliar surface, the presence of trabeculae, the 
heterospory, and the relation to the ligule, are all points in 
which there is a remarkably close agreement between Isoetes 
and Lepidostrobus (Maslen, 1899). This complete correspon¬ 
dence, at once so obvious and so exact, cannot be invalidated 
by a supposed analogy (remote, at the best) with the ventral 
spike of Ophioglossum vulgatum. 
As regards the ligule, its homology with that of Selaginella 
or of the Lepidodendreae would seem manifest. Farmer 
(1890) has endeavoured to dispute this, on the grounds that 
in Isoetes the ligule develops from a single cell, in Selaginella 
from a group of cells, and that it arises earlier, and is more 
elaborately developed in the former genus than in the latter. 
Harvey Gibson (1896) has shown that in the two latter points 
there is no marked difference between the two genera. The 
distinction as regards the number of initial cells holds good, 
but obviously affords no argument against homology in view 
of the close agreement in all other respects. 
The velum of Isoetes has been compared with the indusium 
of a Fern, and regarded as forming the envelope of a ‘ mon- 
angic sorus,’ as in the case of Lygodinm or that of the 
megasporangium of Azolla. No Fern, however, is known in 
which the sporophyll produces a single ‘ monangic sorus,’ and 
that in the ventral position. Until recently the velum of 
Isoetes was in so far a difficulty, that it appeared to be 
an isolated case among the Lycopodiales. Quite lately, 
however, one of us has found that in a Palaeozoic Lyco- 
podinean cone, otherwise identical with a Lepidostrobus , each 
sporangium, whether containing microspores or megaspores, 
became enclosed in an integument, growing out from the 
sporophyll 1 . The integument in the fossil cone differs 
1 See Scott, Studies in Fossil Botany , 1900, p. 180. A short account of this 
fructification ( Lepidocarpori ) has been communicated to the Royal Society, and 
a full description is in preparation. 
