448 Scott and Hill . —Sir tic here of Isoetes Hystrix . 
considerably in detail from the velum of Isoetes , but is suffi¬ 
cient to prove that the formation of a sporangial envelope 
is nothing foreign to the Lycopodiales. Mr. Kidston found 
some indications of a velum, like that of Isoetes , in the cone 
of Sigillaria , but the specimens not having their minute 
structure preserved, the conclusion to be drawn is somewhat 
uncertain. 
One point, respecting the sporophyte, remains. In Isoetes 
most leaves are fertile, and there is no special strobilus. This 
fact scarcely affects the question of affinity, for we have 
almost the same conditions in some species of Lycopodium , 
i. e. L. Selago , where also no strobilus is differentiated, and 
there is a gradual transition from sporophylls to vegetative 
leaves. In the case of Isoetes , we much doubt whether this is 
a primitive character, for the plant altogether suggests, more 
than any other genus now living, the reduced successor of 
some of the gigantic Palaeozoic forms of Lycopodiales, all 
of which, so far as we know, were strobiloid. 
The proposed transference of Isoetes from Lycopodiales to 
Filicales thus finds no support from the characters of the 
mature sporophyte. The prothailus and embryo lie beyond 
the scope of our present work, but we must shortly refer to 
their bearing on the main question. 
As regards the prothailus, recent investigations have broken 
down the distinction formerly believed to exist between 
Selaginella and Isoetes , for the supposed differentiation of 
prothailus and endosperm in the former genus has proved to 
be illusory, and in reality development proceeds in a similar 
way in both genera. 
The archegonia of Isoetes have been compared to those of 
the Eusporangiate Ferns, and no doubt with good reason, but 
the comparison is of no weight as against Lycopodinean 
affinities, for in the genus Lycopodium itself, as Campbell has 
well pointed out, ‘ the sexual organs closely resemble those 
of the eusporangiate Ferns and Equisetum ’ (Campbell, 1895, 
p. 466). Neither does there appear to be any definite distinc¬ 
tion between the archegonia of Isoetes and those of Selaginella. 
