Scott and Hill.—Structure of Isoetes Hystrix. 445 
of very little taxonomic value, and we know, chiefly through 
Williamson’s researches, that it was of common occurrence in 
all groups of Vascular Cryptogams during the Palaeozoic 
period. The anomalous character of the secondary growth 
in Isoetes , however, agrees somewhat more nearly with that 
in certain fossil Lycopods, than with the process as known in 
any of the Filicales. 
The discovery of Bruchmann that the base of the stem 
in Selaginella spimdosa (Bruchmann, 1897, p. 39) shows an 
indefinite growth in thickness, is of great interest, as affording 
a new link between that genus and Isoetes . In this species 
of Selaginella the base of the stem is persistent, and is the 
only seat of root-formation.. The development of secondary 
wood and parenchyma at the base of the stem is closely 
connected with the formation of successive new roots in the 
same region. The conditions, in fact, are closely comparable 
to those which prevail at the base of the stem in Isoetes . 
The minute histology of Isoetes is peculiar, for the tissue- 
elements have been modified in relation to the habit. They 
show no approach to the structure of the corresponding 
tissues in Ferns. 
In their whole structure, and especially in their monarch 
stele, the roots of Isoetes agree exactly with those of Selagi¬ 
nella and with the ‘rootlets’ of the Palaeozoic Stigmaria 
(subterranean organs of Lepidodendron and Sigillaria). So 
far as the dichotomous branching of the roots is concerned, 
the agreement extends to all known Lycopodiales. The 
apical structure of the Isoetes root agrees most nearly 
with that in the genus Lycopodium (Bruchmann, 1874). 
Taking the sum of the root-characters, they are throughout 
so typically Lycopodinean, that even regarded by themselves 
they would go a long way to establish affinity. Against this 
we have to set the fact that in a few species of Ophioglossum 
the roots are monarch, probably by reduction, and are said to 
be dichotomous in their mode of branching. In other species 
of the genus Ophioglossum , and in the majority of the family 
Ophioglosseae as a whole, the roots are diarch or polyarch in 
