1900] SPOROPHYILS AND SPORANGIA OF ISOETES 333 
we must admit, so far as mature leaf structures are concerned, 
that Isoetes occupies an isolated, and in no sense an intermediate 
position. 
The testimony of the young leaves, however, is not so neu- 
tral, The form of the leaf rudiments, their manner of growth, 
and arrangement about the axes are the same in Isoetes as in 
Lycopodium and Selaginella, and quite different from what is seen 
among ferns. The difference is not fully expressed in saying 
that in one case the leaf originates from a single apical cell, and 
grows by means of it, and that in the other case the initiative is 
from a group of cells. The leaves of ferns are distinctly acroge- 
nous, which method of growth gives them the power of assum- 
ing complex forms and allows the successive and often slow 
formation of stipe, pinnz, and pinnules, and their gradual 
unfolding. A leaf which grows as does that of Isoetes has its 
power to assume a complex form limited to the time when it is 
meristematic throughout ; as soon as the apex becomes perma- 
nent tissue the outline of the leaf is determined. The difference 
between such leaves is fundamental and far-reaching. A Lyco- 
podium leaf could easily attain the size of an Isoetes leaf by 
retaining the meristematic power for a longer time, for they dif- 
fer only in degree. The leaf of a fern could become like that 
of Isoetes, or vice versa, only by a radical change in the manner 
of growth. 
The similarity of the leaf rudiments of Lycopodium and 
Isoetes is only a particular instance of a general likeness which 
extends to all their embryonic organs. We have already seen 
how this is true of the sporangia ; and it holds equally good for 
the roots? and stem apex. In none of these organs is there ever 
an apical cell or any concentric segmentation of the apices, such 
aS are characteristic of all the Filicales and Equisetum. A dif- 
ference in this respect in the case of apical-growing organs, like 
the roots and stem, may not lead to important differences in the 
mature Structures, as the variation in the stem apices of Selag- 
nella suffices to show. But a comparative examination of 
*V i : : 
an Tieghem (r), but Bruchmann (1) entertains a different view. 
