PTERIDOPHYTES 
141 
along the triradiate crack through the megaspore wall, along which 
lines the archegonia appear (fig. 322). The archegonium is notably 
broad and short, and the primary neck canal cell usually does not 
divide, resulting in a single uninucleate neck canal cell, which is as 
far as the reduction of the axial row is carried among pteridophytes 
(fig. 322). 
Embryo. The embryo sporophyte differs from that of Selaginella 
and Lycopodium in several important particulars. In the first place s 
there is no suspensor, and this feature 
associates Isoetes with the other pterido- 
phytes. The fertilized egg, however, be- 
haves much as does the embryonal cell in 
Selaginella and Lycopodium, except that 
the quadrant cells are assigned differently, 
two of them forming the foot (as in Lyco- 
podium), and the other two forming leaf 
and root, the stem being the belated 
member. However, it is characteristic of 
Lycopodiales to have some member of the 
body belated in appearance. In Lyco- 
podium the belated member is the root, in 
Selaginella the foot and root, in Isoetes the 
stem. The embryo of Isoetes has long Isoetes: 323, first division of the 
been recognized to have a remarkable re- 'f^f 1 . 6 ^' diffei ; ing f [ ora that 
of Selaginella in that the outer 
semblance to the characteristic embryo of (upper) cell does not form a sus- 
monocotyledons among seed plants; and pensor, but enters into the struc- 
for this reason it was once suggested that *" re f the , emb ;fV' 3 * 4 ' embry 
showing foot (below), terminal 
perhaps Isoetes is a living representative cotyledon (to the right), root (to 
of the ancestors of monocotyledons. In the left), ligule (above) from deep 
Isoetes the axis of the embryo develops the ? otc , h ' and shallow f notch ( between 
J r ligule and root) for stem tip. 
root at one end and a single leaf (cotyledon) After CAMPBELL. 
at the other, the foot arising from the middle 
region and being embedded in the nutritive tissue within the megaspore. 
On the free side of the axis a notch appears, from the bottom of which 
the stem tip arises (fig. 324). The feature of the embryo of mono- 
cotyledons is that the single cotyledon is terminal and the stem tip is 
lateral, and this feature is exactly reproduced in the embryo of Isoetes. 
Summary. A summary of the arguments for and against retaining 
Isoetes among Lycopodiales is as follows: its characters in common 
FIGS. 323, 324. Embryo of 
