PTKRIDOIMIVTKS 



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 



(ng. 3"). 



Embryo. — The embryo sporophyte differs from that of Selaginella 

 and Lycopodium in several important particulars. In the first place, 

 there is no suspensor, and this feature 

 associates Isoeles 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 Isoeles the 

 stem. The embryo of Isoeles has long 

 been recognized to have a remarkable re- 



Figs. 323, 324. — Embryo of 

 Isoetes: 323, first division of the 

 fertilized egg, differing from 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 l " re ° l 



00 showing 



perhaps Isoetes is a living representative 

 of the ancestors of monocotyledons. In 

 Isoetes the axis of the embryo develops the 

 root at one end and a single leaf (cotyledon) 

 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 



ure of the embryo; 324, embryo 

 foot (below), terminal 

 cotyledon (to the right), root (to 

 the left), ligule (above) from deep 

 notch, and shallow notch (between 

 ligule and root) for stem tip. — 

 After Campbell. 



