90 THE MORPHOLOGY OF PTERIDOPHYTES 



commonly consists of thirteen cells (one small prothallial 

 cell, eight jacket cells and four primary spermatogenous 

 cells, of which the latter undergo further divisions to produce 

 128 or 256 biflagellate antherozoids— Fig. 14Y). Within the 

 megaspore, a large vacuole appears, around which free 

 nuclear divisions occur and then, subsequently, a cap of 

 cellular tissue becomes organized beneath the tri-radiate 

 scar. In some species, this cap is continuous with the rest of 

 the prothallus, which later becomes cellular too, but in 

 others a diaphragm of thickened cell walls is laid down, as 

 illustrated in Figs. 14R and 14U (3). Rupturing of the mega- 

 spore allows the cap to become exposed and it frequently 

 develops prominent lobes of tissue, covered with rhizoids, 

 between which are numerous archegonia. It has been 

 suggested that the rhizoids, as well as anchoring the mega- 

 spore, may serve to entangle microspores in close proximity 

 to the archegonia. 



The archegonia are similar to those of Isoetes, except that 

 the neck is shorter and consists of two tiers of cells only. 



The embryology of Selaginella is remarkable for the very 

 great differences that occur between species. These are 

 illustrated in Figs. 14M-X, all of which, for ease of com- 

 parison, are drawn as if the megaspore were lying on its 

 side. The first cross wall is in a plane at right angles to the 

 axis of the archegonium (Fig. 14M) and the fate of the 

 outermost half in the different species is indicated by 

 obUque shading. In S. Martensii (Figs. 14M-P) the outer half 

 gives rise to a suspensor (s), while the inner half gives rise to 

 all the rest of the embryo, with a shoot apex, (x), a root (r) 

 and a swollen foot (f). The axis of the embryo, in this species, 

 becomes bent through one right angle so as to bring the shoot 

 apex into a vertical position. S. selaginoides (Fig. 14Q) 

 is similar, except for the absence of a foot. S. Poulteri (Figs. 

 14R-T) is a species with a well developed diaphragm, through 

 which the embryo is pushed by the elongating suspensor. A 

 curvature through three right angles then brings the shoot into 



