XII LEPTOSPORANGIAT^ HETEROSPORE^ 385 



appearance, and is evidently concerned with the elaboration of 

 the reserved food materials in the large spore cavity. In 

 exceptional cases indications of the formation of cell walls 

 between these nuclei were seen, but usually they remained 

 quite free. Whether a similar state of affairs exists in Sahinia 

 remains to be seen. 



When the first archegonium is ripe, the prothallium is 

 nearly hemispherical, with the originally convex base strongly 

 concave. The central cell of the archegonium is separated b)' 

 one, sometimes two, layers of cells from the spore cavity, and 

 the neck projects considerably above the surface of the pro- 

 thallium. The latter now pushes up between the softened 

 episporic mass at the top of the spore, and the archegonium 

 is exposed. In cross-section the prothallium is more or less 

 triangular (Fig. 198, E), with one angle longer than the others. 

 This longer arm corresponds to the " sterile third " of the 

 prothallium of Sahnnia, and represents the first cell cut off 

 from the prothallium mother cell. 



If the first archegonium is fertilised, no others are formed ; 

 but usually several secondary ones are present. The second 

 archegonium arises close to the primary one ; indeed its 

 central cell is generally separated from it only by a single 

 layer of cells. The third forms near the base of the larger 

 lobe (Fig. 198, E). In case all of these prove abortive, others 

 develop between them apparently in no definite order, and to 

 the number of ten or occasionally more. In the older prothallia 

 these later archegonia are sometimes borne in small groups 

 upon elevations between the older ones. 



The neck canal cell of the archegonium is formed much 

 earlier than Pringsheim describes in Salvinia, and is cut off 

 from the central cell about the time the first divisions take 

 place in the cover cell. Each row of the neck has four cells, 

 as in Salvinia, and the neck canal cell may have its nucleus 

 divide, as in Isoetes and the homosporous Filicinese. This has 

 not yet been observed in Salvinia. 



In Salvinia ^ the prothallium is large and develops a good 

 deal of chlorophyll. It has a very characteristic appearance; and 

 shows the same triangular form that Azolla does, but from \.\\o 

 of the corners long wing-like appendages hang down, and the 

 whole prothallium is saddle-shaped. The side joining the two 



^ Pringsheim (i) ; Prantl (4). 

 2 C 



