XXIIl] 



GAMETOPHYTE 



185 



Pihdaria americana certain elongated but not indurated cells, corresponding 

 in position to the annulus of ScJiisaea, and these he regards as a vestigial 

 representation of it (Vol. I, Fig. 253). The number of the spore-mother- 

 cells is usually eight, but some or all of these may divide again, so that 

 the whole number ranges from eight to sixteen (Campbell). The extreme 

 number of the microspores is 64. In the megasporangium only one of 

 the spore-mother-cells develops beyond the tetrad stage. One of its 

 daughter-cells enlarges greatly at the expense of the others, and forms 

 a single megaspore, with a thick, highly differentiated mucilaginous 

 wall. 



The germination of the spores of Marsilea proceeds very rapidly after 

 evacuation from the sporocarp. Divisions within the microspore separate 

 two basal vegetative cells of the prothallus, 

 while the rest of the contents form two 

 groups of spermatocytes together with 

 tabular cells as their protective walls (com- 

 pare Campbell, I.e. Fig. 245). It appears 

 that here there are probably two antheridia, 

 each with i6spermatoc)'tes: this small num- 

 ber resembles that seen in the antheridia of 

 ScJii:;aea or Ajiemia. The spermatozoids are 

 peculiar in the large number of their spiral 

 coils. The megaspores are essentially tetra- 

 hedral like the microspores, but oval in form; 

 they may be \ mm. in length, and are 

 covered by a thick differentiated mucilagi- 

 nous coat. The apex, which corresponds to Fig. 467. Superficial view of a prothallus 



of Marsilea salvatrix Hanst. //, the 

 prothallus : at its apex the neck of the 

 archegonium. si, the mucilage-layers 

 of the epispore forming a funnel, with 

 many spermatozoids. r, the torn coat 

 of the megaspore. ( x about 150.) (From 

 Engler and Prantl.) 



the centre of the original tetrad, is marked 

 by a hemispherical projection covered by a 

 thin brown wall. Here the female prothal- 

 lus is formed, while the rest of the spore is 

 stored with nutriment (Fig. 468). The 

 prothallus is minute, consisting only of a single archegonium embedded 

 in a cap of tissue fitting over the apex of the large storage cavity within 

 the spore. Seen from without at the time of fertilisation it is as in 

 Fig. 467. 



The embryo undergoes segmentation after the scheme of the Leptospor- 

 angiate Ferns, and the stem, leaf, root, and foot have the normal relations. 

 As the large oval spore will as a rule lie on its side the axis of the arche- 

 gonium will be approximately horizontal. The first or basal wall includes 

 that axis: but, as Leitgeb showed, it can adjust itself in relation to gravity 

 so that there will be an upward-directed, epibasal, shoot-forming half, and 



