GROUP III. PTERIDOPHYTA : FHJCINJE. 



379 



spore-walls are ruptured by the swelling of the cells of the con- 

 tained prothalliura, and the spermatozoids are set free. 



The female individual is a small multicellular prothallium of a 

 green colour, and is developed from a macrospore to which it re- 

 mains attached. The development of the prothallium begins 

 inside the macrospore at its pointed apical end, where there is 

 an aggregation of protoplasm in which the nucleus lies. The 

 nucleus divides, and this is followed by the formation of a cell- 

 wall between the two nuclei, cutting off the apical portion of the 

 spore, as a small cell, from the basal larger portion ; this first 

 wall is termed the diapltragru, and it marks off that portion of the 

 macrospore which gives rise to the prothallium from that portion 

 which takes no part in the process. The small cell then un< 

 repeated division to form the pro- 

 thallium which consists, in the Salvi- 

 niacese, of several layers of cells (at 

 least in the middle region), and in the 

 Marsileaceae of two layers only. As 

 the prothallium developes, the coats of 

 the macrospore split into three valves 

 at the apex, so that now the prothal- 

 lium is in direct relation with the 

 exterior. Whilst in the Marsileacese 

 the prothallium projects but little from 

 the spore, in the Salviniaceae (especially 

 Salvinia), where it is larger, the 

 greater part of it is outside. 



R 



FIG. 226. Gametophyte of Sal- 

 vinia (x 60). A Macrosporangium 

 m with a germinated macrospore sp 



No cell-formation takes place in the (dotted outline) ; pt female pro- 



larger basal portion of the macrospore SEyJSfiLT 71 



below the diaphragm, though nuclear microspore isolated from a micro- 

 division has been observed in Azolla. sporangium ; 8 p spore -,pt male pro- 

 rn,. -n T_ . , thallium ; a antheridia. 



Ihis eventually becomes filled with 



starch and other nutritive substances for the nutrition of the 



embryo. 



The female organ, or archegonium, differs in no essential feature 

 of structure or development from that of the homosporous lepto- 

 sporangiate Ferns. In the Marsileacese, only a single archegonium 

 is developed ; it arises from a cell in the middle of the upper sur- 

 face of the prothallium : in Azolla, if the archegonium does not 

 become fertilised, a few more archegonia may be subsequently de- 

 veloped : in Salvinia, at least three archegonia are always formed, 



