DIV. I 



PTERIDOPHYTA 



529 



corresponds to the rhizoid cell of Salvinia, and into a large cell, which divides 

 successively into eight sterile prothallial or wall cells and two or four central 

 spermatogenous cells (Fig. 496 A). By the further division of the central 

 cells, which represent a single antheridium, numerous spermatozoid mother cells 

 are formed (B-D}. The peripheral cells then break down and give rise to a 

 mucilaginous substance, in which is embedded the central mass of spermatozoid 

 mother cells (E}. The small prothallial cell (p), however, persists. The whole 

 male prothallium is up to this stage still enclosed by the wall of the microspore. 

 This ultimately ruptures, and the mother cells are set free and liberate the club- 

 shaped spermatozoids. Each of these has two long cilia at its pointed end. 



The macrospores in some species similarly begin their development within 

 the sporangia. After the division of the nucleus into daughter-nuclei and their 

 distribution in the apical cytoplasm, the formation of cell walls begins. In this 



FIG. 497,SelagineUa Martensii. A, Ruptured macrospore seen from above showing the prothallus 

 with three groups of rhizoids and several archegonia (x 112). B, Longitudinal section of 

 the prothallus showing two archegonia in which embryos are developing (x 112). (After 

 BRTCHMAXN.) 



way, progressing from apex to base, the spore becomes filled by a process of 

 multicellular formation, with large prothallial cells. At the same time, and 

 proceeding in the same direction, there begins a further division of these cells 

 into smaller cells. In some species the apical disc of tissue is formed first, and is 

 separated by a thickened wall or diaphragm from the rest of the cavity of the 

 spore ; cell- formation occurs in this later. In the tissue at the apex, consisting of 

 small cells, the rudiments of a few archegonia appear, often even before the 

 formation of the prothallium has been completed. The archegonia are usually 

 not formed until the spores have been discharged from the sporangium, but in 

 some cases even fertilisation takes place on the parent plant. 



The wall of the spore eventually bursts at the apex, and the prothallium 

 becomes partially protruded ; it forms a number of rhizoids on three projections 

 of its tissue. The fertilisation of one or two archegonia, which then takes place, 

 is followed directly by the segmentation of the fertilised egg-cells and the forma- 

 tion of the embryos (Fig. 497). 



2M 



