420 



BOTANY 



the mother-cells, thus liberated, set free the club-shaped biciliate sper- 

 matozoids (F). The reduction here exhibited in the formation of the 

 male prothallium resembles that shown by the Hydropterideae (p. 410). 

 The macrospores similarly begin their development within the 

 sporangia. Internally, the spore is filled with numerous proteid 

 grains, while the nucleus lies in the peripheral cytoplasm at the apex. 

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

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

 In this way, progressing from apex to base, the spore becomes filled 

 by a process of multicellular formation, with large prothallium-cells. 

 At the same time, and proceeding in the same direction, there begins 

 a further division of these cells into smaller cells. In the tissue at the 



Fig. 353. — A-E, Selaginella stolonifera, successive stages in the germination of a microspore ; p, 

 prothallium-cell ; w, wall-cells of antheridium ; s, spermatogenous cells ; A, B, D, lateral ; C, 

 dorsal view. In E the prothallium-cell is not visible, the disorganised wall-cells enclose the 

 sperinatozoid mother-cells ; F, spermatozoids of Selaginella cuspidata. (After Belajeff, A-E x 

 640, F x 7S0.) 



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. 



The formation of prothallia, in the case of Selaginella, as also in the 

 related genus Isoetes, is thus effected in a different manner from that 

 in the other Pteridophyta, and it approaches more nearly the corre- 

 sponding process in the Conifers. 



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

 prothallium becomes partially protruded. The fertilisation of one or 

 two archegonia, which then takes place, is followed directly by the 

 segmentation of the fertilised egg - cells in the formation of the 

 embryos (Fig. 354). 



The embryogeny of Selaginella is very similar to that of Lycopodium. 



