286 



THIRD GROUP. VASCULAR CRYPTOGAMS. 



prothallium ; but some weeks after the dispersion of the spores free cell-formation 

 begins beneath this earlier tissue in the cavity of the spore, which results in the filling 

 up of the entire cavity and the production of a large-celled tissue, a secondary 

 prothallium as it may be termed l . The formation of the archegonia begins before 

 the bursting of the exosporium, which ensues as in Isoetes. The first archegonium 

 appears at the apex of the prothallium ; others arise in centrifugal succession on the 

 exposed portions of the prothallium, whether fertilisation is effected in the first or not 2 . 



in 



FIG. 231. Germination of Selagitiella. f III 5. Martensii, A D S. caulescens. I longitudinal section of a nncro- 

 spore filled with the prothallium, in which two embryos e, e> have begun to form ; d the diaphragm. // a young archego- 

 nium still closed. /// an archegonium with the oosphere fertilised and once divided. A a microspore showing the 

 divisions of the endosporium. K. C different views of these divisions. D the mother-cells of the spermatozoids in the 

 matured antheridium. After Pfeffer. 



In both genera the formation of the archegonium begins with the division of a 

 superficial cell of the prothallium parallel with the surface ; the outer of the two new 

 cells divides by cross division into four cells, and each of these divides by obliquely 



1 Pfeffer compared this tissue with the endosperm of the Angiosperms and gave it that name ; 

 but since the homology of the two formations must be doubtful as long as the processes in the 

 macrospore of the Selaginelleae are not better known than they now are, a more definite term is prefer- 

 able. It is probable that the contents of the macrospore divide into two primordial cells, one of which 

 moves to the apex of the macrospore and there produces the primary prothallium, while the other 

 remains at first at the base of the macrospore, and subsequently produces the secondary prothallium. 



2 [Pfeffer, Locomotor. Richtungsbew. d. chem. Reize (Ber. Dent. Bot. Ges. 1883), states 

 that in Sclaginc/Iii crythropus the spermatozoids are attracted by malic acid, and that, as in the' 

 case of the Filices, the presence of this substance in the mucilage of the archegonium is influential in 

 bringing about fertilisation.] 



