XXXII 



PROTHALLUS 



445 



place in the remaining contents (prth 1 } of the spore, pro- 

 ducing a large-celled tissue, the secondary prothallus. 



By the rupture of the double cell-wall of the megaspore 



spsr 



FIG. 117. Reproduction and Development of Selaginella. 



A, diagrammatic vertical section of a cone, consisting of an axis bear- 

 ing close-set sporophylls (sp. ph), on the bases of which microsporangia 

 (mi. spg} and megasporangia (mg. spg} are borne. 



B, section of a microspore, showing the outer coat (mi. sp}, prothallial 

 cell (prth}, and multicellular spermary (spy}. 



c, vertical section of a megaspore, the wall of which (mg. sp) has been 

 burst by the growth of the prothallus (prth} : its cavity (prth l \ contains 

 a large-celled tissue, the secondary prothallus : in the prothallus are 

 three ovaries (ovy}> that to the left containing an ovum, that to the right 

 an embryo (emb) in the polyplast stage, and that in the centre an embryo 

 in the phyllula stage, showing stem-rudiment (st), foot (/"), and two 

 cotyledons (ct} : both embryos are provided with suspensors (dotted) 

 (spsr), and have sunk into the secondary prothallus. 



(Altered from Sachs. ) 



the prothallus is exposed to the air, but it never protrudes 

 through the opening thus made, and is, therefore, like the 

 corresponding male structure, purely endogenous. One or 



