rx:cTi 



PROTHALLUS 



445 



place in the remaining contents {prth^) of the spore, pro- 

 ducing a large-celled tissue, the secondary prothallus. 



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



Fig. 117. — Reproduction and Development oi Sela^inella. 



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

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

 {mi. spf;) and megasporangia {mg. spg) are borne. 



B, section of a microspore, showing the outer coat {mi. sp), prothallial 

 cell (prth), and multicellular spermar)- (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^) 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 {e>nb) in the polyplast stage, and that in the centre an embryo 

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

 cotyledons {ci) : both embryos are provided with suspensors (dotted) 

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



(Altered from Sacl]s. ) 



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 



