304 



EMBRYOLOGY 



prothallus. But, as soon as the root has become established in 

 the soil, and the first leaf has spread out its lamina to the light, 

 this dependence ceases, and soon after that the prothallus withers 

 away (cf. Fig. 169, C). As compared with Bryophj'ta, the rela- 

 tive importance of the two phases in the hfe-history is therefore 

 reversed. The freediving sporophyte usually attains large dimen- 





Fig. 169. — Embryology of the Fern. A, Longitudinal section through an 

 archegonium, showing the octant-stage in the division of the fertilised 

 egg (after Sadebeck). B, Young embryo escaping from the pro- 

 thallus {p.) (after Hofmcister). /., foot ; /., hrst leaf ; r., hrst root ; 

 s., stem. C, Young plant, with the remains of the prothallus {p.) at 

 the base, four unfolded leaves of increasing complexity, and one 

 leaf not yet unfolded (original). 



sions and exhibits a complexity of structure which is only 

 approached by the perennial gametophytes of some few Mosses. 

 On the other hand, the sexual generation is as a general rule 

 shortdived, though in a few Ferns (('.,i,'. the Royal Fern, Osiiiuiida) 

 it persists for some years. Normally the two generations alter- 

 nate regularly with one another, but occasional abnormalities 

 are encountered. 



