28 STRUCTURAL BOTANY 



and a few root-hairs grow out from the prothallus. 

 The mode of development of the prothallus bears a 

 striking resemblance to that of the endosperm of the 

 Spruce Fir or other Gymnosperms, so that we are justified . 

 in calling both by the one name of prothallus (see Part 

 I. p. 272). The archegonia are formed in the same 

 way in both. The prothallus of Picea is developed 

 within the embryo-sac, that of Selaginella within the 

 megaspore. 



d. Fertilisation and Embryology 



The archegonia are fertilised by spermatozoids ; 

 this takes place under water. The spermatozoids, when 

 liberated from the ruptured antheridia, swim actively 

 through the film of water covering the damp earth, and 

 some of them are attracted to the archegonia of any 

 female prothallus which lies near enough. The probable 

 nature of the attraction will be considered when we 

 come to the Ferns (see p. 70). At this time the 

 mucilage formed from the disorganised canal-cells not 

 only fills the canal of the archegonium, but spreads 

 a little beyond its opening (see Fig. 14, ra). The 

 details of fertilisation are not so well known in 

 Selaginella as in the Ferns, but there is no doubt that 

 the spermatozoid becomes caught in the mucilaginous 

 drop and then passes down through the canal to the 

 ovum below. In other plants it has been proved that 

 the spermatozoid unites with the nucleus of the ovum. 



The really important distinction, then, between the 

 fertilisation of a Cryptogam, such as Selaginella, and that 

 of an ordinary Flowering Plant, consists in the mode in 

 which the male cell is conveyed to the ovum. In the 

 Cryptogams, the journey is accomplished by the active 



