EVOLUTION OF SEED-PLANTS 125 



bringing together the male and female spores 

 was to retain the latter within the sporangium, 

 so that the place of meeting might be fixed be- 

 forehand, at a definite point on the parent plant. 

 Various devices thus became practicable for 

 catching the small snores and keeping them in 

 the right position till they discharged their 

 spermatozoids. This stage is represented by 

 some living species of Selaginella, in which 

 microspores and megaspores meet, the sperma- 

 tozoids are emitted and fertilisation takes place, 

 within the megasporangium (see fig. 18, p. 164); 

 an embryo may even develop before the mega- 

 spore is shed. 



The retention of the megapsore within its 

 sporangium and the ultimate shedding of the 

 whole as one organ is, generally speaking, one 

 of the great characteristics of the seed-method 

 of reproduction. The imprisoned megaspore 

 forms its prothallus and pollination takes place 

 on the parent plant; so do fertilisation and the 

 development of an embryo in ordinary cases, 

 but in Cycads, embryo-formation may scarcely 

 begin till after the whole organ is shed. In Palae- 

 ozoic seeds an embryo has never been found, so 

 it is probable that the whole development of the 

 embryo and perhaps even fertilisation itself (as 

 distinguished from the preliminary process of 

 pollination) was postponed to a late stage, im- 

 mediately preceding germination. 



