XIV] SELAGINELLA 57 



takes place as a rule while the female reproductive organ is 

 still attached to the parent-plant and separation does not occur 

 until the ovule has become the seed. 



In a few cases, notably in certain plants characteristic of 

 Mangrove swamps, continuity between the seed and its parent 

 is retained until after germination. The megasporangium of 

 Selaginella dehisces^ along a line marked out by the occurrence 

 of smaller cells over the crest of the wall. It has been customary 

 to describe the megaspores as being fertilised after ejection 

 from the sporangia. This earlier separation from the parent 

 and the absence of any protective covering external to the 

 spore-wall constitute two distinguishing features between seeds 

 and megaspores. In Selaginella apus, a Califomian species, 

 Miss Lyon has shown that fertilisation of the egg-cell usually 

 takes place while the megaspore is still in the strobilus. On 

 examining withered decayed strobili of this species which 

 had been partially covered with the soil for some months after 

 fertilisation of the megaspores, several young plants were 

 found with cotyledons and roots projecting through the crevices 

 of the megasporangia^- From this, adds Miss Lyon, " it seems 

 safe to assume that an embryo may have two periods of 

 growth separated by one of quiescence quite comparable to 

 those of seed plants with marked xerophilous features." 



In another Western American species S. rupestris described 

 by the same writer the cotyledons of young plants were found 

 protruding from the imbricate sporophylls of a withered cone 

 (fig. 131, D). This species is interesting also from the 

 occasional occurrence of one instead of four megaaporangia in a 

 sporangium ; a condition which affords another connecting link 

 between the heterosporous Pteridophytes, on the one hand, and 

 the seed-bearing Phanerogams in which the occurrence of a 

 single embryo-sac (megaspore) in each ovule is the rule. The 

 cones of Selaginella rupestris retain connexion with the plant 

 through the winter and fertilisation occurs in the following 

 spring. After the embryo has been formed the megasporangium 

 " becomes sunken in a shallow pit formed by the cushion-like 

 outgrowth of the sporophyll around the pedicel." It is 

 1 Goebel (05) p. 581. ^ Lyon (01) p. 135. 



