778 LIFE : OUTLINES OF GENERAL BIOLOGY 



The schema summing up the Lycopodium Hfe-history is as 

 follows: » 



Spore — >■ Gametophyte — > Fertilised — > Embryo Sporo > Sporangia 



inconspicuous, egg-cell phyte. for a 



usually in part time dependent 



subterranean, with by means of a 



antheridia and "foot" on the 



archegonia aerial gametophyte 



The Australasian Phylloglossum closely resembles Lycopodium in 

 its gametophyte (a subterranean tuberous portion bearing a green 

 aerial crown with sex organs), but its sporophyte is arrested at a 

 stage corresponding to the very young stage in some Lycopods, and 

 becomes a tuberous stem with a cluster of small leaves and a stalked 

 strobila. 



SELAGINELLA. — This is a very successful genus with about 500 

 species, mostly tropical. It can be traced back to the Carboniferous 

 Period, when there were delicate little forms not very different from 

 many of those now living, but in striking contrast to the giant 

 Lepidodendron trees of the Carboniferous. Then, as now, the 

 Selaginellas produced two kinds of spores, macrospores and micro- 

 spores ; and this gives them a peculiar interest, since it is the first 

 hint of the contrast so marked in Flowering Plants between pollen- 

 grain and embryo-sac. 



The Selaginella sporophyte is a branching leafy stem, with a 

 gradation between foliage leaves and sporophylls, but the sporangia 

 begin on the stem just above the origin of the sporophyll. Near the 

 base of each leaf there is a minute flap or ligule, prominent during 

 development and afterwards inconspicuous, which is a good instance 

 of a minute structural feature that is quite constant. It is present in 

 all Selaginellas and absent from all Lycopodiums. 



All the sporangia begin in the same way, but they soon diverge 

 in development. In some, the microsporangia, usually in the majority, 

 the mother-cells form many microspores, four from each mother- 

 ceU. In others, the megasporangia, only one mother-cell comes to 

 anything, and it develops into a tetrad of four large megaspores. 

 In a word, the Selaginellas are heterosporous, whereas Lycopodium 

 is homosporous. Moreover, when a microspore is shed it develops into 

 a male gametophyte, which is so much telescoped that it never gets 

 free from the spore-wall. The spore divides into (a) a single vegeta- 

 tive cell, all that there is to represent the vegetative body of the 

 gametophyte, and (h) a reproductive cell which forms an anther- 

 idium, eventually giving rise to minute biciliate sperm-cells. It gets 

 carried to a megasporangium, within which a megaspore develops 

 into a female gametophyte. This shows two distinct regions, vege- 



