THE PTERIDOPHYTA : LYCOPSIDA, ETC. 583 



displace the sporophyll. The megaspores are similar in shape to the micro- 

 spores but about twenty times as large, being easily visible to the eye. They 

 have a very thick, cutinized wall, which is rough and warty on the outside, 

 and they also have a ver)' marked triradiate ridge which is prolonged into a 

 beak, at which point dehiscence of the spore later occurs. The mature 

 megaspores are so arranged in the megasporangium that these beaks converge 

 together in the centre of the sporangium. In both cases meiosis occurs 

 during the formation of the spores. 



When the sporangia are ripe they dehisce by a tangential slit across the 

 top and the two halves gape apart. The slit does not go down to the base 



Protoxylem 



Fig. 592. — Selaginella kraussiana. Transverse section of 

 the rhizophore showing the simple concentric stele 

 with internal protoxylem. 



and the contraction of the unsplit basal portion, as it dries, forces the spores 

 out with some violence, so that they fall a few centimetres away from the 

 parent plant. This action is much more powerful in the megasporangia, 

 where the contraction is increased by the presence of a band of thin cells 

 which acts as a hinge between the two contracting sides of the sporangium. 

 This dispersal mechanism may serve to bring spores from different plants into 

 proximity and so promote cross-fertilization. 



The great difference in spore size is correlated with a differentiation in 

 sex. The microspores produce only male prothalli, with antherozoids. The 

 megaspores are purely female, and their large size allows for the storage of 

 food material, in reserve for the nourishment of the young embryo. 

 Selaginella thus marks a very important stage in the evolution of sex in plants 

 and provides an example, among living Cryptogams, of an approach towards 

 the seed habit of the higher plants. Selaginella itself is not genetically related 

 to the seed plants, but their true ancestors in Palaeozoic times must have 



