170 THE EVOLUTION OF PLANTS 



cone being formed. This simple arrangement, 

 though occurring in Lycopodium, is not known 

 among recent Selaginellas. From these facts 

 it would appear that in Coal Measure times 

 some members of the group had already reached 

 the same stage of evolution as the modern genus, 

 while others still lagged behind. It is, however, 

 somewhat surprising to find that another Coal 

 Measure plant, with the habit of Selaginella, 

 went far beyond any living ally, and produced 

 an organ closely analogous to a true seed. This 

 plant, Miadesmia, occurs in a petrified condition 

 in the coal-balls of the Lower Coal Measures of 

 the North of England. Its structure has been 

 studied in detail. The stem is very slender and 

 the leaves appear to have been borne in four rows; 

 their margin is fringed with hairs, and on the 

 upper surface is a remarkably well-developed 

 ligule. The structure of the stem is much like 

 that of one of the simpler species of Selaginella. 



The fructifications are grouped in a loose cone; 

 on the upper surface of each sporophyll is a 

 sporangium, containing a single megaspore of 

 large size, which occupies the whole cavity of the 

 sporangium, like the embryo-sac in a seed (see 

 fig. 19). The prothallus in the megaspore is 

 sometimes preserved. The megasporangium is 

 enclosed in an envelope or coat, by which it is 

 completely covered; only a narrow opening, which 



