56 LYCOPODIALES [CH. 



sporophylls, but in this genus the fertile shoots do not generally 

 form such distinct cones as in many species of Lycopodium. 

 In S.grandis (figs. 130, 3; 131, G) the long and narrow strobili 

 consist of a slender axis bearing imbricate sporophylls in four 

 rows : each sporophyll subtends a sporangium situated between 

 the ligule and the axis of the shoot. The sporangium may be 

 developed from the axis of the cone or, as in Lycopodium, from 

 the cells of the sporophylls In some species the lower sporo- 

 phylls bear only megasporangia, each normally containing four 

 megaspores, the microsporangia being confined to the upper 

 part of the cone. This distribution of the two kinds of sporangia 

 is, however, by no means constant^: in some cases, e.g. S. 

 rupestris, cones may bear megasporangia only, and in the cone 

 of S. grandis, of which a small piece is represented in fig. 131, G, 

 all the sporangia were found to contain microspores. 



The occurrence of two kinds of spores in Selaginella 

 constitutes a feature of special importance from the point of 

 view of the relationship between the Phanerogams, in which 

 heterospory is a constant character, and the heterosporous 

 Pteridophytes. One of the most striking distinctions between 

 the Phanerogams and the rest of the vegetable kingdom lies in 

 the production of seeds. Recent work has, however, shown that 

 seed-production can no longer be regarded as a distinguishing 

 feature of the Gymnosperms and Angiosperms. Palaeozoic 

 plants which combined filicinean and cycadean features re- 

 sembled the existing Phanerogams in the possession of highly 

 specialised seeds. This discovery adds point to the comparison 

 of the true seed with structures concerned with reproduction in 

 seedless plants, which in the course of evolution gave rise to the 

 more efficient arrangement for the nursing, protection, and 

 ultimate dispersal of the embryo. In the megaspore of 

 Selaginella we have, as Hofmeister was the first to recognise 

 in 1851, a structure homologous with the embryo-sac of the 

 Phanerogam. The embryo-sac consists of a large cell produced 

 in a mass of parenchymatous tissue known as the nucellus 

 which is almost completely enclosed by one or more integu- 

 ments. Fertilisation of the egg-cell within the embryo-sac 

 1 Bower (08) p. 315. 2 Hieronymus (02). 



