DEVELOPMENT OF PLANTS 



347 



of Selaginella have a creeping habit Hke that of Lycopodium, 

 and the aerial stems are covered with leaves arranged in several 

 rows, very frequently in four rows of two large and two smaller 

 leaves (Fig. 241, i^). Usually but a single chloroplast appears 

 in each cell, as already noticed in several algae and in the Antho- 



FlG. 241. A common cultivated Selaginella: i, habit of the plant — s, 

 strobili; b, a branch bearing roots, r. lA, portion of stem, showing leaf ar- 

 rangement. — H. O. Hanson. 



cerotales. The roots are frequently developed from the end of 

 naked branches that extend from the stems to the ground (Fig. 

 241, r). 



(a) Sporangia and Spores.- — ^The sporangia are borne on the 

 stems in the axils of the leaves which form a strobilus, as in 

 Lycopodium. However, they differ radically from those of the 

 Lycopodium, in that two kinds of spores are formed, small spores 

 or microspores and larger ones or megaspores. The formation 

 of two kinds of spores, or heterospory, is not confined to the 

 Lycopodiales. It occurs among certain genera of the Filicales 



