CH. XIV] LYCOPODIUM 31 



in their life-histories a well marked alternation of generations. 

 The sporophyte (asexual generation) is characterised by the 

 relatively small size of the leaves except in the genus Isoetes 

 (fig. 132) and in the Australian and New Zealand genus 

 Phylloglossum. The stems are usually erect or trailing, pen- 

 dulous in epiphytic species or small and tuberous in Isoetes 

 and Phylloglossum. The repeated forking of the shoots 

 (monopodial and dichotomous branching) is a prominent feature 

 of the group. The vascular tissue of the stem usually assumes 

 the form of a single axial strand (stele) (fig. 125), but the 

 shoots of some species of Selaginella often contain two or more 

 distinct steles (fig. 131). The group as a whole is characterised 

 by the centripetal development of the xylem composed almost 

 entirely of scalariform tracheids: secondary xylem and phloem 

 of a peculiar type occur in Isoetes, and the production of secondary 

 xylem elements in a very slight degree has been noticed in one 

 species of Selaginella (S. spinosay The roots are constructed on 

 a simple plan, having in most cases only one strand of spiral 

 protoxylem elements (monarch structure). In Lycopodium, in 

 which stem and root anatomy are more nearly of the same type 

 than in the majority of plants, several protoxylem strands may be 

 present. The sporangia are axillary or, more frequently, borne 

 on the upper surface of sporophylls, which are either identical 

 with or more or less distinct from the foliage leaves ; in the 

 latter case the sporophylls often occur in the form of a well 

 defined strobilus (cone) at the tips of branches. 



The gametophyte (sexual generation) is represented by 

 prothalli which, in the homosporous genera, may live under- 

 ground as saprophytes, or the upper portion may develop 

 chlorophyll and project above the surface of the ground as an 

 irregularly lobed green structure (e.g. Lycopodium cernuumy. 

 In the heterosporous forms the prothalli are much reduced and 

 do not lead an independent existence outside the spore by the 

 membrane of which they are always more or less enclosed. The 

 sexual organs are represented by antheridia and archegonia; 



1 Bruohmann (97). 



2 Treub (84—90) ; see also Lang (99) and Bruohmann (98). 



