310 



BOTANY 



tory by Psilotum triquetrum, which is found sparingly in Florida and 

 the adjacent region. A second genus, Tmesipteris, occurs in the Aus- 

 tralasian region. They are usually epiphytes of peculiar habit, and 

 their affinity with the Lycopodiales is somewhat doubtful. In the 

 stem-structure they show a resemblance to the extinct class Spheno- 

 phyllales, with which they may possibly be remotely related. Un- 

 fortunately nothing is certainly known of the gametophyte. 



The sporophyte in Psilotum (Fig. 275) is leafless, and the roots 

 are replaced by creeping rhizomes from which the dichotomously 



branched shoots de- 

 f 



-" ** velop. The large spo- 



rangia are in groups of 

 three, and probably 

 take the place of a 

 leaf. It is question- 

 able whether or not 

 the whole synangium 

 is the equivalent of a 

 single sporangium in 

 Lycopodium. 



Ord. III. 

 Selaginellineae 



The majority of the 

 Lycopodiales belong 

 to the Selaginellineae, 

 which includes the 

 single large genus 

 Selaginella, most of 

 whose species are trop- 

 ical, although a small 

 number occur in tem- 

 perate regions. In 

 general aspect, Sela- 

 ginella closely resembles Lycopodium, but it differs in one very 

 important particular, viz., it is markedly heterosporous. Some of 

 the species have the leaves all alike, and arranged spirally about 

 the stem e.g. S. rupestris; the greater number have prostrate 

 stems with four rows of dorsal leaves, two large and two small 

 e.g. S. apus, S. Kraussiana, etc. (Fig. 279). The creeping forms 

 usually develop peculiar leafless pendent branches (Rhizophores), 

 from which the dichotomously branched roots are produced. The 

 apparently dichotomous branching of the shoots is really monopodial. 

 The sporangia are borne in the axils of slightly modified leaves 



FIG. 276. A, B, C, three views of the young anther- 

 idium of Selaginella Kraussiana (x 450) ; 'x, pro- 

 thallial cell. D, an older antheridium (X480). 

 E, F, S. stolonifera (X 480). G, spermatozoids of 

 S. cuspidata (X 1170). (After BELAJEFF.) 



