510 . MOSSES AND FERNS ' chap. 



firm woody walls marked by numerous pits. Occasionally the 

 septum is partially absent and the loculi are thus thrown more 

 or less completely into communication. The spores are usually 

 of the bilateral form, like the microspores of Isoetes, but may- 

 also be of the tetrahedral type. 



Bower regards the whole synangium as homologous with 

 the single sporangium of Lycopodiurn, and also calls attention 

 to its resemblance to the sporangium of Lepidodendron, with 

 which the Psilotacese also show resemblances in the structure 

 of the stem. 



The Affinities of the Psilotacece {Bower (21), Ford (i), 



Scott (i)) 



While the Psilotacese are usually united with the Lycopods, 

 there has been of late a tendency to remove them from this class, 

 and to aSsume a somewhat near affinity with the fossil Spheno- 

 phyllales, whose relationships are usually considered to be with 

 the Equisetales. The undoubted anatomical resemblances be- 

 tween the Psilotacese and Lycopodiacese cannot be overlooked, 

 and the question then remains whether these resemblances are 

 anything more than analogies. 



The anatomy of the smaller shoots of the Psilotacese un- 

 doubtedly recall the stem-structure of Sphenophyllum, and there 

 seems to be also important points of resemblance in the sporan- 

 gial structures. (Bower (21), Thomas (3)). 



Miss Ford ((i), p. 603), whose work on Psilotum is the 

 most recent, considers the Psilotaceae to be much reduced forms, 

 probably owing to their saprophytic habit. They are "some- 

 what closely allied to the fossil group of the Sphenophy Hales." 



The Selaginellace^ 



Unlike the Filicineae, the heterosporous Lycopodinese out- 

 number very much the homosporous forms, but all of the former 

 may be reduced to a single genus, Selaginella, which contains 

 nearly five hundred species, and, except for the presence of 

 heterospory, approaches closely the genus Lycopodium, to which 

 it is clearly not very distantly related. The great majority of 

 the species of Selaginella belong to the tropics, and form a 



