480 A TEXTBOOK OF THEORETICAL BOTANY 



Although this arrangement probably represents the phylogenetic sequence 

 of types, we shall consider the Pteropsida first because, although the anatomy 

 of the Filicales is the most complicated, they illustrate best the alterna- 

 tion of generations characteristic of the Pteridophyta and are therefore 

 usually selected for junior study. 



PTEROPSIDA * 



The Pteropsida are distinguished from the other groups of the Pterido- 

 phyta by their megaphyllous habit, that is to say, their leaves are normally 

 large in relation to the size of the stem, and the leaves rather than the stems 

 are the dominant feature of the plant's architecture. Along with this 

 morphological difference goes the anatomical character that in most cases 

 the stele in the stem of adult plants shows leaf gaps, where the leaf traces 

 are given off. This character is naturally absent or very slightly developed 

 in stems with solid steles, and even in other cases it is subject to much 

 variation according to the relative size of the leaves. Nevertheless, it is so 

 widespread in the group, occurring even in very reduced forms, that it must 

 be considered to be a distinguishing group character. 



FILICALES 



There is only a single order, the Filicales, and they therefore exhibit 

 the same general characters as those already enumerated for the Pteropsida. 

 Since it will be necessary for us to consider a number of examples of this 

 order, on account of their marked differences, it is desirable to go somewhat 

 more fully into the classification of the Filicales which we shall describe in 

 this chapter. 



Eusporangiatae . 



Homosporous. Sporangia developing from a group of cells ; the 

 sporangial wall consisting of several layers of cells. 

 Marattiaceae, e.g., Marattia. 

 Ophioglossaceae, e.g., Botrychium. 



Leptosporangiatae . 



Homosporous. Sporangia developing from a single epidermal cell ; 

 the mature sporangium wall consisting of a single layer of cells. 

 Osmund aceae, e.g., Osmunda. 

 Hymenophyllaceae, e.g., Hymenophylliim. 

 PoLYPODi aceae, e.g.. Dry opt er is, Pteridium. 



Hydropteridae . 



Heterosporous. Sporangia enclosed within a modified leaf segment 

 or a closed indusium. 



Salviniaceae, e.g., Azolla. 



* Used here in a restricted sense to cover the megaphyllous Pteridophyta only. For 

 the wider application of the term by Jeffrey see p. 649. 



