PTERIDOPHYTA. 205 



Class 3. Lycopodinae (Club-Mosses). --Roots generally 

 branching dichotomously. The leaves are scattered or opposite, 

 and in proportion to the stem very small, undivided, and simple. 

 They are scale-like and triangular, tapering from a broad base to 

 a point. The sporangia are situated singly (except in Psilotacece), 

 and almost in every case on the upper side of the leaf or in the 

 axil of a leaf; but in some cases they are borne on the stem, just 

 above the leaf-axil. The sporangia arise from groups of epidermal 

 cells. The sporophylls are often modified, and differ from the 

 foliage-leaves ; they are then arranged in cones placed terminally 

 on branches, thus limiting their growth. 



A. Isosporpus : Sub-Class 1. Lycopodieas. 



I?. Heterosporous : Sub-Class 2. Selaginellese. 



Class 1. Filicinae (Ferns). 



The characteristics of this class have already been given on page 

 204. 



The class is divided into two sub-classes : 



1. The TRUE FERNS, FILICES, have one kind of spore which 

 generally developes monoecious prothallia, relatively large and 

 green. The sporangia are most frequently situated in groups 

 (sori), which are often covered but not enclosed by an indusium. 



2. WATER FERNS, HYDROPTERIDE.E, have micros porangia with 

 many (4x16) microspores, and macrosporangia, each with one mac- 

 rospore. The prothallium is small, and projects but slightly from 

 the germinating spore. The sporangia are situated in groups 

 (sori), which are either enclosed by an indusium, or enveloped in 

 a portion of a leaf, to form " fruits " termed sporocarps. 



The old name for the Hydropterideas, "Rhizocarpeaa," i.e. the " root-frnited," 

 originated from the erroneous supposition that the sporocarps were borne on 

 the roots. 



Sub-Class 1. Filices (the True Ferns). 



Of the eight orders (with about 4,000 species) comprised in this 

 sub-class, the Polypodiaceas is the largest (having about 2,800 

 species) and the most familiar; for this reason it will be taken as 

 typical. 



The sexual generation. When the spore germinates, the 

 external covering (exospore) is ruptured, as in the Mosses. The 

 internal cell-wall (endospore) grows out as a filament, which soon 

 divides and gives rise to the prothallium, a flat, cellular expansion 

 resembling the thallus of a Liverwort. In its fully developed state 



