_B_. =Heterosporous=: Sub-Class 2. Extinct forms. Class 3. =Lycopodinæ= (=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 _Psilotaceæ_), 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_. =Isosporous=: Sub-Class 1. Lycopodieæ. _B._ =Heterosporous=: Sub-Class 2. Selaginelleæ. Class 1. =Filicinæ= (=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 monœcious 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, HYDROPTERIDÆ, have microsporangia with many (4 × 16) microspores, and _macrosporangia, each with one macrospore_. 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 Hydropterideæ, “Rhizocarpeæ,” _i.e._ the “root-fruited,” 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 Polypodiaceæ is the largest (having about 2,800 species) and the most familiar; for this reason it will be taken as typical.