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.