[Illustration: FIG. 300.—ISOËTES, showing habit of plant at _a_; _b_, base of leaf, showing sporangium, velum, and ligule. ] Isoëtes bears two kinds of spores, large roughened ones, the _macrospores_, and small ones or _microspores_. Both kinds are formed in _sporangia_ borne in an excavation in the expanded base of the leaf. The macrospores are formed on the outer and the microspores on the inner leaves. A sporangium in the base of a leaf is shown at _b_. It is partially covered by a thin membrane, the _velum_. The minute triangular appendage at the upper end of the sporangium is called the _ligule_. The spores are liberated by the decay of the sporangia. They form rudimentary prothallia of two kinds. The microspores produce prothallia with _antheridia_, while the macrospores produce prothallia with _archegonia_. Fertilization takes place as in the mosses or liverworts, and the fertilized egg-cell, by continued growth, gives rise again to the isoëtes plant. CLUB-MOSSES (Pteridophyta) The club-mosses are low trailing plants of moss-like looks and habit, although more closely allied to ferns than to true mosses. Except one genus in Florida, all the club-mosses belong to the genus _Lycopodium_. They grow mostly in woods, having 1-nerved evergreen leaves arranged in four or more ranks. Some of them make long strands, as the ground pine, and are much used for Christmas decorations. The spores are all _of one kind_ or form, borne in _1-celled sporangia_ that open on the margin into _two valves_. The sporangia are borne in some species (Fig. 301) as small yellow bodies in the axils of the ordinary leaves near the tip of the shoot; in other species (Fig. 302) they are borne in the axils of small scales that form a catkin-like spike. The spores are very numerous, and they contain an oil that makes them inflammable. About 100 species of lycopodium are known. The plants grown by florists under the name of lycopodium are of the genus _Selaginella_, more closely allied to isoëtes, bearing two kinds of spores (microspores and macrospores). [Illustration: FIG. 301.—A LYCOPODIUM WITH SPORANGIA IN THE AXILS OF THE FOLIAGE LEAVES. (_Lycopodium lucidulum._) ] [Illustration: FIG. 302.—A CLUB-MOSS (_Lycopodium complanatum_).] INDEX Aborted seeds, 166. Abutilon, 156. Accessory fruit, 164, 169. Adaptation to environment, 6. Adventitious roots, 36; buds, 114. Aërial roots, 34. Aggregate fruit, 168. Air plants, 35. Akenes, 165. Algæ, 179, 183, 195. Alternation of generation, 179. Anemophilous, 149. Annual plant, 17. Anther, 135, 144, 180. Antheridium, 178, 186, 198, 200, 202, 203. Apical dehiscence, 166. Archegonium, 178, 198, 200, 202, 203. Arum family, 140. Ash, 92. Assimilation, 97. Axil, 112. Axis, plant, 15. Bacteria, 39, 109, 182.