Ferns. In the Isoetes (a Lycopod) we see, for the first time in our brief survey of the vegetal kingdom, a stem presenting woody layers one inside of the other, one of the distinguishing features of trees hke the Oak, Walnut, Chestnut, Pine, Fir, Cycas. This fact is an important one, as will appear later. The reproduction of the Lycopodiaceae and Rhizocarpse differs from that of any plants of which we have yet spoken. There are found in the Lycopodiaceae both large and small spore-cases, of which the former (Fig. 128) contain only four large spores, the latter (Fig. 128) many small ones. In Selaginella the leaves are spike-shaped, and at the base of the leaf is found either a large or small spore-case. Each spore of the large spore-case may produce within its cavity a Prothallus like that of the Fern; but it will be remembered that the Prothallus of the Fern is produced outside of the spore, whereas the Prothallus of the Lycopod is developed inside the large spore. In the Prothallus of the Lycopod, Archegonia, with their embryo-cells, alone are found; the Antheridia, with their spiral filaments, coming only from the small spores. Finally the large spore bursts, freeing its Prothallus. The spiral filament of the Antheridium of the small spore, coming in contact with the embryo-cell in the Archegonium in the Prothallus of the large spore, gives rise to the new Lycopod, which, in Selaginella, is a little stem supporting two leaves, one on each side. (Fig. 129.) This kind of reproduction is seen in the Rhizocarpae. By comparing the reproduction noticed in the Fern, Horse-tail, Rhizocarp, and Lycopod, the following series becomes apparent: the Ferns and Horse-tails produce one kind of spore; from this spore is developed a Prothallus containing both Archegonia with their embryo-cells, and Antheridia with their spiral filaments. The Rhizocarpae and Lycopodiaceae produce two kinds of spores, large and small; in the Pillwort and Pepperwort the large and small spores are
