Each spore a single cell which develops with three others in tetrads (groups of four). Very numerous tetrads enclosed in a spore case or sporangium which develops on a leaflike segment called the sporophyll. Each spore germinates independently of the others after being scattered, all being of the same size. Common in fossils and living Pteridophytes.] [Illustration: Fig. 53.—Spores Each a single cell like the preceding, but here only one tetrad in a sporangium ripens, so that each contains only four spores. Compared with the preceding types these spores are very large. Otherwise details similar to above. Some fossils have such sporangia with eight spores, or some other small number; living Selaginellas have four. In the same cone sporangia with small spores are developed and give rise to the male organs.] [Illustration: Fig. 54.—“Spores” of Seedlike Structure Out of a tetrad in each sporangium only one spore ripens, S in figure, the others, _s_, abort. The wall of the sporangium, _w_, is more massive than in the preceding cases, and from the sporophyll, flaps, _sp f_, grow up on each side and enclose and protect the sporangium. The one big spore appears to germinate inside these protective coats, and not to be scattered separately from them. Only found in fossils, one of the methods of reproduction in _Lepidodendron_. Other sporangia with small spores were developed which gave rise to the male organs.] [Illustration: Fig. 55.—“Seed” In appearance this is like a seed, but differs from a true seed in having no embryo, and is like the preceding structure in having a very large spore, S, though there is no trace of the three aborting ones. The spore develops in a special mass of tissue known as the nucellus, _n_, which partly corresponds to the sporangium wall of the previous types. In it a cavity, _p c_, the pollen chamber, receives the pollen grains which enter at the apex of the “seed”. There is a complex coat, C, which stands round the nucellus but is not joined to it, leaving the space _l_ between them. Only in fossils; _Trigonocarpus_ (see p. 122) is similarly organized. Small spores in fern-like sporangia, called pollen grains.] [Illustration: Fig. 56.—“Seed” Very similarly organized to the above, but the coat is joined to the nucellus about two-thirds of its extent, and up to the level _l_. In the pollen chamber, _p c_, a cone of nucellar tissue projects, and the upper part of the coat is fluted, but these complexities are not of primary importance. The large spore S germinated and was fertilized within the “seed”, but apparently produced no embryo before it ripened. Small “spores” in fern-like sporangia form the pollen grains. Only in fossils, _e.g._ Lagenostoma. (See p. 119.)] [Illustration: Fig. 57.—Seed