Fig. 52.—Spores
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.
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.
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.
