it will be seen that the leaflets are contracted and are densely covered beneath with brown bodies. These bodies are collections of sporangia or spore-cases.
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Fig. 256.—Common Polypode Fern. Polypodium vulgare.
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Fig. 257.—Sori and Sporangium of Polypode. A chain of cells lies along the top of the sporangium, which springs back elastically on drying, thus disseminating the spores.
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Fig. 258.—The Brake Fruits underneath the Revolute Edges of the Leaf.
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Fig. 259.—Fruiting Pinnules of Maidenhair Fern.
The sporangia are collected into little groups, known as sori (singular, sorus) or fruit-dots. Each sorus is covered with a thin scale or shield, known as an indusium. This indusium separates from the frond at its edges, and the sporangia are exposed. Not all ferns have indusia. The polypode (Figs. 256, 257) does not; the sori are naked. In the brake (Fig. 258) and maidenhair (Fig. 259) the edge of the frond turns over and forms an indusium. The nephrolepis or sword fern of greenhouses is allied to the polypode. The sori are in a single row on either side the midrib (Fig. 260). The indusium is circular or kidney-shaped and open at one edge
