PTERIDOPHTTA. 225 



vascular bundle. The internal cells of the larger end of 

 these hairs undergo subdivision, and thus give rise to a 

 number of spores. The hairs are thus spore-cases {lep- 

 tosporangia). In some ferns these clusters of spore-cases 

 are naked (Pig. 133, A), while in others they are covered 

 by a special outgrowth of the epidermis (Fig. 133, B, C), or 

 by a folding of a part of the leaf (Pig. 133, D), etc. 



419. The mature spore-case in most common ferns has a 

 ring of thicker cells extending around it. When these be- 

 come dry, they contract in such a way as to break open the 

 spore-case and thus set the spores free. 



420. The spores soon germinate, upon moist earth. 

 The sexual plant thus produced is generally heart- 

 shaped, flat, and green, adhering closely to the earth by its 

 root-hairs. After som'e weeks or months little "seedling" 

 ferns may be found, with one or two minute leaves. Un- 

 der favorable conditions every such fernlet will give rise to 

 a strong and long-lived fern. 



Among our common ferns are tlie common Polypody (Polypodium 

 Tulgare, Fig. 133), the Golden Fern (Gymnogramme triangularis) of 

 California, the Maidenhair of tlie North. (Adiantum pedatum) and of 

 the South (A. capillus-veneris), the common Brake (Pteris aquilina), 

 the Spleenworts (Asplenium) of many species, the Shield-ferns (Aspi- 

 dium), also of many species, the curious little Walking-leaf (Campto- 

 sorus rhizophyllus), the Bladder-fern (Cystopteris fragilis), the large 

 Ostrich-fern (Onoclea struthiopteris), the " Flowering Ferns" (Osmun- 

 da) of several species, and, most beautiful of all, the Climbing Fern 

 (Lygodium palmatum) of the Appalachian region. 



In the Coal Period the ferns were much more numerous than at 

 the present. Many families which flourished then are now extinct. 

 The ferns of that period were often tree-like and of large size. 



421. The Pepperworts (Order 31, HTDEOPTERiDBiE) are 

 small aquatic or semi-aquatic plants, producing spores of 

 two kinds, viz., small ones (microspores) which are very 

 numerous, and large ones (macrospores) which are less 



