278 PLANT MORPHOLOGY 



7. Polypodiaceae. The Polypodiaceae constitute the highest and 

 hirgest family of true ferns. Although chiefly tropical, it includes nearly 

 all the ferns of temperate regions. About 150 genera and 6,000 species 

 are known. The following list includes most of the large genera: Pteris, 

 Adiantum, Athyrium, Cheilanthes, Dryopteris, Polystichum, Asplenium, 

 Blechnum, Elaphoglossum, and Polypodium. 



Sporophyte. The sporophyte of the Filicales displays great variation 

 in size, ranging from small delicate herbs to trees 18 m. or more in height. 

 Most members of the group are terrestrial, but some are climbing, some 

 epiphytic, and a few^ aquatic. The stem may be subterranean or aerial, 

 erect or horizontal, and branched or unbranched. In most true ferns, and 

 in all the common species of temperate regions, the stem is a creeping rhi- 

 zome without aerial branches. The leaves of tropical species are ever- 

 green. Those of temperate species, with few exceptions, die at the end of 

 the growing season, new ones appearing each spring. The tree ferns of 

 tropical regions have an erect, woody, unbranched stem bearing a ter- 

 minal cluster of large leaves (Fig. 233). 



Some true ferns have simple leaves but most of them have large, char- 

 acteristic, pinnately divided leaves often called fronds. Their leaflets, 

 termed pinnae, are usually again divided, the smaller segments being 

 known as pinnules. The leaves are generally firm and leathery but are 

 often thin and membranaceous, being very delicate in the "filmy ferns" 

 (Hymenophyllaceae). Stipular wings are present at the base of the pet- 

 iole in the Osmundaceae but not in the other families. In unfolding from 

 the bud, the leaves uncoil from the base toward the apex and continue to 

 grow at the tip until they have reached their full size. This familiar 

 behavior, known as circinate vernation, is very characteristic. The leaves 

 have an elaborate system of branching veins, the branching being nearly 

 always dichotomous and open, but sometimes reticulate (Fig. 240). 

 Branching of the rhizome is usually monopodial but occasionally dichoto- 

 mous; that of the roots is always monopodial. 



In nearly all the Filicales the root tip displays a large tetrahedral apical 

 cell that undergoes very regular segmentation, cutting off cells from the 

 three sides and also from the forward face to form the root cap (Fig. 234). 

 The stem tip likewise grows by means of a large apical cell that nearly 

 always is tetrahedral, cutting off segments in regular succession, but only 

 from the three lateral faces. In the bracken (Pteridium aquilinum) a 

 modified form of dolabrate apical cell is present, forming segments right 

 and left. 



Vascular Anatomy. The anatomy of the roots and leaves is essentially 

 similar to that of the spermatophytes. The leaves have an upper and a 

 lower epidermis with stomata usually confined to the lower surface, meso- 



