Ferns and Their Allies 195 



the rough, silica-bearing stems, they may be used for scouring purposes 

 and are commonly known as scouring rushes. The vascular system con- 

 sists of vascular bundles composed of phloem and xylem. 



The terminal homosporous strobili (cones) consist of whorls of shield- 

 shaped (umbrella-like) sporangia phores, each of which bears five to ten 

 elongated, saclike sporangia (spore cases). The spores possess jour 

 elators which are ribbon shaped and hygroscopic. The elators respond 

 to differences in moisture and may assist in spore dispersal (Fig. 49). 



Germinating spores form small, green, irregularly lobed, thalloid game- 

 tophytes. These bear rhizoids and usually both male antheridia and 

 female archegonia. The antheridium produces coiled, multiflagellated 

 sperms which swim to the egg in the archegonium where fertilization 

 produces a zygote. The latter develops into an embryo from which a 

 new sporophyte with its leaves, stems, and roots is formed. Hence, there 

 is an alternation of generations (Fig. 49). 



FERNS 



1. Pteridium (te-rid'ium) (Gr. ptero, wing or feather). — The ferns 

 belong to the subkingdom Embryophyta; phylum Tracheophyta; sub- 

 phylum Pteropsida (ter -op' si da) (Gr. pteris, wing, or feather; opsis., 

 appearance) because of the winglike or featherlike appearance of certain 

 species. 



The brake ferns (brackens) (bracken, fernj are common species in 

 temperate regions and many belong to the genus Pteridium (Fig. 50). 

 The slender, underground stem (rhizome) continues to grow at its an- 

 terior end, while it dies at the opposite end. It may even separate into 

 pieces, thus producing independent plants. The underground stem is 

 quite well developed, consisting of internal parenchyma cells, mechanical 

 tissues, vascular bundles (with phloem and xylem), and epidermis (Fig. 

 69). Long, slender roots arise from the underground stem. A young 

 leaf arises from the stem as a tightly coiled structure which pushes 

 through the soil. When in the air it uncoils and continues to grow to 

 form a slender, central petiole and a much-divided blade. The entire 

 leaf of a fern is referred to as a frond (L. frons, leaf), and the small 

 leaflets are called pinnae (L. pinnae, feather) (Fig. 68). In general, 

 the internal structure of the leaf is similar to that of higher plants, being 

 composed of epidermis, stomata, guard cells with chlorophyll, veins, 

 spongy tissue, and a palisade layer. 



Certain of the green leaves of a bracken bear numerous sporangia 

 (spore cases) on the edge of the undersurface of each leaflet. Such 



