198 Plant Biology 



and primary leaf. Although male antheridia and female archegonia are 

 present on the same gametophyte, most of the antheridia develop and 

 discharge their sperms before the eggs are mature in the archegonia of 

 the same plant. Hence, cross-fertilization between different plants 

 usually occurs. The life cycle of the bracken shows alternation of gen- 

 erations between the independent sporophyte generation (with 2N 

 chromosomes) and the independent gametophyte generation (with N 

 chromosomes). A contrast between ferns and mosses may be observed 

 below : 



2. Polypodium (pol i -po' di um) (Gr. polys, many; podion, small 

 foot).— The common polypody ferns of the genus Polypodium (Figs. 51 

 and 68) have rather simple but lobed, leaf blades. The thick rhizome 

 is horizontal and possesses numerous slender, fibrous, adventitious (un- 

 usual) roots. . Several leaves (fronds) arise from the rhizome. Dotlike 

 aggregates of sporangia known as sori (Gr. soros, heap) are present on 

 the undersurface of the leaves. The particular arrangement of the sori 

 varies with the species. In many ferns each sorus is covered by a pro- 

 tective, membranous indusium (in-du'zium) (L. induere, to put on). 

 Each sporangium consists of a thin-walled capsule borne on a stalk. A 

 hygroscopic annulus is composed of a band of moisture-sensitive cells. 

 Some of the walls of the annulus cells are thick, while the other walls 

 are thin. Their response to moisture changes causes the annulus to bend, 

 thus hurling the spores from the capsule. 



QUESTIONS AND TOPICS 



1. List the characteristics of (1) the phylum Tracheophyta, (2) subphylum 

 Pteropsida, and (3) class Filicineae. 



2. Learn the pronunciation, derivation, and meaning of each new term used in 

 this chapter. 



3. Discuss the economic importance of ferns, horsetails, and club "mosses." 



4. Explain how the sporophytes and gametophytes differ in ferns, horsetails, and 

 club "mosses." 



5. Make a diagram of a typical fern life cycle showing the stages in correct 

 sequence and the chromosome numbers in the sporophyte and gametophyte 

 generations. 



