CHAPTER VI 

 PTERIDOPHYTA 



The pteridophytes are a comparatively small group of plants today, 

 but in past geologic times they were much more numerous. They are 

 represented by over 9,000 living species, very unequally divided among 

 the four classes Psilophytinae, Lycopodiinae, Equisetinae, and Filicinae. 

 Most pteridophytes are terrestrial plants, but some are epiphytic and a 

 few are secondarily aquatic. They grow in a wide variety of habitats. 



Pteridophytes may be characterized as vascular plants without seeds. 

 Like the bryophytes, they display a distinct alternation of generations. 

 Their great advance lies in the development of an independent sporophyte 

 with complex roots, stems, and leaves, and one in which a prominent 

 vascular system is present. The sporophyte has now become the dom- 

 inant generation, the gametophyte always being small and inconspicuous. 



The sporophyte presents a great range in size and habit, although one 

 not so extreme as in the spermatophytes. Nearly all existing pterido- 

 phytes are herbaceous or somewhat woody, the tree ferns being a notable 

 exception. Branching of the stem, where present, is dichotomous in 

 some members, monopodial in others. Elongation of the root and stem 

 generally occurs through the activity of an apical cell; in some forms this 

 is replaced by a meristem. The spores are produced in sporangia, which 

 are usually borne in connection with the leaves. Most living pterido- 

 phytes are homosporous, all the spores being alike. Some are heterospo- 

 rous, with spores of two different kinds, these always being produced in 

 separate sporangia. As in all bryophytes and spermatophytes, the reduc- 

 tion in chromosome number occurs in connection with the formation of 

 spores. Consequently the sporophyte is the diploid generation, while the 

 gametophyte, produced by a spore, is the haploid generation. 



In the homosporous pteridophytes the gametophyte is either a simple 

 green thallus or a tuberous body that is subterranean, colorless, and 

 saprophytic. In the heterosporous forms the gametophytes are sexually 

 differentiated and greatly reduced in structure, developing entirely or 

 largely within the spore wall. The sex organs of pteridophytes are essen- 

 tially similar to those of bryophytes, but are simpler. Generally both the 

 antheridia and archegonia are embedded structures. Swimming sperms 

 are universally present. Following fertilization, the embryo develops 

 within the venter of the archegonium, which forms the calyptra. The 



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