676 TEXTBOOK OF BOTANY 



uncoil when dry. The spores contain chlorophyll, are short-lived, and 

 cannot withstand desiccation. Consequently, they survive longest and 

 germinate best on shaded moist banks. 



The gametophyte. The gametophytes of equisetums are irregularly 

 lobed, thalloid structures growing on moist substrates near streams or 

 in bogs. The spores from the sporangia are essentially alike in appear- 

 ance, and the gametophytes of most species, if not all, are potentially 

 bisexual. But the expression of sex in bisexual gametophytes is influenced 

 by environmental factors, and may be controlled by experimental condi- 

 tions. Some species such as E. arvense, which may be bisexual under 

 certain conditions, are usually unisexual. Archegonia usually develop on 

 the larger gametophytes, and antheridia on the smaller ones. Fertiliza- 

 tion takes place when a motile sperm fuses with an egg. The resulting 

 zygote germinates at once, and an embryo sporophyte is formed. 



Summary. The ferns, club mosses, quillworts, and equisetums are 

 classified in one group, the pteridophytes. The sporophyte of these 

 plants, in sharp contrast to liverworts and mosses, has a vascular system 

 and is much more conspicuous than the gametophyte. The vegetative 

 phase of the sporophyte generally consists of definite leaves, stems, and 

 roots; but modern pteridophytes rarely bear seeds. The gametophyte is 

 generally thalloid; it may be wholly green and grow on the soil or in 

 other moist situations; it may be subterranean, devoid of chlorophyll, 

 and hence saprophytic; or it may be partly green and partly non-green. 

 The terrestrial ferns and the lycopods produce one kind of spore only 

 (homospory); aquatic ferns and Selaginella produce microspores and 

 megaspores ( heterospory ) . The spores usually gemiinate on the ground 

 or in the water, resulting in the formation of a gametophyte separated 

 from the sporophvte. The megaspore of Selaginella, however, sometimes 

 germinates in the megasporangium. This is a step in a series of events 

 that leads to the formation of a seed. Geologically the pteridophytes are 

 very ancient. 



REFERENCES 



Blomquist, H. L. Ferns of North Carolina. Duke Univ. Press. 1934. 

 Brown, W. H. The Plant Kingdom. Ginn and Company. 1935. 

 Clute, W. N. The Fern Allies. Willard N. Clute and Company. 1928. 

 Fames, A. J. Morphologij of Vascular Plants. McCraw-Hill Book Company, 



Inc. 1936. 

 Frye, T. C. Ferns of the Northwest. Metropolitan Press. 1934. 

 Schaffner, J. H. Papers on Equisetums in American Fern Journal. 1930-36. 



