[Chap. XLIX FERNS, CLUB MOSSES AND EQUISETUMS 667 



All the eggs and sperms produced in the same gametophyte have the 

 same chromosome complement. If self-fertilization occurs, the resultant 

 zygote and sporophyte will be completely homozygous. If reduction 

 division is regular, all of the spores formed in this sporophyte will have 

 the same chromosome complement; and so will all of the cells of all of 

 the gametophytes that develop from these spores. 



Mutation, hybridization, and parthenogenesis occur in ferns as in 

 flowering plants. Changes in chromosome number in ferns, however, 

 may also occur in another peculiar way. Gametophytes sometimes origi- 

 nate vegetatively from the diploid cells of the sporophyte; and sporo- 

 phytes sometimes originate vegetatively from monoploid cells of the 

 gametophyte. 



Alternating phases in the life cycle of plants. In the life cycle of many 

 algae, and of all seed plants, bryophytes, and pteridophytes, a diploid 

 spore-bearing phase, the sporophyte, ordinarily alternates with a mon- 

 oploid gamete-bearing phase, the gametophyte. In the ferns and brown 

 algae either phase may continue to multiply vegetatively. In the bryo- 

 phytes and such algae as Oedogonmm, vegetative multiplication is lim- 

 ited largely to the gametophyte, while in the seed plants it usually occurs 

 only in the sporophyte. There are no alternating phases in the blue- 

 green algae. 



The sporophyte phase of seed plants, pteridophytes, and brown 

 algae is the conspicuous one. The gametophyte phase (pollen and 

 embryo sac) of the seed plants and of some of the pteridophytes is 

 very small. The gametophyte phase of the bryophytes is the conspicuous 

 one, and the smaller sporophyte phase is wholly or partially dependent 

 upon it as a source of food. 



The gametophyte phase of the seed plants develops within the meg- 

 asporangium (ovule), whereas that of non-seed plants develops on the 

 soil or in water. The first step in the formation of seeds is the germina- 

 tion of the spores within the sporangia of the sporophyte. If these spores 

 fall upon the soil, no seeds are formed. Some of the ancient ferns had 

 seeds, and some of the modern pteridophytes occasionally have seeds or 

 structures closely resembling seeds. 



The aquatic ferns. The water ferns constitute a small group consisting 

 of four genera that are considerably unlike the plants so far described 

 in this chapter. Salviriia and Azolla are floating, while Marsilia and 

 Piliilaria are rooted at the bottom of small ponds or sluggish streams. 

 Salvinia has a whorl of three leaves at each node, two of which are 



