CHAPTER XIV 



GAMETOGENESIS AND SPOROGENESIS 



In plants there are two principal kinds of specialized reproductive 

 cells: spores, which develop singly, and gametes, which undergo sexual 

 fusion and form a zygote, this in turn developing into a new individual. 

 As already pointed out (p. 122), gametogenesis and sporogenesis are 

 commonly associated with the alternation of two generations in the life 

 cycle of plants, the gametophyte producing gametes and the sporophyte 

 spores. In some of the thallophytes, however, both spores and gametes 

 may be produced rather freely by the same individual, no clear alternation 

 of generations being present. ^ In the higher animals there are reproduc- 

 tive cells of but one principal kind, namely, the gametes. 



The present chapter deals with a few sample cases from among the 

 many diverse types of reproductive phenomena exhibited by different 

 organisms. Discussion of the actual fusion of the gametes and of the 

 behavior of chromosomes in sporogenesis is deferred to the following two 

 chapters. 



Algae. — The two gametes which fuse sexually may be morphologically 

 similar or dissimilar, and motile or non-motile. In Ulothrix and Ecio- 

 carpus they are similar and motile, the two cilia of each gamete being 

 equal and terminal in the former but unequal and lateral in the latter 

 (Fig. 138, A, B). In Spirogyra the gametes are but little-modified vege- 

 tative cells with no special motile apparatus aside from contractile 

 vacuoles (p. 228). 



Examples of unlike male and female gametes are afforded by (Edo- 

 gonium, Fucus, Vaucheria, and Polysiphonia. In no two of these genera 

 are the male gametes of the same morphological type. In (Edogonium 

 certain small cells of the filament produce two ovoid, green spermatozoids, 

 each with a ring of cilia around one end, while other cells enlarge and 

 become eggs (Fig. 138, C). The egg contains plastids and reserve food 

 materials, while a more or less clear "receptive spot" may often be made 

 out at the point where the spermatozoid is to enter. In Fucus the pri- 



1 For general accounts of the life cycles of plants, see general textbooks of botany, 

 especially Holman and Robbins (1928), Smith, Overton et al. (1928), Gager (1926), 

 Robbins and Rickett (1929), and Fitting, Sierp, Harder, and Karsten (1930). For 

 algae, see Oltmanns (1922-1923), Bonnet (1914), West (1927), and G. M. Smith 

 (1933); for fungi, Gaumann-Dodge (1928) and Fitzpatrick (1930); for seed plants, 

 Coulter and Chamberlain (1903, 1910). For the cytology of bryophytes, see Motte 

 (1932) and Hoefer (1932). 



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