CHAPTER XVIII. 



SEXUAL REPRODUCTION. 



369. Cell union. — All methods of sexual reproduction 

 consist in the formation of a single cell by the union of two 

 specialized cells, known, respectively, as the male gamete and 

 the female gamete. The essential step in their union is the 

 coalescence of the nuclei. The cell thus formed is capable 

 of de^-eloping into a new plant under suitable conditions, 

 and is, consequently, a spore. Such sexually produced spores 

 must not be confounded with non-sexual spores (see ^[ 304)- 



370. Origin. — It is scarcely to be doubted that the earliest 

 methods of reproduction were \egetative, and that sexuality 

 has been acquired by a gradual modification of cells previ- 

 ously devoted wholly to ordinary processes of growth. The 

 probable history of the origin of sexual cells and sex organs 

 can only be inferred from the fact that the simplest plants 

 show no sexuality, others show imperfect sexualit^r, and still 

 others complete sexuality. The data are \-ery imperfect, but 

 they enable us to form at least an intelligent idea ot how 

 sexuality viay have been acquired. 



Theory of sexuality. 



371. Rejuvenescence. — Among the processes of growth 

 in tlie simpler plants, especially the fission-alga: (^ 11), on 

 of the most striking is that known as rejuvenescence. In this 

 process the proto|)lasni of tlie cell escapes from the cell-wall, 

 and acquires special motor organs known as cilia, which en- 



268 



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