CHAPTER XVII 



REPRODUCTION IN PLANTS 



Although the reproductive processes of plants are fundamentally similar 

 to those of animals, certain striking differences occur. 1 In animals we 

 found it convenient to differentiate between reproduction in unicellular 

 and in multicellular forms and to base our classification of metazoan 

 reproduction upon the distinction between germ and soma. In plants the 

 distinctions between unicellular and multicellular and between germ and 

 soma are much less clear-cut, and other criteria have been adopted for 

 the classification of reproductive processes. 



The alternation of generations. Except for the most primitive forms, 

 the typical life cycle of any plant includes two stages or generations — a 

 haploid gametophyte generation, which reproduces sexually by gametes, 

 and a diploid sporophyte generation, which reproduces asexually by means 

 of spores. Here sexual reproduction involves the fusion of two gametes 

 to form a diploid zygote, which develops into a sporophyte individual, 

 and asexual reproduction involves the formation of minute haploid cells, 

 which develop into gametophyte individuals. 2 



Simple vegetative reproduction. In addition to the reproductive 

 processes that are associated with the alternation of generations, nearly 

 all plants are capable of a direct and clearly asexual vegetative reproduc- 

 tion. By a process that is essentially like the budding or fission of the 

 lower metazoans, individuals of the gametophyte generation may produce 

 other gametophyte individuals, and individuals of a sporophyte genera- 

 tion may produce other sporophyte individuals. In many plants, both 



1 Among multicellular forms, the divergence in the plants' and the animals' ways 

 of life has involved their reproductive practices. At least a part of the difference 

 appears to be due to the fact that nearly all the nonmotile plants have utilized repro- 

 duction to provide for the dispersal of their progeny. 



2 This is perhaps the most fundamental difference between the reproductive cycles 

 of the Metazoa and the multicellular plants. In the Metazoa, the haploid stage is 

 confined to the unicellular gametes (sperm and egg), which immediately unite to form 

 a zygote; in plants, the reduction division takes place at spore formation, and the 

 haploid cells have a more or less prolonged existence as an often long-lived gameto- 

 phyte generation. 



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