^^Reproduction in 

 Multicellular Plants 



ALTERNATION OF GENERATIONS 



Higher plants reproduce in two ways: (1) 

 sexually, by forming gametes; and (2) asexu- 

 ally, by forming spores. The sexual stage of 

 the plant alternates regularly with the 

 asexual stage; and usually the sexual form 

 differs so widely from the asexual form that 

 it is difficult to recognize the two genera- 

 tions as stages in the life cycle of the same 

 species. But the sexual generation always 

 produces eggs, each of which (when fertil- 

 ized) develops into an asexual plant; and the 

 asexual plant always produces spores that 

 give rise again to the original sexual plant. 

 Accordingly the sexual generation, which 

 produces gametes, is called the gametophyte; 

 and the asexual generation, which produces 

 spores, is called the sporophyte. Moreover, 

 the cells of the gametophyte generation are 

 always haploid, while the cells of the sporo- 

 phyte are always diploid. 



LIFE CYCLE OF A FERN 



An alternation of generations is the com- 

 mon evolutionary heritage of all higher 



plants, although the character of the life 

 cycle differs in detail in the different plant 

 groups. First to be described will be the life 

 cycle of a fern. All ferns (about 4000 species) 

 display a similar cycle, and in a broad sense, 

 the fern life cycle will serve as the prototype 

 for higher plants generally. 



The Sporophyte. The large, familiar fern 

 plant (Fig. 12-1) represents the sporophyte 

 generation — that is, the diploid asexual form 

 of the fern. The green parts of the fern, 

 which are seen above the ground level, con- 

 sist entirely of the leaves, or fronds. Each 

 complexly subdivided leaf arises from a hori- 

 zontal underground stem (Fig. 12-1), and 

 numerous fine roots grow downward from 

 the stem into the soil. The sporophyte car- 

 ries on an independent holophytic nutrition. 

 It may live for a number of years, producing 

 one crop of spores annually. 



When the sporophyte of the fern reaches 

 maturity a number of small, regularly ar- 

 ranged, dark-brown bodies (Fig. 12-2) appear 

 on the undersurface of the leaves. Each 

 brown "spot" as a whole is called a sorus; 

 when magnified, each sorus is seen to be a 



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