LIVERWORTS 



401 



spores out of the ripe sporogonium. The spores are light and 

 may then be scattered by currents of air. Thus you see the 

 advantage which it is to the plant to have its sporogonia 

 borne on stalked organs whose 

 stalks elongate after fertilization 

 has occurred. 



This little history of the sex 

 method of reproduction in a liver- 

 wort, and of what happens after 

 the egg is fertilized, illustrates 

 what is called the alternation of 

 generations, and this is a matter 

 important enough to have a head- 

 ing of its own. 



B. Alternation of Generations. 

 - In animals the fertilized egg 

 produces a form like its parent. FlG - J 9 8 - Section through the 



T-< ,1 r i i sporogonium of Marchantia. 



From the egg of a chicken we get T ? his % tructure hangs down 

 another chicken. There is no al- 



from beneath the top of the 

 ripe archegoniophore. Note 

 that it has three parts. The 

 largest part is the capsule. It 

 contains the spores. Scattered 

 among the spores the spiral 

 elaters are seen. Above the 

 capsule there is the stalk or 

 seta, and above that there is 

 the foot which is embedded in 

 the tissue of the archegonio- 



not succeeded by another just p ore ' 

 about like it ; it is succeeded by one which is not like it at 

 all. Then this new, entirely different generation produces 

 in turn, not itself, but the generation which produced it. 

 Let us see how this is illustrated in the liverwort. You 



ternation of generations. The gen- 

 erations are all alike. 



But in plants, at least from 

 liverworts up, the fertilized egg 

 does not produce a form like its 

 parent. It produces a form like its 

 grandparent. One generation is 



