76 ELEMENTARY STUDIES IN BOTANY 



stalk may be very long. This spore-case individual is usually 

 without chloroplasts, so that it cannot manufacture food. 

 It obtains food from the plant that produced the egg, usually 

 by the end of its stalk becoming imbedded in the tissue of 

 that plant. This imbedded part of the stalk often be- 

 comes enlarged and is called the foot, and it is through the 

 foot that food enters the spore-case individual, which is 

 therefore a parasite. 



When the spores formed by this individual germinate, they 

 do not produce other spore-case individuals, but they pro- 

 duce the green liverwort body. 



The life-history of a liverwort, therefore, includes two in- 

 dividuals that alternate with one another. One individual 

 is green and bears the sex-organs (containing gametes), and 

 hence is called the gametophyte (" gamete-plant ") ; the other 

 is a parasite and produces spores, and hence is called the 

 sporophyte (" spore-plant "). The fertilized egg of the game- 

 tophyte produces the sporophyte, and the spore of the sporo- 

 phyte in turn produces the gametophyte. The life-history 

 formula, using G for gametophyte and S for sporophyte, 

 thus becomes G=2>o S o G=> o S, etc. This is alter- 

 nation of generations, meaning that two individuals (gen- 

 erations) alternate in the life-history. This is a most 

 important fact in connection with Liverworts, because all 

 of the higher plants continue this alternation, and their 

 advance has depended upon the modification of these two 

 generations. It must not be supposed that Liverworts 

 introduced alternation of generations, for it was started among 

 Algae, but Liverworts established it, and all plants afterwards 

 retained it. 



It will be noticed that alternation of generations involves 

 a division of labor. Among Algae that do not possess it, 

 the same individual manufactures food, produces spores, 

 and forms gametes. In Liverworts, the gametophyte manu- 

 factures food and forms gametes, while the sporophyte 



