822 ECOLOGY 



The significance of alternating generations. — The chief advantage 

 in the alternation of generations has been supposed to be that one genera- 

 tion, the sporophyte, produces asexual spores in great abundance, thus 

 facilitating multiplication and dispersal, while the other generation, the 

 gametophyte, produces gametes, thus facilitating the merging of char- 

 acters of different individuals. The advantages of- the sporophytic 

 generation are obvious enough, but those of the game tophy tic generation 

 are less apparent, depending solely upon such advantages as may inhere 

 in sexuality. There are some obvious disadvantages in alternation; 

 for example, in certain mosses, as previously noted, the conditions for 

 the fusion of gametes often are lacking, hence development is impossible 

 for the sporophyte with its numerous asexual spores, however well- 

 fitted they may be for multiplication and dispersal. In the ferns, where 

 considerable moisture is required by the gametophytic generation and 

 particularly for the fusion of gametes, the sporophytic generation, which 

 often is well-suited for xerophytic situations, can grow only where a 

 gametophytic generation has preceded it.' In the seed plants, the 

 alternation of generations means that seed formation, in addition to 

 favorable conditions in the soil and the climate, depends upon polli- 

 nation, and therefore upon various pollinating agents, such as wind 

 and insects. 



Apogamy and apospory. — In some cases one of the alternating gener- 

 ations in whole or in part is eliminated; if the eliminated structure or 

 process is gametophytic, the phenomenon is called apogamy; if sporo- 

 phytic, it is called apospory. In Pteris cretica, Nephrodium molle, and 

 in many other ferns the sporophyte may develop from a bud on the 



simple fission. After a time, individuals of one line of ancestry conjugate with those of 

 another, and there appears to be an exchange of substance between the individuals, which 

 later separate and again reproduce by fission. Since cultures of Paramoecium in which 

 conjugation does not take place show a gradual decrease in size and activity after many 

 generations, it has been urged that sexuality thus is shown to be advantageous, at least in 

 animals. Indeed, in the older experiments, cultures in which conjugation was prevented 

 could not be maintained for more than 140 generations {i.e. about three months), although 

 parallel cultures with conjugation remained vigorous indefinitely. More recently it has 

 been shown that it is not lack of conjugation which causes death, but probably some 

 deterioration in the culture media, since by varying the media from time to time, 1500 

 generations have been secured without conjugation and with no loss of vigor. It is now 

 believed that cultures of Paramoecium thus can be kept indefinitely without conjugation, 

 and it is to be noted that the changes introduced in the culture media probably arc much 

 less than are those which occur in natural habitats. 



' Of course such a plant as Pteris may migrate from its place of origin by rhizome 

 propagation. 



