820 ECOLOGY 



in a significant increase of individuals, asexual spores or propagules 

 chiefly being responsible for such increase. 1 Thus multiplication, which 

 is the feature of chief significance in other forms of reproduction, usually 

 is not conspicuous in sexual reproduction. Gametes and the spores 

 resulting from their fusion (except in the thallophytes) are among the 

 most delicate of plant structures, so that fitness for endurance through 

 severe periods is not one of their characteristics, as it is of many asexual 

 spores. Furthermore, neither the gametes nor the resulting spores are 

 particularly efficient disseminules ; the female gamete, in particular, 

 whose position determines the place of the next generation, is for the 

 most part immotile. Hence none of the three features commonly as- 

 sociated with reproduction, namely, multiplication, endurance, and 

 dispersal, are of especial significance in sexual reproduction. 



It is believed commonly that sexual reproduction makes possible the 

 advantageous merging in one individual of the qualities of two races, 

 hence sometimes the phenomenon is known as amphimixis. In Ulo- 

 thrix the advantage gained has been thought to be one of size, since plants 

 developing from zygospores are larger than those which develop from 

 gametes that fail to fuse. In other cases (apart, perhaps, from seed 

 plants, p. 866) the advantages of sexuality appear more hypothetical 

 than real, but even hypothetically the crossing of two races might fairly 

 be expected to introduce into a given strain disadvantages as well as 

 advantages. The chief reason for believing that sexuality is of no partic- 

 ular advantage (at least in the lower plants) is that its absence seems 

 to bring no disadvantage. In the bacteria and blue-green algae, in some 

 green algae, and in many fungi, true sexuality is absent, but no plants 

 are more successful than these ; in many fungi (as in Saprolegnia and in 

 the Ascomycetes) , and in some liverworts and mosses there is excellent 

 evidence of diminishing sexuality (see p. 883), but none of diminishing 

 success. 2 Even in the higher plants, where sexuality is much more 



1 Such increase as there is among these plants is most conspicuous among the bryo- 

 phytes, where single gametophytes may bear several (rarely many) sporophytes. Some- 

 times (especially in gymnosperms) two or more embryos' develop from one sexual spore or 

 from one sporiferous center of a gametophyte, a phenomenon known as polyembryony ; 

 this is of little significance, however, inasmuch as but one embryo, as a rule, is able to 

 mature. 



2 Somewhat recently there have been discovered modified forms of sexuality in the rusts 

 and smuts and in various other fungi, but in these cases there is no crossing, so that 

 true .amphimixis with its supposed advantages necessarily is excluded. In many algae, 

 there occurs inbreeding or automixis, which is well illustrated in Spirogyra, where 

 fusion may take place between gametes of adjoining cells in the same filament, and in 



