REPRODUCTION AND DISPERSAL 819 



perhaps, because of dioecism and partly, it may be supposed, because the elevated 

 female organs make more uncertain the presence of sufficient water for sperm mo- 

 tility. In hermaphroditic mosses dichogamy is rare, hence close inbreeding is the 

 rule rather than the exception. 



The advantages arid disadvantages of heterospory. — In heterosporous 

 pteridophytes the proximity of male and female gametes is a matter of 

 uncertainty, since it depends upon the chance of microspores and meg- 

 aspores lodging near one another. In most living species the difficulty, 

 perhaps, is slight, since the plants are so small that often the spores must 

 fall near the parent plant and hence near each other; furthermore, 

 a number of the species are hydrophytes, and hence the motile sperms 

 have a favorable medium. In past ages, hovs^ever, there have been many 

 heterosporous trees among the pteridophytes, and the waste of both 

 microspores and megaspores must have been enormous. This is the 

 only known ecological group of past ages that is unrepresented among 

 living forms, and it well may be that its disappearance was due in part, 

 at least, to its disadvantageous heterospory, coupled, perhaps, with 

 extensive land emergence and with the consequent lessening of habitats 

 favorable for the fusion of gametes. In contrast vdth this extinct group 

 are the seed plants, whose greater success probably is due in part to the 

 retention of the megaspores instead of their dispersal, with the enormous 

 consequent waste; the waste even of microspores is reduced largely in 

 the great group of insect-pollinated plants. However, the seed plants 

 do not equal the homosporous ferns and the lower plants in ease of 

 dispersal, as appears from the fact that the homosporous constituents of 

 any two widely separated floras are much more alike than are their 

 heterosporous constituents. Obviously, the great advantage of heteros- 

 pory, if such there be, must be sought along other lines, even in the 

 seed plants. 



The significance of sexual reproduction. — In the simplest cases (as in 

 Ulothrix) the result of the fusion of gametes is a decrease of potential 

 individuals, since two cells resembling zoospores and having, perhaps, 

 the possibility of growing into two plants unite and form a spore that can 

 grow into but one plant. However, since a single algal filament may 

 produce a number of gametes, considerable multiplication is possible 

 through sexuality; this is conspicuous especially in those groups which 

 are without asexual spores {viz. the Conjugales, Charales, and Fucales). 

 But in plants with a well-defined alternation of generations {viz. in 

 bryophytes, ferns, and seed plants) sexual reproduction rarely results 



