REPRODUCTION AND DISPERSAL 



817 



from different gamete-producing organs (gametangia) , and in Dasy- 

 cladus, only between gametes from different plants, though it is impossible 

 in any of these to distinguish male and female 

 characters. However, in the Conjugates, one of 

 the gametes often is immotile, while the other 

 migrates from a neighboring filament through a 

 passageway made by the fusion (or conjugation) 

 of two lateral outgrowths (figs. 107-109). From 

 analogy with the higher plants, the immotile 

 gamete may be called female and the motile 

 gamete, male. In some conjugating forms, as 

 Mucor, there is no such distinction, the two 

 gametes moving equally and meeting in the 

 passageway between the filaments (figs. 163- 

 166). Usually zygospores are thick- walled rest- 

 ing cells closely packed with food and well able 

 to exist over severe periods (figs. 50, no, 166). 



Heterogamy. In the great majority of plants, 

 including many thallophytes and all the higher 

 plants, the two gametes are unequal; this con- 

 dition is known as heterogamy, and the spore re- 

 sulting from the fusion of unequal gametes is 

 called an oospore. It is in the heterogamous 

 plants that one may speak of true sex differenti- 

 ation and of the development of male gametes or 

 sperms and of female gametes or eggs (fig. 1135). 

 In nearly all bryophytes, pteridophytes, and 

 heterogamous algae the sperms are relatively 

 small, ciliated, actively motile bodies (figs. 28, 

 119, 320, 349, 415), whereas in the seed plants 

 (except in Ginkgo and in the cycads, fig. 455), 

 they are non-ciliated, and exhibit but little true 

 locomotion (fig. 479). Eggs commonly are much 

 larger than sperms, and, except in the case of a 

 few algae where they float freely in the water, 

 they are essentially immotile (figs. 31, 77, 481). 

 Often the male and female gametes are borne in special organs, the 

 antheridia and the oogonia (or archegonia), respectively. In many 

 thallophytes the oospore is a thick- walled resting cell (fig. 70). 



FIGS. 1133, 1134. 

 Zoospores and isoga- 

 mous gametes in Ulo- 

 thrix : 1133, a part of a 

 filament from which a 

 4-ciliate zoospore and 

 several smaller biciliate 

 gametes are escaping ; 

 JI 34i gametes pairing 

 and fusing ; highly mag- 

 nified. From Co ULTER. 



1135 r 



FIG. 1135. Heterog- 

 amy; an egg of Fucus, 

 surrounded by a swarm 

 of biciliate sperms; 

 highly magnified. 

 After THURET. 



