LIFE HISTORIES OF ALG^E 



247 



to as tetrasporic plants. When the tetraspores are set free 

 they soon become attached to some solid object, and, like 

 the fertilized eggs, develop into plants that externally 

 resemble, at maturity, those bearing tetraspores. Thus, 

 the plants produced by the fertilized eggs and by the tetra- 

 spores closely resemble each other in all vegetative characters; 

 they differ externally only in the kind of reproductive 

 organs they bear. 



233. Alternation of Generations. Although the Dic- 

 tyota plants developed from zygotes and spores look alike, 



FIG. 182. Dictyota dichotoma. A, Vertical section, transverse to the 

 axis of the thallus, showing a polar view of the nuclear plate in the first 

 division of the antheridium. C, Similar view of the first division of the 

 oogonium; B, similar view of the first nuclear division of the fertilized egg. 

 Note that the reduced (haploid) number of chromosomes in A and C is 16, 

 while the fertilized egg (J5) shows the diploid number (32). (Redrawn 

 from J. Lloyd Williams.) 



it is obvious that the products of the tetraspore, since they 

 bear gametes, and never spores, are gametophytes; and 

 the products of the fertilized egg, since they bear spores 

 only, and never gametes, are sporophytes. These facts 

 have only recently been established by careful experi- 

 mental cultures. There is thus a true alternation of 

 generations, although, in marked contrast to the ferns and 

 mosses, the plant bodies of the two generations are 



