LIFE HISTORIES OF 
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 (B) 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 
