LIFE HISTORIES OF \LGM 

 OUTLINE OF LIFE HISTORY OF DICTYOTA 



249 



Antheridial-plant 

 (Male gametophvte) 



i 



Antheridium 



I 



Oogonial-plant 

 (Female gametophyte) 



Oogonium 

 i 



Sperm Fertilization 



Egg 



Tetra- 

 spore 



i 



Male 



Oosperm (zygote) 



ii 



Tetrasporic-plant 

 (Sporophyte) 



Sporangium 



Spore-mother-cell 



Tetra- 



Tetra- 



Tetra- 



[ Reduction 



spore spore spore 

 ^ ^ ^ 



Male Female Female 

 gameto- gameto- gameto- gameto 

 phyte phyte phyte phyte 



236. Effect of Environment. — How is the similarity in 

 external appearance of the alternating generations of 

 Dictyota to be explained? We cannot say for certain. 

 The two generations start from reproductive cells (zygotes 

 and spores) that are profoundly unlike in their internal 

 organization, but it must be borne in mind that both 

 generations develop under practically identical external 

 conditions — the surrounding ocean-water, with alternate 

 exposure and immersion at low and high tides. This 

 would tend to influence the plant bodies alike, and has 

 been suggested by some botanists as an explanation of the 

 resemblance of the two kinds of generations. In the 

 case of ferns and mosses the environment of the two 



