LIFE HISTORIES OF ALG^ 249 



OUTLINE OF LIFE HISTORY OF DICTYOTA 



Antheridial-plant Oogonial-plant 



(Male gametophyte) (Female gametophyte) 



4, 4. 



Antheridium Oogonium 



4- 4. 



Sperm Fertilization 



Oosperm (zygote) 



4.4, 



Tetraspo ric-plant 

 (Sporophyte) 



II 



Sporangium 



Spore-mother-cell 



\, \. \, \. \Reduction 



Tetra- Tetra- Tetra- Tetra- 



spore spore spore spore 



4^ 4* 4" 4' 



Male 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 



