LIFE HISTORIES OF ALG^E 249 
OUTLINE OF LIFE HISTORY OF DICTYOTA 
Antheridial-plant Oogonial-plant 
(Male gametophyte) (Female gametophyte) 
Antheridium Oogonium 
4 4 
Sperm Fertilization Egg 
Oosperm (zygote) 
44 
Tetrasporic-plant 
(Sporophyte) 
4.4 
Sporangium 
44 
Spore-mother-cell 
4, 4 44 ^Reduction 
Tetra- Tetra- Tetra- Tetra- 
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 
