PERIODICITY IN ALGA 



103 



one kind of these reproductive cells. The rudiments of 

 the tetraspores are borne singly or in small groups, while 

 the eggs and spermatozoids are produced in sori — appearing, 

 when mature, as black or white patches distinctly visible to 

 the naked eye. The tetraspores are produced without regu- 

 larity, spores of all ages being found on the same plant at 

 any time during the fruiting season. 



In some localities the sexual cells, however, show a 

 remarkable periodicity in their production, appearing in 

 regular crops at definite intervals — all the eggs and spermato- 



FIG. 4. — Male and female plants of Dictyota dichotoma showing the 

 fruits in patches on the surface. About one-fourth size. This photo- 

 graph is the property of the Bureau of Fisheries and is published with 

 the permission of Hon. George M. Bowers, U. S. Pish Commissioner. 



zoids on all the plants in the same locality beirtg, at any 

 one time, at about the same stage of development. These 

 crops follow one another in regular succession and bear a 

 definite relation to the spring tides. 



The spring tides are, of course, those of greatest range, 

 having the highest floods and the lowest ebbs, and occur 

 every two weeks, about the times of the new and the full 

 moons. The neap tides are those of least range, having the 



