104 THE PLANT WORLD 



lowest floods and the highest ebbs, and occur between the 

 springs. 



This periodicity in the fruiting of Dictyota was first 

 described by J. Lloyd Williams. According to this author, 

 ( I ) the sexual plants of Dictyota on the coast of Wales 

 and England fruit at fortnightly intervals, the sexual cells, on 

 the coast of Wales, being initiated a few tides below the least 

 neap and liberated a few tides after the greatest spring tide. 

 Williams concluded, from careful observations, that the fac- 

 tor regulating this periodicity is the increased amount of 

 light obtained by the plants during the low water of spring 

 tides. 



Observations made at the laboratory of the Bureau of 

 Fisheries at Beaufort, North Carolina, during the past two 

 summers show that, at this place also, Dictyota dichotoma 

 produces its sexual cells in regular crops bearing a definite 

 relation to the tides. These crops are, however, borne 

 at monthly, instead of fortnightly intervals, appearing only at 

 alternate spring tides. Rudiments of sexual organs are first 

 observed about the day of the greatest tide, and general 

 liberation of the sexual cells regularly occur on the sixth day 

 after the greatest tide of each alternate set of springs. ^ 



This succession of crops occurs with the utmost regu- 

 larity throughout the summer and is very exactly associated 

 with the range of tides given in the tide tables of the United 

 States Coast and Geodetic Survey. This association is so 

 exact that it was possible, at the beginning of last summer, 

 to predict accurately the stage in the development of the 

 sexual cells which the Beaufort plants of Dictyota would 

 show on every day during the entire season. Until one has 

 seen rudiments of sexual cells appear on practically all the 

 sexual plants on the same day, has watched this crop pass 

 rapidly to maturity, has seen the discharge of the eggs and 



1. Annals of Botany, 19: 531-560. 1905. 



2. The authoi- is indebted to Hon. George M. Bowers, U. S. Fis.'i 

 Commissioner, for the privilege of working in the Fisheries Laboratory at 

 Beaufort, North Carolina. 



