PERIODICITY IN ALGA 105 



spermatozoids on the morning of the sixth day after the 

 greatest tide, and then has seen crop after crop follow one 

 another in regular order on corresponding days during each 

 alternate set of spring tides, one can scarcely realize the 

 striking nature of the phenomenon. 



A fuller account of the fruiting of Dhtyota at Beaufort 

 with a chart showing the relations of the fruiting periods to 

 the tides may be found in the Botanical Gazette for June, 

 1907. ^ 



It is not possible, with our present knowledge, to explain 

 this periodicity or the differences exhibited in different locali- 

 ties. The time of production of the crops at Beaufort is very 

 exactly related to the range of the tides given in the tide 

 tables, but does not seem to be affected by the actual height 

 of the water, or by the number of days intervening between 

 the spring tides. Expermients made by the author show that 

 the periodicity is transmitted to new portions of the thallus 



produced under conditions removed from the influence of 

 the tides. 



A few observations indicate the possibility that plants 

 of Dictyota dichotoma growing a few miles from Beaufort 

 may fruit with a periodicity slightly different from that of 

 the plants at Beaufort. We do not know whether these 

 differences, if they are constant, are to be explained by 

 differences in the environment or by seme other factor — 

 for example, geographical isolation. 



Specimens of several species of Dictyota from three 

 localities in Jamaica indicate that a modified periodicity 

 obtains there; while all the sexual cells on one plant 

 were of one age (or of two very distinct ages), the sexual 

 cells on different plants were of different ages. Further 

 study is needed, however, before we can form conclusions 

 about the fruiting plants of this region. 



Our knowledge of this entire subject is very scanty. 

 Much work remains to be done. We should discover whether 



1. Botanical Gazette 43: 383-392. 1907. 



