LIFE HISTORIES OF ALG^E 245 



230. Discharge of Gametes. As in Ascophyllum, both 

 kinds of gametes are set free at the same time, before 

 fertilization. Recent studies have disclosed the very 

 interesting fact that their discharge occurs at rhythmic 

 intervals of about two weeks, synchronizing with the 

 periods of high and low tide. The advantages of this, if 

 any, are not apparent, and the periodicity persists in plants 

 placed in jars of sea- water in the laboratory, and even 

 with branches newly developed in the laboratory, and thus 



FIG. 1 80. Dictyota dichotoma (Huds.) Lamx. Longitudinal section of an 

 oogonial sorus. (After Bornet and Thuret.) 



never (as branches) subjected to the variations of the tide. 

 So close, however, is the harmony between tidal periods 

 and discharge of gametes, that the exact day of their dis- 

 charge, at any given station, can be predicted (with an 

 error, at most, of only one day) by consulting the tide- 

 tables for the given station. 



231. Fertilization. From the freshly liberated eggs 

 there diffuses through the water some unknown substance 

 which attracts the sperms. The latter respond to this 

 stimulus ]by swimming toward the egg. One of them 



