LIFE HISTORIES OF 
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 che 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 
