148 THE ALGAE 



the Welsh coast the gametes are set free just after each series of 

 high spring tides during July to October, and it has been suggested 

 that light plays the part of the determining factor during the inter- 

 tidal periods. However, when plants were removed to the labora- 

 tory, it was found that tlie periodicity was maintained, so that it 

 must be inherited, whilst plants from CaroHna likewise retained 

 their periodicity when transferred to the laboratory, the specimens 

 fruiting at the same time as those living under natural conditions. 

 The mean tidal differences vary considerably in the four localities, 

 11-18 ft. in England, 0.8 ft. at Naples, 3.0 ft. in North CaroHna, 

 and 0.8 ft. in Jamaica. These differences preclude either Hght or 

 tidal rise from being the controlling factor because the English and 

 NeapoHtan plants behave similarly even though there is a great 

 difference in the tides. Regularity of the tidal cycle, however, may 

 modify the reproductive cycle, because where the tides are some- 

 what irregular, as in Jamaica, the reproductive rhythm is also ir- 

 regular. This rhythmic behaviour is probably not due to any one 

 factor but has been acquired over a long period of time as a response 

 to the environment and is now inherited. The phenomenon is not 

 confined to Dictyota because regular or irregular periodic cropping 

 has been recorded for species of Halicystis^ Sargassum, Cystophyl- 

 lum, Padina and Nemoderma. Culture experiments are required in 

 order to determine whether the habit persists in successive genera- 

 tions when they are grown under completely non-tidal conditions, 

 but unfortunately Dictyota has not proved very amenable to cul- 

 tural conditions. Finally, it can be argued that tides and Hght may 

 have no control over this rhythm and that it may be associated in- 

 stead with lunar periodicity, in which case even cultures wiU be of 

 no avail. It has been observed that the plants in North Carolina 

 always fruited at the time of fuU moon, and it is a weU-known fact 

 that a number of marine animals spawn regularly at such a period. 

 The lunar explanation appears to have some satisfactory features, 

 but even so produces difficulties when the behaviour of the species 

 in Jamaica is considered. 



REFERENCE 



Fritsch, F. E. (1945). Structure and Reproduction of the Algae, Vol. 11, 

 pp. 302-22. Camb. Univ. Press. 



