i66 PHAEOPHYCEAE 



each of the three locahties. It is, however, possible that there is a 

 genetical distinction between the plants from the different localities 

 and an investigation along these lines might prove very profitable. 

 Wherever the plants occur the bulk of the gametes (60-70 %) are 

 usually liberated in a single hour at about daybreak. On 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 intertidal 

 periods. However, when plants were removed to the laboratory it 

 was found that the periodicity was maintained, so that it must be 

 inherited, whilst plants from Carolina 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, 

 ii-i8ft. in England, o-8 ft. at Naples, 3-0 ft. in North Carolina, 

 and 0-8 ft. in Jamaica. These differences preclude either light or 

 tidal rise from being the controlling factor because the English and 

 Neapolitan 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 

 irregular. 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 Sargassum, Halicystis, Cysto- 

 phyllum, Padina and Nemoderma. Culture experiments are 

 required in order to determine whether the habit persists in suc- 

 cessive generations when they are grown under completely non- 

 tidal conditions, but unfortunately Dictyota has not proved very 

 amenable to cultural conditions. Finally, it can be argued that 

 tides and light may have no control over this rhythm and that it 

 may be associated instead with lunar periodicity, in which case even 

 cultures will be of no avail. It has been observed that the plants in 

 North Carolina always fruited at the time of full moon, and it is a 

 well-known fact that a number of marine animals spawn regularly 

 at such a period. At present the lunar explanation would appear to 

 be the most satisfactory, but even that produces difficulties when 

 the behaviour of the species in Jamaica is considered. 



