DICTYOTALES (ISOGENERATAE) 165 



to divide into four spores but germinates as a whole and this 

 phenomenon probably explains the abundance of sporophytic 

 plants in certain localities, although the conditions that cause this 

 abnormality have not yet been discovered. Whilst the sex organs 

 are produced in rhythmic crops there is no such periodicity in the 

 case of the tetraspores, and here again there is scope for further 

 research. 



In the related genus Taonia the asexual plant bears tetrasporangia 

 and hairs in zonate bands across the thallus, and there is some evidence 

 for a correlation between the tides, or perhaps the Hght conditions 

 of each intertidal period, and the development of the zones. Each 

 zone probably corresponds to a single tidal period because a plant 

 30 days old was found to possess sixty zones of tetrasporangia. The 

 period between the initiation of each new crop is probably required 

 in order that the plant may accumulate the necessary food material. 

 In Taonia also the asexual plants are frequently more abundant than 

 the sexual, but this is partly accounted for by the persistence of a 

 sporophytic rhizoidal portion that can give rise to new plants. 

 More commonly, however, the tetrasporangium fails to divide and 

 the whole structure germinates before meiosis has taken place. 

 Plants formed in this way are found to be more resistant and 

 vigorous than the plants produced from normal tetraspores, and this 

 may be due to the larger supply of food material available from a 

 complete sporangium. 



Three kinds of rhythmic periodicity for the sex organs of 

 Dictyota have been described from different localities : 



{a) In Wales the sori require 10 to 13 days to develop whilst in 

 Naples 15 or 16 days are necessary, the gametes being liberated 

 about once a fortnight in both areas. 



{h) In North Carolina liberations occur once a month, at the 

 alternate spring tidal cycles, although only 8 days are required for 

 the development of the sex organs. This suggests that the plants 

 are exhausted after each fruiting and a resting period is necessary 

 in order to recuperate. 



{c) In Jamaica the successive crops take a very long time to 

 mature, e.g. very little change can be seen even after 22 days. This 

 results in almost continuous fruiting with two successive crops 

 overlapping. One very significant feature is that the commonest 

 species, D, dichotoma, apparently behaves as described above in 



