EUPHYCOPHYTA 147 



however, always die in culture, though it is possible that in nature 

 they may persist. The sex organs are produced in regular crops, 

 the new sori appearing between the scars of the old, and when the 

 whole of the surface has been used up the plant dies. 



After fertilization the zygote develops into a morphologically 

 similar plant which reproduces by means of tetraspores that are 

 formed in tetrads in superficial sporangia. At sporangixmi forma- 

 tion an epidermal cell swells up in aU directions, and after a stalk 

 cell has been cut off the sporangium initial divides twice to give the 

 four tetraspores, during which the thirty-two diploid chromosomes 

 are reduced to the haploid nimiber of sixteen. A tetraspore at the 

 time of Uberation is an elongated body and grows at once into a new 

 sexual plant. In some cases, however, the tetrasporangium fails to 

 divide into four spores but germinates as a whole and this pheno- 

 menon probably explains the abundance of sporophytic plants in 

 certain locaHties, although the conditions that cause this abnor- 

 maUty 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. 



Three kinds of rhythmic periodicity for the sex organs of Die- 

 tyota have been described from different locahties : 

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

 Naples 15 or i6 days are necessary, the gametes being 

 liberated about once a fortnight in both areas. 

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

 alternate spring tidal cycles, although only 8 days are re- 

 quired 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 Uttle change can be seen even after 22 days. 

 This results in almost continuous fruiting with two succes- 

 sive crops overlapping. There is obviously a considerable 

 difference in the behaviour of species of Dictyota. Further 

 investigations would be desirable in order to ascertain 

 whether one species can behave differendy under different 

 tidal regimes. 

 Wherever the plants occur, the bulk of the gametes (60-70 per 

 cent) are usually Uberated in a single hour at about daybreak. On 



