176 PHAEOPHYCEAE 



into the tube whilst the other degenerates, but at present the 

 significance of this phenomenon is obscure : it would hardly seem 

 to be associated with meiosis because this process takes place in the 

 zoosporangium. Both kinds of gametophyte show much variation 

 in shape and size, the male gametophyte being the smaller through- 

 out as it is built of smaller cells that contain dense chromatophores. 



The gametophytes can be cultivated in the laboratory, but for 

 successful cultivation the water must be sterilized and the cultures 

 placed close to a north window in winter and 2 or 3 m. distant in 

 summer. Reproductive organs are only formed at low temperatures, 

 2-6° C, whilst above 12-16° they are rarely produced, this fact 

 perhaps accounting for their temperate and arctic distribution (cf. 

 fig. 118). It is also known that the eggs may develop partheno- 

 genetically to give a haploid sporophyte which has an irregular 

 shape, whilst attempts to produce hybrids by artificial fertilization 

 have so far met with no success. Schreiber (1930) found that the 

 ratio of male to female gametophytes was always 1:1, and he sub- 

 sequently showed that of the thirty-two zoospores produced in 

 each sporangium sixteen gave male and the other sixteen female 

 gametophytes. The male gametophyte of L. religiosa is reported to 

 bear unilocular and plurilocular sporangia, but this is so abnormal 

 and has never been confirmed or reported for any other species that 

 it can hardly be accepted without further evidence. The ova of 

 L. saccharina are reported to be capable of producing dwarf fila- 

 mentous diploid plants which reproduce by means of unilocular 

 sporangia. If this is confirmed it may be that here we have an 

 example of a reversion to a primitive filamentous diploid progenitor, 

 a feature which might help considerably in indicating their ancestry. 



The most important characteristics of the gametophytic genera- 

 tion are : 



(i) the male gametophyte always has smaller cells; 



(2) the male gametophyte always consists of more than three cells 

 whereas the female may consist of only one cell, the oogonium. 

 Under good nutrient conditions both become much branched ; 



(3 ) the antheridia are unicellular and produce only one antherozoid ; 



(4) any cell of the female gametophyte may function as an 

 oogonium ; 



(5) the male gametophyte degenerates after the gametes are shed 

 whereas the female gametophyte persists. 



