258 REPRODUCTION, EVOLUTION, ETC. 



Kylin (1933), on the other hand, whilst agreeing with an origin for 

 the Dictyotales from the Phaeosporeae suggests that the Fucales 

 have not arisen from that source. It is possible to imagine a line of 

 evolution, not only on morphological, e.g. the cable type of con- 

 struction, but also on reproductive criteria, commencing from 

 Ectocarpus-^Castagnea^Chordaria-^Chorda^Laminaria, whilst an 

 alternative source for the Laminariales could also be found in 

 parenchymatous genera such as Dictyosiphon or Punctaria. It is 

 extremely tempting to consider whether the Fucales may not have 

 been evolved from the Laminariales because of the existence of 

 forms such as Durvillea, and if the oogonia and antheridia of the 

 Fucales are regarded as modified unilocular sporangia, e.g. micro- 

 and macrosporangia, then this becomes a possibility. On the other 

 hand, the oogonia and antheridia might be regarded as more closely 

 allied to the tetrasporangia of the Dictyotales which must then be 

 regarded as a possible ancestral source. 



From the above table it would seem that the evolution of the 

 Fucales is associated with the development of heterospory in much 

 the same way as the evolution of the seed habit is often said to be 

 associated with the development of heterospory. The origin of such 

 a habit forms a very distinct problem because there is very little 

 evidence for such a development among the other phaeophycean 

 groups. In the Laminariales heterospory but not heterangy is 

 recorded for Macrocystis, and if it has arisen once it may have been 

 present in some of the ancestral Laminariales, forms from which 

 perhaps the Fucales arose. It is also possible that we are pursuing 

 a false scent in trying to establish heterospory as a feature of the 

 Fucales, and that if the actual spores are considered, e.g. the 

 products of the first two divisions in the micro- and megasporangia. 



