EUPHYCOPHYTA 193 



would really form a basis for placing the Fucales in the Hetero- 

 generatae. The alternative interpretation is that the sexual genera- 

 tion has been completely suppressed and is solely represented by 

 the gametes, so that whilst there is a cytological alternation of 

 generations there is only one morphological generation (cf. also 

 Chapter IX). Recent cytological work has revealed that the 2n 

 number of chromosomes is 64 except for Sargassum horneri where 

 it is 32, Halidrys siliquosa (55-60) and Pelvetia canaliculata (22). 

 It seems not unHkely, therefore, that the Fucales represent a poly- 

 ploid series. The sex organs are borne in flask-shaped depressions 

 of the thallus called conceptacles, each of which is lined with para- 

 physes and opens to the surface by means of an ostiole. The plants 

 of the different species may be dioecious, monoecious or herma- 

 phrodite. In a number of the fucoids the ova, when they are ex- 

 truded through the ostiole, remain attached by means of a thin 

 gelatinous stalk. These stalk-forming fucoids, e.g. Scytothalia, 

 Marginariella, Sargassum, Bifurcaria, etc., have been studied in 

 some detail and they possess certain features in common : 

 (i) The stalk is formed from the mesochiton or middle wall of 

 the oogonitmi (see p. 202). 



(2) They are inhabitants of deep water or in pools near low 

 water springs {Bifurcaria hrassicaeformis of South Africa is 

 an exception here). 



(3) The conceptacles are unisexual. 



(4) They have only one oosphere per oogonium {B. laevigata is 

 an exception). 



(5) Maturation takes place at a late stage. 



(6) The fertilized ovum begins development within the jelly of 

 the attachment stalk. 



In addition, those belonging to the family Fucaceae {Bifurcaria, 

 Marginariella) have : 



(7) Conceptacles borne on specialized laterals. 



(8) The gelatinous stalks are hollow. 



(9) They are specialized endemic genera or species. 



It would seem from the above that the possession of this stalk- 

 forming characteristic is probably an advanced feature phylogene- 

 ticaUy. 



The number of primary rhizoids in the embryo depends first 

 upon the species and also upon the size of the rhizoidal cell, which 

 in turn bears a relation first to the size of the egg, and secondly to 



