ECOLOGY OF SALT MARSHES 325 



is a free-living form found on American salt marshes, in Scotland 

 and on the shores of Strangford Lough in Ireland, whilst ecad 

 scorpioides is a partially embedded form found on the Essex marshes 

 and on the shores of Strangford Lough. All these forms probably 

 originated as a result of vegetative budding, although it is also 

 possible that they have developed from fertilized oogonia that 

 became attached to phanerogams on the marsh. There is definite 

 evidence that Ascophyllum nodosum ecad scorpioides arises by 

 vegetative budding from fragments of the normal plant, whilst it 

 has been suggested that conditions of darkness may be favourable 

 for the development of ecad Mackaii. As a group the marsh fucoids 

 are characterized by 



(i) vegetative reproduction as the common means of per- 

 petuation ; 



(2) absence of any definite attachment disk ; 



(3) dwarf habit; 



(4) curling or spirality of thallus. 



In the embedded forms derived from Fucus vesiculosus the three- 

 sided juvenile condition (cf. p. 197) of the apical cell is retained, the 

 cryptostomata are marginal and division in the megasporangia is 

 only partial or else does not occur. It is suggested that these 

 features are due to 



{a) exposure, which results in a dwarfing of the thallus ; 



{b) lack of nutrient salts which induces a narrow thallus ; 



{c) the procumbent habit and consequent contact with the soil 

 causes spirality because growth takes place more rapidly on the side 

 touching the soil. 



The cause of sterility may either be a result of the high humidity 

 (according to Baker, 1912, 1915) or, more probably, because of the 

 persistence of the juvenile condition as represented by the apical 

 cell and cryptostomata. The marsh fucoids occur most frequently 

 either as pioneers on the lowest marshes or else as an undergrowth 

 to the phanerogams. 



One of the more striking physiographical features of salt marshes 

 is the salt pan. The number, shape and size of these on the diflFerent 

 salt marshes varies very considerably, but they generally contain a 

 certain number of algae, especially those pans which occur on the 

 lower marshes. They are important because they provide a much 



