356 THE ALGAE 



shows that five communities are each confined to one type of 

 habitat. This relationship may be due to : 



(a) Association with a particular phanerogamic community, e.g. 

 Phormidium autumnale (IX) and Halimione portulacoides. 



(b) Dependence upon certain edaphic conditions, e.g. Muddy 

 Chlorophyceae (Ic). 



(c) Dependence upon the physical character of the environ- 

 ment, for example slope, exposure, wave action, e.g. Mar- 

 ginal Cyanophyceae^ (III), Vernal Ulothrix (IV) and the Pan 

 Association (XVI). 



The factors operating on salt marshes are in many respects 

 similar to those found on a rocky coast. The principal factor is the 

 tidal one with all the derived factors that are associated with rise 

 and fall of the tide (i.e. saUnity changes, water loss, temperature 

 changes, metabolic changes). Since salt marshes are restricted to 

 protected coasts, there are not the changes that take place with 

 transition to an exposed coast, as on rocky shores. However, there 

 is relative microprotection as between the open low-lying bare sand 

 or mud flats and the higher phanerogamic covered marshes. It is 

 worth noting that in the only comparative analysis of the tidal 

 factors operating on salt marshes (Norfolk (England), Lynn (Mas- 

 sachusetts) and Cold Spring Harbour (Long Is.)j it became evident 

 that for some of the species common to the areas the controlling 

 factors must be the same and are indeed the tidal factors. 



Before leaving the subject of salt marshes, it should be noted 

 that some of the communities so characteristic of these physio- 

 graphic features are also to be found in mangrove swamps. Thus 

 the wide-spread Catenella-Bostrychia community occurs clothing 

 the aerating roots (pheumatophores) and lower trunks of man- 

 groves. Different species of Catenella and Bostrychia are associated 

 in different parts of the world. A Myxophycean community is also 

 wide-spread on the pneumatophores, and on the mud itself species 

 of Vaucheria may form green patches. 



^ Myxophyceae is probably preferable: see p. 278. 



