350 ECOLOGICAL FACTORS, ETC. 



coast and hence it is impossible to estimate its importance, but 

 towards high-water mark it must operate in preventing the upward 

 spread of a number of species. 



(5) Temperature. Rees (1935) as well as Knight and Parke 

 (1931) consider that changes of temperature throughout a season 

 are probably responsible for the upward and downward migration 

 of some species on the shore. Temperature would only appear to 

 operate in the summer on high salt marshes where there is a low- 

 growing vegetation, because it will then result in much evaporation 

 with a consequent increase in desiccating conditions together with a 

 rise in salinity. 



(6) Salinity. This probably varies but little on a rocky coast, 

 except perhaps in the case of tide pools, but it is important on the 

 higher salt marshes in summer when the values rise so high in the 

 surface layers of the soil that probably only a few algae can tolerate 

 the conditions. The salinity of the marsh soil has also been invoked 

 in order to explain the origin of spirality in the marsh fucoids. On 

 all types of shore the incidence of fresh water flowing down from 

 the land always produces a local modification in the flora. 



(7) Substrate. The nature of the substrate, whether solid rock, 

 boulders, pebbles, sand, mud or peat, is of fundamental import- 

 ance in connexion with anchorage, the general aspect of the flora 

 being largely determined by this factor. On the rocky coasts the 

 angle of slope may aflfect the occurrence locally of some species, or 

 even whole belts of vegetation (cf. p. 314). 



(8) The movement of water, apart from the ebb and flow, plays a 

 great part in determining the luxuriance of the vegetation. In 

 many cases there may be considerable local currents and in salt 

 marshes there is the continual flow of water in the creeks which 

 persists even when no tide is present. The action of surf may also be 

 included here, and there are several species, e.g. Postelsia, which are 

 known to be surf-loving, whilst there are also those species which 

 cannot tolerate surf. In places where the water carries a heavy 

 load of silt there may be some modification in the flora because it is 

 probable that some species are not able to tolerate consistent 

 deposition of a muddy covering. Recent work shows that turbulence 

 has a depressing eflFect upon the respiration of marine algae which 

 is particularly marked in the case of the littoral species, and this 

 may indirectly be related to the zonation. 



