392 THE ALGAE 



of cracks may affect the occurrence locally of some species, either 

 operating as a presence or absence factor or as a modifying factor. 

 The geological nature of the rock may also be important, e.g. soft 

 rocks such as sandstone rarely carry large algae. 



(b) Pressure 



Normally this is not an important factor, but Damant (i937) has 

 shown that in the case of vesicular species, such as Fucus vesiculosus, 

 there is a limit to the depth at which bladders will exist, because 

 the pressure of the water forces the gas out. This depth will there- 

 fore delimit the approximate lower limit to which the species can 

 descend, since the vesicles represent a flotation mechanism and 

 keep the thallus near the surface when submerged. A similar pheno- 

 menon has been observed in the case ofEgregia where the shape of 

 the bladders appears to be related to water depth, the deeper plants 

 having spherical bladders and the shallow plants pear-shaped ones. 



(c) Temperature 



Temperature can affect biogeographic distribution (see p. 433), 

 so for any given locaHty it operates as a presence or absence factor. 

 In that locaUty, however, there may be seasonal changes in tem- 

 perature which can affect the composition of the flora (presence or 

 absence factor again). Thus at Cape Lookout in North Carolina 

 the winter temperatures are such that the flora is essentially that of 

 New England: in the summer the temperatures rise by eighteen 

 degrees and the winter flora is replaced by sub-tropical species. 



Changes of temperature can also bring about seasonal migra- 

 tions, i.e. temperature operates as a modifying factor. Because of 

 the temperature changes, some species migrate up and down on the 

 shore. On salt marshes temperature operates at the upper levels 

 when there is low growing vegetation, because it results in much 

 evaporation with a consequent increase in desiccating conditions 

 together with a rise in sahnity . On rocky shores the day temperature 

 of the belts usually only responds to changes of air temperatures 

 during the simimer, and then it is always less than that of the air. 

 The temperature change is least in belts, e.g. Myxophyceae, 

 Chrysophyceae, that retain more moisture. If, at high levels, the 

 period of insolation is at all long in the summer then mats of Myxo- 

 phyceae, Chrysophyceae and Enteromorpha become cracked and 



