526 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



are favoured by exposure, being at best stunted in sheltered 

 situations. 



Particularly marked is the zonation of Algae living in the intertidal 

 belt, on shores that are alternately left uncovered by the ebbing tide 

 and reflooded by the flow. Thus the Algae are regularly emersed 

 and submersed, their conditions of life being subject to drastic 

 fluctuations. Owing to variation in the amplitude of the tides them- 

 selves, there are found, above the low-water mark of the lowest 

 spring tides, practically all temporal degrees of emersion ; these 

 take a leading part in determining the localization, at levels where 

 each finds favourable conditions, of different groups of shore Algae. 

 Whereas the Algae of permanently submerged zones below low 

 spring tide-mark are liable to die after short exposure to the air or 

 to diluted sea-water, those living between tide-marks are more re- 

 sistant, being often able to withstand emergence for many days on 

 end, while some (especially among Blue-green Algae) can lose so much 

 of their water as to become brittle without injury. Again, certain 

 species die if they are kept constantly submerged ; an example is 

 Pehetia canalicuJata, which passes most of its life in the air. Yet 

 others are protected by growing among dense mats of larger Algae 

 (such as species of Fucus). These Algae exposed to emersion, how- 

 ever, in general have to withstand considerable degrees of desiccation, 

 and, in addition, marked changes in salinity and temperature. It 

 is apparently a combination of these varying factors that produce, 

 on sea-shores, the characteristic zonations in more or less regular 

 horizontal bands of different Algae at particular levels which, how- 

 ever, may vary according to the locality, degree of shelter, and so 

 on. As pointed out by Mr. F. T, Walker {in lift.), quantitative 

 variation may be not only seasonal and with depth but also cyclic, 

 over periods of years, while at least for Laminariaceae in Scotland 

 ' decrease in seaweed density has been found to be more the result 

 of a reduction in the number of plants per unit area than of the weight 

 of individual plants '. 



Various delimitations and systems of nomenclature have been pro- 

 posed for the main recognizable zones or belts inhabited by benthic 

 marine Algae. These may be usefully designated, in line with our 

 earlier subdivision of the ocean's margin (Fig. 170), as follows : 

 (i) Siipralittoral zone, lying between the upper limit of marine 

 vegetation and the high-water mark of ordinary spring tides, the 

 plants being bathed in sea-water only during storms or unusually 

 high or equinox tides, and in Europe characteristically including 



