532 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



the main period of vegetative activity in the boreal and austral regions 

 (though of course involving antithetic times of the year in the nor- 

 thern and southern hemispheres). Many perennial types, however, 

 reserve their main reproductive activity for winter, a few, such as 

 species of Fiicus, being in this respect independent of the season, 

 while again relatively few, such as species of the Rhodophycean genus 

 Polysiphonia, reproduce mainly in summer. Particularly striking 

 differences between winter and summer states are exhibited by those 

 types which shed their assimilating ' fronds ' at the beginning of the 

 cold season, the frondless residue being often covered with repro- 

 ductive organs. Others, such as many Laminariales, have an inter- 

 calary zone of growth producing a new ' frond ', the old one being 

 cast off as a whole in the spring : this commonly happens at about 

 the same time to all the representatives of a species in a particular 

 region. 



The most characteristic sequence on open shores and, for example, 

 in the English Channel, shows a supralittoral zone formed for the 

 most part by Lichens, with girdles of yellow or orange Xatithoria 

 parieiina and Caloplaca marina above, and of blackish Verrucaria 

 mama below. The eulittoral is characterized by a series of girdles 

 formed largely by individual Brown Algae — for example, successively 

 from top to bottom, by Pehetia canalicidata, Fuciis spiralis, F. 

 vesiciilosus, and F. serratiis. Lower down, uncovered at most only 

 by the lowest spring tides and forming the upper sublittoral, are 

 girdles of large Kelps (Laminariales). Commonly Laminaria digitata 

 is dominant near low-water mark and L. cloustonii farther down, or 

 L. saccharina may replace the latter where the substratum is more or 

 less sandy. To these normal girdles which have analogies elsewhere, 

 as for example on the Atlantic coast of North America, various 

 facultative ones may be added ; nor is there any exact correlation 

 with tidal level, for, especially on strongly exposed rocks, wave 

 action induces a displacement upwards of the upper girdles to a 

 height roughly corresponding with that attained by the waves. 



In some circumstances, various Green Algae can largely take the 

 place of the characteristic tidal girdles of Brown Algae. Ecological 

 factors can also favour the presence of quite a range of other algal 

 communities that are not restricted to such narrow bands and 

 definite levels along the coasts : such are, particularly, the communi- 

 ties of tidal pools and grottoes, and of local run-oft areas. Even 

 sandy and muddy shores may be well populated in the upper 

 sublittoral by Eel-grasses accompanied by characteristic epiphytic 



