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A TEXTBOOK OF THEORETICAL BOTANY 



which is their most typical habitat, certain groups Hve exclusively in the sea, 

 while others, such as Cladophora, adapt themselves readily to either salt or 

 brackish water, to life on salt marshes and similar places. Recent investiga- 

 tions have shown that many Algae can be isolated from the soil, and it seems 

 clear that many of the unicellular types live primarily in damp soils. 



One of the most interesting aspects of algal distribution, however, is the 

 zonation to be seen on the seashore. If a study be made of the succession 

 of types met with, from high tide mark to well below the lowest levels of the 

 spring tides, it will be found that the genera and species will generally appear 

 in the same order, provided other factors are the same. Exposed on the 

 surface of rocks, only covered by the spring tides, will be found the Channelled 

 Wrack {Pelvetia canaliculata) and the Spiral Wrack {Fuciis spiralis) (Fig. 176). 



n, Pelvetia ranaliailata. 



h. Funis spiralis. 



c, Laminar ia digitata. 

 AscophyUiim nodosum 

 Fucus vesiculosus. 

 Hinianthalia lorea. 



d, 

 f. 



g, Fucus serratus. 



h, Laminaria hyperborea. 



i, Laminaria saccharina. 



j, Alaria esculeiita. 

 k, Saccorrhiza bulbosa. 



I, Halidrys siliquosa. 



Low water mark 



Fig. 176. — Zonation of Algae on sea coast, according to depth below high tide level. 

 {After Darbishire.) 



Somewhat lower down on these rocks the Knobbed Wrack {Ascophyllum 

 nodosum), the Bladderwrack [Fucus vesiculosus) and the Serrated Wrack (Fucus 

 serratus) form a zone. These Algae are only immersed at high tide and have 

 to withstand considerable exposure to sun and rain. 



As we pass further down between tide marks we find that various Red 

 Algae make their appearance, Laurencia pinnatifida, Gigartina stellata and 

 Corallina officinalis being the most common and making another recognizable 

 zone. At about low tide mark species of the Kelps or Tangles {Laminaria 

 digitata and Laminaria saccharina) appear ; sheltering under their great 

 fronds are many small Brown and Red species. Slightly lower still the 

 Seathongs {Himanthalia lorea) appears, with its great thong-like branches 

 sometimes 6 ft. long, and the Badderlocks {Alaria esculenta), with a broad 

 frond sometimes as much as 6 ft. in length. Lowest of all the Brown Algae 



