CHAPTER XXIII. 



BROWN ALGAE (PHAEOPHYCEAE). 



The Brown Algae, or Phaeophyceae, include a large proportion of the 

 Seaweeds commonly found between the tide-marks, and extending 

 downwards to greater depths. Some of them are delicate filamentous 

 growths, branched or unbranched (Ectocarpales). Others are larger 

 and more complicated in structure, with ribbon-shaped thallus 

 (Dictyota). Some, of leathery texture, attain gigantic dimensions, 

 the Tangles of the colder oceans being among the largest of living 

 beings (Laminaria, Nereocystis, Macrocystis). The most familiar 

 are the species of Fucus found on all British coasts, of which F. vesi- 

 culosus is the Common Bladder Wrack. The smaller forms show 

 gradual steps of increasing complexity, from the simple septate 

 unbranched filament, through various modes of branching and 

 cortication, to the massive tissue-formation seen in the larger Tangles. 

 Even the largest of them may thus be referred ultimately in origin 

 to the simple filament 



The thallus of the larger forms is usually flattened, and bilaterally 

 symmetrical (p. 205). It shows forked branching, often very perfectly 

 dichotomous, and in a single plane. The result is that the whole 

 frond is fan-shaped, as is seen particularly well in the native species 

 Fucus serratus (Fig. 280). The thallus is attached by a holdfast to 

 some firm body such as a rock ; but the gulfweed (Sargassum bacciferum) 

 is exceptional in floating freely, in which state, however, it probably 

 propagates vegetatively : organs of sexual reproduction only occur on 

 attached plants. The holdfast of Tangles applies itself so closely 

 to the irregularities of the surface that the stalk will often break before 

 it would come away. In Fucus it is discoid ; in Laminaria and others 

 it may be branched and root-like. But its function is only mechanical, 

 not absorbent. The thickness of the stalk which arises from it is 

 proportional to the size of the thallus it has to moor. A plant so 



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