CHAPTER XXIV. 



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 (Ectocarpeae). 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. 

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

 symmetrical (p. 172). 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. 316). The thallus is attached by a holdfast to 

 some firm body such as a rock ; but the gulfweed (Sargassum bacci- 

 ferum) is exceptional in floating freely. The holdfast applies itself 

 so closely to the irregularities of the surface that the stalk will often 

 break before it would come away. In Fucus the holdfast 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 constructed is well fitted to 

 resist the swirl of the waves, keeping its hold, and though pliant, 

 retaining the form of its leathery frond. If exposed between tide- 



377 



