BROWN ALG^ (PH^OPHYCE^) 199 



water from dead specimens, which then assume a green colour. 

 It is possible that these pigments serve to screen the chlorophyll 

 against the strong light to which Seaweeds are at times exposed, 

 although other functions have been attributed to them. 



Many Brown Seaweeds [PhcBophycecB), of which Eciocarpus 

 (Fig. 119, /) has already furnished a relatively simple example, 

 attain to very considerable 

 dimensions, are far bulkier 

 than any Algse yet noticed, 

 and evince marked division 

 of labour. Thus in Lamin- 

 aria, which occurs in the 

 zone just below low- tide 

 level, the thallus consists of 

 three distinct regions (Fig. 

 106) : viz. a richly branched 

 holdfast, by which the Alga 

 is anchored to rocks, a stout 

 cylindrical stalk which may, 

 in some species, be several 

 feet in length and as much 

 as an inch in diameter, and 

 a correspondingly large 

 leathery blade. The latter 

 either takes the form of a 

 broad ribbon (L. saccliarina) 

 or of a deeply divided frond 

 like the palm of a hand (L. 

 digitata, Fig. 106). Whilst 

 the stalk and attaching organ 



are perennial, the blade is renewed, usually in the spring of each 

 year, by means of a small-celled meristem situated at the top of 

 the stalk ; the new frond therefore appears at the base of the old 

 one, which eventually becomes detached by the action of the waves. 

 The holdfast is composed of numerous cells with strongly thickened 

 walls, and develops additional branches as the plant grows older, 

 whilst simultaneously the stalk slowly increases in thickness. ^ 



' This Alga not infrequently develops on rock-fragments that arc too 

 small to anchor the adult, in consequence of which the thalli and attached 



Fig. 106. — Young plant of Laminaria 

 digitata, about one-fifth natural 

 size. 



