230 



developed genera, such as Sargassum (Ag.) and Durvillaga, there is a 

 more or less advanced differentiation of lamina or leaf from stipe or stem ; 

 and the ' leaves ' are even arranged spirally and furnished with a rudi- 

 mentary midrib. 



The cell-wall often consists of two distinct layers, an inner firm 

 compact but thin layer, and an outer gelatinous one which swells greatly 

 in fresh water, filling up the intercellular spaces, and causing the 

 slimy character which the Fucaceae commonly assume after they have 

 lain for some time in fresh water. The cells contain chlorophyll ; but 

 the green colour is in all cases entirely masked by a pigment of a brown 

 or olive colour, which can be extracted from the dead plant by cold fresh 

 water. The nature of this pigment has been investigated by Millardet, 

 Rosanoff, Sorby, Schiitt, and others. According to Millardet, an olive - 

 green alcoholic extract may be obtained from quickly dried and powdered 

 specimens. If this is then shaken up with double its volume of benzine 

 and allowed to settle, the upper layer of benzine is coloured green by 

 having taken up the chlorophyll, while the lower alcoholic layer is yellow, 

 and contains phycoxanthin in solution. Thin sections of the thallus, 

 after complete extraction by alcohol, still yield a reddish brown substance 

 which in fresh cells adheres to the chlorophyll-grains, but can be dis- 

 solved out with cold water, especially if the frond be first reduced to 

 powder. To this reddish brown substance, the spectrum of which has one 

 absorption band between E and F, Millardet gives the name phycophczin. 

 Schiitt states that the spectrum of phycophaein has no characteristic 

 absorption bands, but a regular increase of absorption from the red 

 towards the blue end. He proposes to limit the term phycophaein to 

 the portion soluble in water, and phycoxanthin to that soluble in alcohol, 

 while the entire compound pigment of the Fucaceae and Phaeosporeae 

 he would call phceophyll. Sorby applies the term fucoxanthin to the 

 principal colouring matter of the olive-green seaweeds (Fucaceae and 

 Laminariaceae). It is soluble in bisulphide of carbon, imparting to it a 

 beautiful amber colour ; its spectrum shows two obscure absorption- 

 bands in the yellow. Hick (Journ. of Bot, 1885, pp. 97 and 354) has 

 detected continuity of protoplasm in several species of Fucus, both in 

 the cortical layers and in the central medullary tissue. The intercom- 

 munication of the protoplasmic contents of contiguous cells is also 

 effected, as in ;the Laminariaceae (see p. 244), through structures of the 

 nature of sieve-plates. 



In many of the Fucaceae air-bladders are formed in the frond by the 

 hollowing out of large cavities in the interior of the tissue, which serve 

 to float the frond in the water, and to aid in the process of fertilisa- 

 tion. These are especially noticeable in the common * bladder-wrack ' 



