CHAPTER XIII 



DIFFERENTIATION OF TISSUES. FUCUS : THE 

 SEA-WRACK 



Besides the Green Algas, which are mostly unicellular or 

 filamentous fresh- water forms (the " sea lettuce "is an 

 example of a green alga which is marine and has a large 

 thin body or thallus, which is soft and membranous, 

 and composed of two layers of green cells), there are 

 two other large groups of algae, commonly called the 

 Red and the Brown Seaweeds, because they are 

 respectively red and brown in colour and they live in 

 the sea, mostly in the intertidal zone or not very far 

 below low-water mark. The plastids of the cells of these 

 seaweeds contain the chlorophyll pigments, but in a 

 different proportion to that in which they exist in 

 green chlorophyll, and also other pigments in addi- 

 tion. For instance in the Brown Seaweeds there is a 

 smaller proportion of the pure green pigments (chloro- 

 phyll a and P) and a larger proportion of the yellow 

 and orange constituents (xanthophyll and carotin), 

 and, in addition, a special orange pigment, fucoxanthin 

 (C^oR^fig), peculiar to the Brown Seaweeds. This 

 combination gives the phceoplasts,^ as they are called, 

 a brown or olive green colour. 



The Brown Algae vary from unicellular forms, through 

 a considerable series of filamentous types, either con- 



' Greek ^aui;, greyish. 

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