EXTERNAL . STRUCTURE OF FUCUS 215 



sisting of single rows of cells or of stouter threads 

 several cells thick, to large forms ' with quite bulky 

 bodies consisting of more or less differentiated tissues. 

 One of these latter forms we shall now study in some 

 detail, because it illustrates yery well the fundamental 

 principle of differentiation of function and corresponding 

 differentiation of structure leading to the origin of 

 distinct tissues. 



Several different species of the genus Fucus live 

 attached to rocks, mainly between tide marks, on 

 the coasts of the cooler countries of the northern 

 hemisphere. They are the commonest seaweeds on the 

 rocky British coasts, and frequently cover the rocks so 

 thickly as to make them very slippery to walk upon 

 just after the tide has ebbed, leaving the mucilaginous 

 surface of the seaweed wet and slimy. 



The individual plants of Fucus vary from a few 

 inches to several feet in length. The body or thallus 

 consists of a cylindrical stalk or stipe attached to the 

 rock substratum by a more or less branched spread- 

 ing holdfast which cements itself firmly by means of 

 the gluelike mucilaginous walls of the surface cells 

 to the rock or stone on which the plant grows. Above, 

 the stipe passes into the flat-forked frond by the gradual 

 appearance of thin wings distinct from the thickened 

 midrib, which is a direct continuation of the stipe. At 

 the end of each branch of the frond is a groove or 

 depression, often containing hairs consisting of single 

 chains of cells, and it is by division of the cells at the 

 base of the groove (Fig. 33, A) that the branches grow. 



The end portions of some of the branches are 

 swollen, and thickly set in the swollen frond are slightly 



I Some of the Brown Seaweeds, for instance MacrocysHs and 

 Nereocysiis, living in the Antarctic and Pacific Oceans, are immense 

 plants, hundreds of feet long. 



