30 THE SEAS 



become adapted to these conditions ; if the shore is rocky 

 and there are pools in this region they are filled with the 

 green weed, Enteromorpha, consisting of long tubular 

 fronds. In the summer the pools are apt to dry up, the 

 weed is killed and only a line of white marks this Entero- 

 morpha belt. About the region of high-water mark there 

 is a belt, in breadth from a few feet to about five yards 

 varying according to the slope of the shore, of the yellow or 

 olive coloured " Channelled wrack " (Pelvetia canaliculata) , 

 distinguished from the other brown weeds by its narrower 

 fronds. (Plate 9.) From the base of this belt to low- 

 water mark the rocky shore is covered with a tangle of 

 brown weeds known generally as the fucoid zone, because 

 most of the weeds are members of the genus Fucus. The 

 various kinds of Fucus are arranged in quite a definite 

 series. Next to the Pelvetia comes the broad fronded 

 Fucus platy carpus, followed by a broader belt of Ascophyl- 

 lum nodosum or " Knotted wrack," with exceptionally 

 long fronds bearing a single row of bulbous air bladders 

 down the centre. This weed is easily to be distinguished 

 because on it invariably grows a small red weed (Polysi- 

 phonia fastigiata). The next belt consists of Fucus 

 vesiculosus or " Bladder wrack " (Plate 9), to be distin- 

 guished by the pairs of air bladders along the fronds which, 

 when dried, " pop " sharply when pressed. Nearest low- 

 water mark is the commonest and most strongly growing 

 of all these fucoids, Fucus serratus, the " Notched wrack " 

 which, as its name tells us, has toothed serrations along the 

 edge of the fronds. These different belts of Fucus are not 

 always present, depending on local conditions, the amount 

 of fresh water, exposure, etc., while there are several less 

 common species which we have not mentioned, but generally 

 speaking the above types will be found on any typical 

 rocky shore. They are terminated, quite sharply, at low- 



