16 PLANT-LIFE ON LAND [CH. 



zonation, and it is a familiar matter of observation 

 how the dense tassels of the yellowish Pelvetia 

 almost take possession of the highest levels on our 

 shores, followed downwards by definite zones of 

 tangles, such as Fucus platycarpus and Ascophyllum 

 nodosum, while the strange Himanthalia, with its 

 thong-like fruiting branches rising from the cup- 

 shaped base, are only met with below half-tide 

 mark. Similar sequences of zonation may be noted 

 in the Red Seaweeds also. 



One point of distribution is specially worthy of 

 remark: it is that certain of the Green Algae are 

 stimulated to active growth by sewage impurities, 

 which seem to affect the others adversely. In the 

 neighbourhood of a harbour, and conspicuously 

 near to the drainage-outfalls, the Green Seaweeds 

 are in the ascendant : so much so that their presence, 

 in quantity, may often serve as an indication and a 

 warning. In recent years the growth of vast masses 

 of the bright green Sea-Lettuce (Ulva latissima), and 

 its subsequent decay as the season progresses, has 

 been a cause of serious nuisance along the shores 

 which border on the relatively impure waters of 

 Belfast Lough and Dublin Bay. No method has 

 yet been devised for checking its growth. 



The Sea-Lettuce is common all round the British 

 coasts, being found attached to rocks, piles and 

 piers. It consists of a thin filmy expansion of 



