30 INTRODUCTIOX. 



they will grow for years, though they get no nourishment 

 except what they derive from the air by their surface. AYe 

 doubt not, then, tliat in some instances, such as the floating 

 Sargassum, sea-plants grow without deriving any nourish- 

 ment by roots, but we are disposed to regard such cases as 

 exceptions from the general rule. We cannot persuade 

 ourselves tliat they draw no nourishment from the sub- 

 stances to wliich they are attaclied. They show a decided 

 preference for certain kinds of rocks and Alga3, and their 

 growth is more luxuriant when they are found on those they 

 prefer. Limestone seems a great favourite with some, and 

 the abundance of limestone rock on many parts of the 

 Irisli coast, is probably one reason why many of the Irish 

 specimens put to shame the English and Scottish dwarfs of 

 the same species. It is well known also that there are 

 several species that grow both on rocks and on large Alga3, 

 and it is as well known that, in the same habitat, those 

 growing on rocks and those on Algce differ considerably in 

 appearance, and tliis seems to be owing to difl'ercnt nourish- 

 ment by roots, as other circumstances are the same. 



