THE GEEAT GROUPS OF ALGiE 



249 



teristic coast forms. All of them are anchored by holdfasts, 

 which are sometimes highly developed root-like structures ; 

 and the yellow, l)rowu, or olive-green floating 

 bodies are buoyed in the water usually by the 

 aid of floats or air-bladders, which are often 

 very conspicuous. The kelps are most highlj 

 developed in the colder waters, and form much 

 of the " wrack," " tangle," etc., of the coasts 

 The group is well adapted to 

 live exposed to waves and cur- 

 rents with its strong holdfasts, 

 air-ljladders, and tough leathery 

 bodies. It is what is known as 

 a specialized gr 0112} — that is, one 

 which has become highly organ- 

 ized for certain special condi- 

 tions. It is not our 

 purpose to consider 

 such a specialized 

 group in any detail, 

 as it does not usual- 

 ly help to explain the 

 structures of higher 

 groups. 



170. The plant 

 body. — There is very 

 great diversity in the 

 structure of the 

 jjlant body. Some 

 of them, as Ectucar- 

 ^j(/,s (Fig. 217), are fil- 

 amentous forms, like 

 the Confervas among 

 the Green AlgK, but 



others are very much more complex. The thallus of Lam- 

 inar ia is like a huge floating leaf, frequently nine to ten 



Fui. "217. A brown alga <,Ectocarpus). showing a 

 body consisting of a simple filament which puts 

 out branches (.4), some sporangia (_B) contain- 

 ing zoospores, and gametangia {C) containing 

 gametes.— Caldwell. 



