THALLOPHYTA: ALGAE 



67 



factory fossil evidence of their existence before the Jurassic. The 

 Phaeophyceae number almost 1,000 species, nearly all of which are con- 

 tained in six main orders: Ectocarpales, Sphacelariales, Cutleriales, 

 Dictyotales, Laminariales, and Fucales. 



1. Ectocarpales 



The Ectocarpales include the simplest of the brown algae. They 

 occur along all rocky seacoasts, growing attached to rocks, piers, and 

 other plants. They include over 60 genera and 300 species, forming a 

 diverse assemblage that is often broken up into 

 several smaller orders. Of the many genera, 

 perhaps the two that are best known are Ecto- 

 carpus and Pylaiella. 



Ectocarpus. Ectocarpus is a simple brown 

 alga, widely distributed along seacoasts, where it 

 grows attached to rocks or to other algae. It is 

 filamentous and usually much branched, the older 

 portions sometimes being surrounded by rhizoid- 

 like branches. Otherwise the body is strictly 

 mono sipho nous, each branch consisting of a single 

 filament. An alga composed of parallel bundles 

 of filaments is said to be polysiphonous. Growth 

 of the filaments occurs mainly by intercalary 

 cell divisions. Each cell contains a single nucleus 

 and a number of small brown plastids. 



Zoospores and isogametes are borne in spo- 

 rangia and gametangia, respectively. These de- 

 velop from the terminal cell of a short lateral 

 branch, but may be either stalked or sessile. The 

 sporangium is globular or somewhat elongated 

 (Fig. 54A). It is unicellular and contains many 

 (32 or 64) zoospores. It is at first uninucleate, 

 becoming multinucleate and forming zoospores by 

 cleavage of the cytoplasm. The gametangium 



is longer than the sporangium and often ovate or cylindrical (Fig. 545). 

 It is divided by cell walls into many small cubical cells, in each of which 

 an isogamete is formed. Both the zoospores and gametes are laterally 

 biciliate, the cilia being of unequal length. The pairing gametes are 

 generally of the same size but, in some species, one is slightly larger than 

 the other and swims less vigorously. Where this slight tendency toward 

 heterogamy exists, all the gametes in a gametangium are either smaller 

 (male) or larger (female). As in all the brown algae, the zygote germi- 

 nates without going into a resting stage. 



Fig. 54. A sporangium 

 (A) and a gametangium 

 (5) of Ectocarpus, X400; 

 also a single escaped zoo- 

 spore, more highly magni- 

 fied. 



