CRYPTOGAMS 



329 



Class VII 



Phaeophyceae (Brown Algae) 



With exception of a very few fresh-water species, the Phaeophyceae 

 are only found in salt-water. They include over 160 genera, are all 

 fixed, and attain their highest development in the colder waters of 

 the ocean. They show great diversity in the form and structure of 

 their vegetative body. The simplest representatives of this class (e.g. 

 the genus Ectocarpus) closely resemble the Gonfervoideae, in having a 

 filamentous thallus consisting of a branched or unbranched row of 

 simple cells. Some Phaeophyceae, again, have a cylindrical, copiously 

 branched, multicellular thallus (e.g. Cladostephus, whose main axes are 

 thickly beset with short multicellular branches, Fig. 7, p. 12); while 

 in other cases the multicellular thallus 

 is band - shaped and dichotomously 

 branched (e.g. Dictyota, Fig. 8, p. 13). 

 Growth in length in both of these forms 

 ensues from the division of a large 

 apical cell (Fig. 7, p. 12; Fig. 160, p. 

 148). Other species, again, are charac- 

 terised by disc-shaped or globose thalli. 



The Laminariaceae and Fucaceae in- 

 clude the most highly-developed forms 

 of the Phaeophyceae. To the first family 

 belongs the genus Laminaria found 

 in the oceans of northern latitudes. 

 The large-stalked thallus of the Lami- 

 narias resembles an immense leaf ; it 

 is attached to the substratum by means 

 of branched, root-like hold-fasts, devel- 

 oped from the base of the stalk. 



In the case of the Laminaria digitata 

 (Fig. 252), and similarly in other species, 

 a zone at the base of the palmately 

 divided leaf-like expansion of the thallus 

 retains its meristematic character, and 

 by its intercalary growth produces a 

 succession of new laminse. Each older 

 lamina becomes pushed up and gradu- 

 ally dies, while a new one takes its 

 place and becomes in turn palmately 

 divided by longitudinal slits. The large size of their thalli is also 

 characteristic of the Laminarias : L. saccharina (North Sea), for instance, 

 is frequently 3 m. long and the stalk more than 1 cm. thick. 



Fio. 



252. — Laminaria digitata, forma 

 North Sea. (Reduced J. 

 Officinal.) 



