196 THE ALGAE 



each other just as in the medulla of Laminaria (see p. 175). These 

 h5rphae extend right up to the apex, thus differing from genera such 

 as Fucus. There is also a mechanism involved in thallus sphtting in 

 much the same way as there are thallus-splitting mechanisms in 

 the Laminariales. Prior to sphtting, new cortical tissue is formed. 



In the most widespread species, D. antarctica, the adult lamina 

 is characterized by large internal air spaces separated from each 

 other by septa. These air spaces are considered to arise as a result 

 of an active internal splitting process which may or may not be 

 associated with degeneration of the tissues. In this genus, growth 

 is locaHzed to the tips of the laminae, the surface layer forming the 

 primary meristematic layer. 



The ends of the older laminae become frayed and broken off by 

 wave action, whilst the holdfast may attain a diameter of 2 ft. 

 through the addition of new tissue annually. If this secondary 

 growth did not occur the plant would soon be torn from its moor- 

 ings because the holdfast is continually becoming riddled with 

 holes made by boring molluscs. The oogonia and antheridia, which 

 are borne in conceptacles on different plants, as the genus is dioe- 

 cious, occur over the whole of the lamina, this condition being re- 

 garded as the primitive state for the Fucales. The oogonia (macro- 

 sporangia) are usually borne on branched hairs, whereas in all 

 other genera they are borne directly on the wall. This condition in 

 Durvillea is comparable to the antheridial structure generally in the 

 Fucales and must be regarded as the primitive condition. The ripe 

 oogonium contains four ova. The plant is known as the *bull kelp' 

 and forms submarine forests in deep waters off New Zealand, the 

 Antarctic islands and the southern part of South America. 



FUCO-ASCOPHYLLEAE 



* Fucaceae: Fucus (a seaweed). Figs. 111-13 



This genus contains a number of species that are widely scat- 

 tered over the world with the majority in the northern hemisphere, 

 many of them exhibiting a wide range of form with numerous 

 so-called varieties. When two or more species occur in the same 

 area they are generally present in different zones on the shore, 

 probably dependent upon the degree of desiccation that they can 

 tolerate (cf. p. 387). The plants are attached by means of a basal 

 disc and there is usually a short stalk, which continues on to form 

 the mid-rib of the frond in those regions where the expanded wings 



