194 THE ALGAE 



the complexity of the thallus. On this basis a series of increasing 

 embryonal complexity may be traced, e.g. Fucus — Ascophyllum — 

 Pelvetia — Cystoseira — Sargassum. 



Geographically the original centre of distribution was un- 

 doubtedly the southern Pacific in the waters of Australia and New 

 Zealand where the greatest number of species are now to be found. 

 This makes an interesting comparison with the preceding order 

 whose original centre of distribution was the northern Pacific in the 

 waters around Japan and Alaska. The Fucales are classified into 

 five groups, the classification being based primarily upon the struc- 

 ture of the apical growing cell or cells : 



(i) Durvilleae. A group comprising two genera, Durvillea and 

 SarcophycuSi from AustraHa and Patagonia, both without any 

 means of apical growth. 



(2) FucO'Ascophyllae. Growth is determined in the adult stage 

 by one four-sided apical cell. Recently it has been found that 

 in Marginariella urvilliana the apical growing cell is three- 

 sided in the adult condition. The apical growing cell of the 

 salt marsh fucoids (see p. 349) has also been shown to persist 

 in the three-sided condition. Despite these exceptions, how- 

 ever, the basis of the classification remains very useful. 



(3) Loriformes. Growth is due to one three-sided apical cell which 

 gives rise to a long whip-like thallus. 



(4) Cystoseiro-Sargassaceae. The apical cell is again three-sided 

 but there is copious branching which results in bilateral, 

 radial and bilaterally radial thalli. 



(5) Anomalae, composed of one genus, Hormosira, confined to 

 the Antipodes. Growth is brought about by a group of cells 

 instead of a single cell. 



In most of the Fucales the apical cell is sunk in an apical pit or 

 groove. In the Fucaceae proper, this groove is usually parallel to 

 the plane of fiattening of the thallus, but in Halidrys and Himan- 

 thalia it is at right angles to it. In the adult the hairs in the young 

 apical pit are deciduous, but in Himanthalia, Bifurcaria and Xipho- 

 phora hairs can be found growing out from the apical groove of 

 mature plants. 



Durvilleae 

 DuRViLLEACEAE : DurvUka (after J. D. D'Urville). Fig. no 

 The sporophyte is a dark ohve brown or black in colour and 



