204 THE ALGAE 



vesicles may be present in some species but normally they are 

 absent, especially in the European P. canaliculata, which grows on 

 rocky shores forming a zone near high-water mark or even above, 

 so long as it is reached by the spray. ^ The thallus of this species is 

 said always to be occupied by an Ascomycete, Mycosphaerella pel- 

 vetiae. Modified salt-marsh forms derived from P. canalicuhata are 

 also recorded but these are confined to Great Britain (cf. p. 348); 

 like the marsh forms ofFucus they are characterized by the general 

 absence of fruiting receptacles, reproduction being primarily 

 vegetative. The structure of the thallus is essentially similar to that 

 of Fucus, but the Californian Pelvetia fastigiata also possesses a 

 few cryptostomata which are otherwise absent from the genus. 

 The oogonia are similar to those of Fucus except that normally 

 only two ova mature, the remaining six nuclei being extruded from 

 the cytoplasm into the wall, though in Pelvetia fastigiata one may 

 occasionally find four ripe ova or else ova that contain two nuclei. 

 In P. canaliculata the two mature eggs are arranged one above the 

 other, whilst in the Japanese species. P. wrightii, they are placed 

 side by side. This difference is probably dependent upon the 

 relative position of the two megaspores which germinate. In the 

 former species gametes are liberated at spring tide periods. 



Vvcaceae: Ascophy Hum {asco, wine-skin ;/)/z3;//wm, leaf). Fig. 115 



The plants of this genus are large, often attaining several feet in 

 length, and are commonly to be found on sheltered coasts at about 

 mean sea-level. The thallus of the common species, A, nodosum^ 

 which sometimes bears nodular galls caused by the eel-worm 

 Tylenchus fucicola^ is more or less perennial, and regenerates each 

 year from a persistent base or from the denuded branches. As in 

 the two previous genera free-living or embedded forms have 

 evolved in salt-marsh areas (cf. p. 349), and these differ consider- 

 ably from the common parent species, Ascophyllum nodosum, not 

 only vegetatively but also in the absence of sporangia. The normal 

 fronds have a serrated margin but no mid-rib and commonly bear 

 vesicles which are known as pneumatocysts, but when the vesicles 

 are borne on the little side branches they are termed pneumato- 

 phores. Recent work has shown that in some regions at least one 

 pneumatocyst and its associated piece of axis is produced annually, 



1 The mean growth rate of this species is 3-2 cm. per year, and plants live for 4-5 

 years (Subrahmanyan, i960). 



