198 



PHAEOPHYCEAE 



adult plant. It is perhaps of interest to note that in Fucus vesiculosus 

 it has been shown that the mature sporophyte contains the diploid 

 number of sixty-four chromosomes, which appears to be the usual 

 number in all the Fucales so far examined, with the exception of 

 Sargassum Horneri in which 2^ = 32. 



*Fucaceae: Pelvetia (after the French botanist, Dr Pelvet). Fig. 

 132. 

 The fronds in this genus have no mid-rib and are linear, com- 

 pressed or cylindrical with irregular dichotomous branching. Air 





Fig. 132. Pelvetia canaliculata. A, plant ( x |). B, megasporangium (x6o). 

 C, mature fertilized sporangium ( x 72). we = rejected nuclei. D, germinating 

 oospores ( x 72). E, microsporangia (x 156). (A, original; B-E, after Scott.) 



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. Modified salt-marsh forms 

 derived from P. canaliculata are also recorded but these are con- 

 fined to Great Britain (cf. p. 324); like the marsh forms of Fucus 

 they are characterized by the general absence of fruiting receptacles, 

 reproduction being primarily vegetative. The structure of the 

 thallus is essentiallv similar to that of Fucus, but the Californian 

 Pelvetia fastigiata also possesses a few cryptostomata which are 



