198 THE ALGAE 



summer, the proportions of contained oxygen and nitrogen varying 

 accordingly : carbon dioxide appears to be absent. Sometimes whole 

 portions of the frond may be inflated in an irregular manner, but 

 the factors causing this phenomenon are not known, although it is 

 possible that contact with rock or soil provides the necessary 

 stimulus. In at least one species, F. serratus, the overall growth rate 

 not only varies with season but also with the degree of shelter. 



With increasing age the lower portions of the alae may be frayed 

 off by wave action, leaving only the mid-rib, which then has the 

 appearance of a stipe. The whole of the expanded thallus is covered 

 with sterile pits or cryptostomata similar to those of Saccorhiza, 

 but in fruiting plants it is only the ends of the branches that become 

 swollen and studded with the fertile conceptacles. In F. spiralis 

 these conceptacles are hermaphrodite, containing both oogonia and 

 antheridia; in F. vesiculosus and F. serratus the plants are dioecious, 

 the two types of sex organs occurring on separate plants, whilst in 

 F. ceranoides either state may be found. 



In the two species that have been studied, F. vesiculosus and F. 

 serratus^ the peak of reproduction in the former occurs in spring 

 and summer and in the latter in autumn and winter. This may pos- 

 sibly be related to differences in germination time and survival of 

 plants as in Laminaria (see p. 178), though no such evidence has 

 been reported. The mortality rate of sporelings is extremely high in 

 the early stages of growth (this also may possibly be seasonal), the 

 principal offenders being limpets. A number of very peculiar forms 

 have been found which commonly occur on salt marshes: these 

 rarely fruit, reproduction being secured principally by means of 

 vegetative proUferations (cf. p. 349). The age of Fucus plants has 

 not been studied in much detail but the following figures (Table 3) 

 may be cited from the results of tagging plants. 



Table 3 

 Age of Fucoids 



Species F. spiralis F. serratus F. vesiculosus Ascophyllum 



nodosum 



Max. age (yrs.) 4 4 2^ 19 



Av. age (yrs.) li 2 i 3-4 



Recent work from Russia suggests that the maximimi age of 

 Fucus plants may be ra±er greater than that given above. In the 



