200 



PHAEOPHYCEAE 



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

 which sometimes bears nodular galls caused by the eel-worm 

 Tylenchus fiickola, 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. 324), and these differ considerably 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 pneumatophores. The 

 axis is beset by simple, clavate, compressed branchlets that arise 

 singly or in groups in the axils of the serrations. These are later 

 converted into or are replaced by short-stalked, yellow, fertile 

 branches which fall off after the gametes have been liberated from 

 their conceptacles. The macrosporangia each give rise to four ova, 

 the remaining four nuclei degenerating. 



The method of branching is perhaps best understood from an 

 inspection of fig. 133. In spring the main branches divide dicho- 

 tomously as in Fucus, after which opposite pairs of fertile recep- 

 tacles or sterile tufts of hairs are produced in notches that are 

 formed as follows on both sides of the thallus. The apical cell (A) 

 cuts off another apical cell (^1) that remains dormant for a time, 

 during which period it is carried up the edge of the groove to the 

 side of the thallus by the activity of the primary apical cell. The 

 limiting layer immediately around A^ does not undergo further 

 growth and so it also comes to lie in a groove. Later on, tertiary 

 (AA-^) and quaternary (AA^) apical cells are cut off from ^1, the 

 tertiary cell becoming the apical cell of a sterile or fertile branch. 



Fucaceae: Seirococcus {seiro, chain; coccus, berry). Fig. 134. 



The mode of branching in this southern-hemisphere genus can 

 be explained if it is assumed that the lower side of a notch, com- 

 parable to one of those found in Ascophyllum, develops into a leafy 

 member (cf. fig. 134). The apical cell cuts off segments on either 

 side, ^1 and A<^ , which are secondary apicals that become separated 

 from A through growth of the epidermis. These secondary apicals 

 divide to give tertiaries, A^ , after which they become separated from 

 each other by a new leaf organ (/) that develops as a result of the 



