Fig. 112 Fucus. A-Cj origin of conceptacles in F. serratus. h = basal 

 cell, I = initial. D, juvenile conceptacle of Cystoseira. ^=hair. 



(After Oltmanns.) 



Barents Sea F. vesiculosus plants reach their maximum size at the 

 greatest depth for the species (300 cm.). 



Morphologically the primary thallus is built up by the activity of 

 the apical growing cell (see below) and by the surface layer of cells, 

 the limiting layer or meristoderm. Below the limiting layer is a 

 cortex composed of several layers of parenchymatous cells which 

 become more and more elongate and mucilaginous towards the 

 centre, and these probably form the storage system. In the very 

 centre the cells are extended into hyphae which are interwoven into 

 a loose tangled web, but they do not extend right up to the apex. 

 This central tissue is called the medulla and probably acts as a con- 

 ducting system, because the transverse walls of the hyphae are 

 frequently perforated with the same type of pit that is to be found 

 in some of the Laminariaceae. The primary medullary hyphae are 

 relatively thin-walled, but when secondary growth of the thallus 

 takes place the new hyphae which result from this process are very 

 thick-walled and so are probably mainly mechanical in function. 

 Secondary growth is due to the activity of the Umiting layer and the 



199 



