202 FUCUS (THALLUS) 



small cavities {conceptacles) in the thallus. These are lined with 

 hairs which often project as a minute tuft through the tiny aper- 

 ture leading to the exterior. Usually some of the branches of the 

 frond have swollen ends (Fig. io8) provided with much more 

 conspicuous (fertile) conceptacles enclosing the sexual repro- 

 ductive organs (cf. p. 224). 



The Serrated Wrack (Fiicus serratus), which is equally common 

 on rocky shores, is distinguished by the toothed margin of the 

 frond, the absence of air-bladders, and the less distinct swelling 

 of the ends bearing the fertile conceptacles (Fig. 108, right). 



A transverse section through the frond of any Fiicus (Fig. 

 121, B, p. 225) presents three regions. At the outside is a small- 

 celled assimilating zone {a.), of which the most obvious part is the 

 palisade-like surface-layer whose cells show occasional tangential 

 division-walls. The cells of the central medulla {m.) are con- 

 spicuous for their thick mucilaginous walls, by which the small 

 protoplasts are widely separated. The elements in question, 

 whose function is partly mechanical and partly conducting, 

 are of considerable length, but roughly follow the direction of 

 the thallus, and hence appear more or less oval in transverse 

 section. Between the medulla and the surface region is a zone 

 of relatively large storage-cells (S.) with highly refractive contents 

 that are presumably products of assimilation. The outermost 

 layer of the thallus is meristematic and adds to the assimi- 

 latory region, whose innermost cells gradually enlarge to form 

 elements of the storage tissue. Similarly the medulla slowly 

 increases at the expense of the adjacent storage-cells, so that, 

 although there is an obvious differentiation into three regions, 

 the same cells may perform different functions in successive 

 periods of the life of the thallus. 



A cross-section of the stalk or frond of Laiiiinaria shows 

 essentially the same construction, and here the high specialisation 

 of the Brown Algfe is evidenced by the presence of sieve-like 

 areas, analogous to those of sieve-plates, on the cross-walls of 

 many of the elongated elements of the medulla. Similar struc- 

 tures occur in Fncus, but are not so easily recognised. 



Clothing the rocks, which for the greater part of the season 

 are only reached by spray, one finds the Seaweed Pclvdia, which, 

 though closely allied to Fncus, is much smaller, and in which the 



