PH^EOPHYCE^E 45 



secondary growth in thickness. The central strand of 

 long narrow cells bears a resemblance to the vascular 

 portion of the axis of the higher plants, and continuity 

 of protoplasm has been demonstrated through the pits 

 on the longitudinal walls. This strand is often 

 massively developed in the stem, and traverses the 

 leaf-stalk, where its cell-walls become thinner, into the 

 blade of the leaf, where subdivision of the strand takes 

 place, the subdivisions gradually disappearing and 

 becoming lost in the parenchyma as they approach 

 the edge of the blade. This subdivision frequently 

 begins in the leaf-stalk, but on the other hand the 

 elements of the strand often maintain sufficient 

 coherence to give the appearance of a midrib to the 

 blade. Not only, then, is there a striking external 

 conformation of parts differentiated in these higher 

 forms of the Fucacece, but the internal structure of 

 the tissues attains a considerable diversity. The 

 mode of branching is either lateral or dichotomous, 

 or even a mixture of both. Most of our British 

 Fucacece (e.g. the genera Fucus, Ascophyllum, &c.) 

 occupy a lower level of vegetative differentiation, as 

 do also such remarkable genera of the southern seas 

 as Iformosira, with its simple, beaded, necklace-like 

 fronds, without lateral foliar expansions or special 

 receptacular branches. In the lowest rank of all 

 comes the quite undifferentiated thallus of such 

 genera as our native Himanthalia, resembling a 

 stalked button, from which, however, long dicho- 

 tomous receptacular branches spring. 



The attachment of the thallus to the substratum 

 is most frequently effected by means of a sucker-like 



