PH HOPHYCE & 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 Fucacce, 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 
Fucacee (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 Hormosira, 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 HMimanthalia, 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 
