56 
FUCACEtE — Sargassum. 
iv. 
** Frond branched , imperfectly leafy or pinnatifid. Air-vessels formed in certain parts 
of the leaves or branchlets. 
II. Phyllosrora. Leaves distichous, nerveless. Air-vessels formed in the petioles 
of the leaves. 
III. Halidrys. Frond pinnatifid, leafy below, filiform above. Air-vessels formed 
in the ultimate branchlets, podlike, of several air-cells. 
* * * Frond branched , imperfectly leafy or filiform. Air-vessels either absent , or 
formed irregularly by the occasional swelling of the branches. 
IY. Cystoseira. Frond much branched, bushy ; the branches filiform. Recep- 
tacles filiform, slender, terminal ; their substance formed of small cells. 
Y. Fucus. Frond dichotomous, flat or compressed. Receptacles filled with mucus, 
which is traversed by a net- work of jointed filaments. 
* * * * Frond reduced to a top-shaped , or cup-shaped vesicle. 
YI. Himanthalia. Receptacles strap-shaped, dichotomously branched. 
I. SARGASSUM, Ag. 
Root a conical disc. Frond much divided ; having a distinct stem, branches, 
leaves, air-vessels, and receptacles. Branches filiform or flat, alternate, lateral, 
more or less distinctly pinnate. Leaves horizontal, or very rarely vertical and 
decurrent, mostly furnished with a midrib, and muciferous pores. Air-vessels 
stalked, axillary, formed from transformed leaves, pointless or tipped with a slender 
process. Receptacles small, linear, tuberculated, axillary, racemose or dichotomous, 
composed of a densely cellular substance ; having numerous pores, beneath which 
are placed the spherical conceptacles (or spore cavities.) Spore-cavities mostly 
dioecious. Spores one or more in each conceptacle, to whose walls they are attached, 
obovoid, subsessile, having a hyaline perispore. Antheridia roundish, on branched 
filaments, racemose. Paranemata simple or forked, clothing the walls of the con- 
ceptacle. 
The frond originates in a single leaf, having a lamina and midrib. This first leaf 
lengthens, and either continues undivided or becomes forked at the extremity. 
Afterwards the lamina gradually disappears from the lower portion, while the 
midrib thickens and becomes the commencement of the future stem ; and the 
upper portion, still extending, is again divided and each of its divisions forms the 
starting point of a branch. All the young stems and branches, which in this 
manner are formed out of the midrib of the first formed leaves, are in their early 
growth winged with the remains of the lamina of the transformed leaf ; but as this 
soon decays away and is not renewed, the branches as they extend upwards become 
