50 
FUCACEflL 
IY. 
development ; the cup-like frond being wholly formed and perfected before the 
branching fructification begins to be evolved. 
Rising to still higher types of the Order we find (in Fucus , Haliclrys , Cystoseira, 
&c.,) plants with branching, pinnate, or more commonly, dichotomous stems, either 
filiform or imperfectly leafy, having usually their leaf-like portions strongly 
midribbed, and forming their fructification in portions of the branches ; generally 
in the extremities, which at first resemble ordinary parts of the frond, but 
afterwards swell, become succulent, and are converted into more or less distinct 
receptacles. 
Lastly, (in Sargassum , and its allies,) there is a branching stem ; distinct 
midribbed, rarely ribless, leaves, which are, in a few instances, decurrent, developed 
in a distichous or subspiral order ; and receptacles which are, from their origin, 
set apart as organs of fructification (not formed by swellings of the branches,) and 
placed, either in the axils or along the edges of the leaves or branches. 
In a large number of the plants of this Order, air vessels ( vesiculate j or floats 
designed to give buoyancy to the stem and branches, are present. In the least 
perfect, (as in Himantlialia , Fucus , and Cystoseira) the air vessels are formed 
by simple swellings of portions of the branches, the swollen portion becoming 
hollow and filled with air. In Halidrys several of these hollow swellings placed 
close together in the ramuli become confluent into a compound moniliform vesicle, 
which is evidently only an extreme development of the chained vesicles of 
Cystoseira. In Pliyllospora the air vessel is formed in the leaf-stalk, the lamina 
being a crest to its summit. Such is likewise the case in Sargassum , the highest 
type in the order, but in this genus the lamina of these vesicular leaves is either 
wholly abortive or reduced to a slender mucro ; so that here the air vessel appears 
like a distinct organ. It usually accompanies the receptacles of fructification, 
and is, in fact, properly a floral leaf or bract, interposing between the ordinary 
leaves and those appropriated to the fructification. 
On most parts of the frond, but especially on the expanded portions of the stem 
in the less organized types, and on the leaves in the more fully developed ones, 
will be found minute dot-like pores , from which, while the plant is under water and 
in a growing state, a pencil of delicate, colourless, jointed hairs is seen to protrude. 
These pores, called the muciferous pjores by early writers, are found in all the 
Fucacece, and are one of their most definite characters. Under each pore is placed 
a minute hollow chamber, of a spherical form, from the inside of whose walls the 
colourless fibres originate. It is possible that these hairs may exercise an impor- 
tant physiological office, acting on the aerated water as the stomates of aerial 
leaves do upon the air ; nothing, however, has been ascertained on this point. But 
whatever be the use of these hollow chambers and their contents in the vegetating 
parts of the frond, in those appropriated to fructification they are enlarged, and 
transformed into the spherical cavities within which the spores and antheridia are 
lodged. 
In the less organised genera, as has been already mentioned, the spore-cavities 
( scaphidia , Ag. — conceptacula , Mont. — Endl. — angiocarpia, Kiitz,) are dispersed over 
the whole frond ; in the more perfect, they are confined to limited portions of the. 
