108 , BRITISH MARINE ALGA. 
are seated on little stalks. P. fibrata, so named from the tufts of fibres 
which terminate the ramuli of every filament, is a very pretty species, 
found very generally on aJl the European shores, and is generally regarded 
as one of our commonest species. The fronds are densely tufted and very 
much branched, being gradually attenuated upwards to a hair-like fineness. 
The articulations and siphons of this species may be very distinctly seen 
under a lens. They differ in length somewhat in different parts of the 
stem and branches. The siphons are however four, surrounding a small 
central colourless tube (c, Fig. 101). The structure of this species is pretty 
well represented in the drawings of magnified portions at Fig. 103 ; (a) is 
a ramulus or branchlet containing tetraspores in its central articulations, 
and is crowned with a tuft of branching fibres; (b) represents an ovate or 
egg-shaped capsule, containing within it a tuft of pear-shaped spores ; 
(c) is @ more highly magnified branchlet, at the tip of which, and at the 
base of the apical fibres, are three large oblong bodies, which in the living 
plant are of a bright yellow. These are antheridia, filled with active 
granules or antherozoids. These antheridia are frequently so abundant on 
this species, that the branchlets which bear them seem as though 
they were crowned with a tuft of golden fruit. These bodies, which are 
supposed to be the representatives of stamens in flowering plants, are 
found on many species of seaweeds; but, as Dr. Harvey remarks, ‘‘ how 
they act, or whether they act on the spores at all, has not been ascer- 
tained.’”’ A transverse section of the stem of P. fibrata, which contains 
four siphons arranged around a central colourless tube, is represented at 
c., Fig. 101. P. fastigiata is another common species on which antheridia 
are very frequently found. ‘They are produced in tufts at the tips of the 
little forked filaments of the plant, and are so conspicuous that they give 
quite a yellow tint to the plant. This species of Polysiphonia is parasitic 
on Fucus nodosus (Fig. 35), or the “ knobbed wrack.”’ It grows in dense 
brownish tufts on the upper branches of the Fucus, encircling the stems 
of the plant, its little intertwined branches pointing upwards, about 2in. 
in height, and every one of them terminating in a tiny fork. Tetraspores 
are immersed in the terminal branchlets. Spores are contained in egg- 
shaped conceptacles. The stem contains no less than eighteen siphons, 
arranged around a central cavity, which is filled with endochrome. This 
cavity is, however, not, as in other species of Polysiphonia, a continuous 
tube, buta series of bags of colouring matter, which are separated from each 
other at the very slight divisions which occur at the articulations or joints 
of the stem and branches, all of which are shorter than their diameter. 
A transverse section of the stem is seen at b, Fig. 101. This species 
invariably turns black in drying, and adheres very imperfectly to paper. 
P. urceolata, so named from the urceolate or pitcher-shaped form of its 
spore-vessels, is found growing on the stems of Laminaria digitata (Fig. 
46), and some times fringing the shady sides of rock pools, its long red 
silky filaments mingling with those of the green Enteromorphw or 
Cladophora, with occasionally an olive frond or two of a young Laminaria, 
