t.- 
* 
396 REPRODUCTIVE ORGANS OF ALGZ.—FUCUS. 
protoplasm is to be seen at its end ; the cell-wall then opens at 
this point, and the contents at the same time retract from the 
cell-wall and what is termed the oosphere is formed. The anthe- 
ridium opens at the same time as the oogonium, and the anthe- 
rozoids escape, reach the oosphere, mix with it, and then dis- 
appear ; and the oosphere is transformed into an oospore. The 
oospore thus formed acquires a distinct cell-wall of its own, and 
its colour also changes to a reddish hue. By the germination of 
the oospore, a new Vaucheria thallus may be formed. 
Fucus.—This genus includes numerous species, which form 
the various plants commonly known as Seaweeds. The thallus 
(fig. 5) is usually long, very much branched, and of a greenish 
brown colour. In structure, it is made up at the surface of 
closely packed small cells, but towards the interior the cells are 
more elongated, and joined end to end, so as to form filaments 
Fic. 865. 
Fig. 865. Vertical section of a female conceptacle of Fucus vesiculosus con- 
taining oogonia and paraphyses. After Thuret, 
which are interwoven amongst one another (fig. 865). The walls 
of the constituent cells are peculiar in consisting of two parts, 
an inner firm layer, and an outer one which is generally more 
or less swollen by imbibition of water. (See page 73.) 
Reproduction is effected by a sexual process, which takes 
place in the following way :—Numerous little cavities, known 
as conceptacles, appear sunk in the surface of large swollen recep- 
tacles (fig. 5, t, t), on the ends of the longer forked branches of 
the Fucus, and in these are contained the antheridia, or oogonia ; 
or both of these organs, together with abortive filaments or 
paraphyses. Some species, as Fucus platycarpus, are monoecious, 
i.e. contain both antheridia and oogonia in the same conceptacle; 
but in others, as Fucus vesiculosus, either only antheridia or 
