276 



OEDOGONIUM: FUCUS. 



396. In the " macrandrous " species, whether monoecious or 

 dioecious, the aiitheridia appear as short disc-like cells, usually 

 in a series ; the contents of each usually divide to form two 

 antherozoids, which have very little chlorophyll and are much 

 smaller than the zoogonidia, but resemble them in their ciliation, 

 and are set free in the same way, ultimately fertilising an oosphere. 



397. In the " nannandrous " species the antherozoids are formed 

 in a curiously roundabout way. Short cells are formed in the 

 (female) filaments, either singly or in chains, and from each of these 

 androgonidangia there is produced an androgonidium, inter- 

 mediate in size between zoogonidium and an antherozoid. The 

 androgonidium swims about, and then settles by its clear ciliated 

 end either on an oogonium or on a cell near one, acquires a cell- 

 wall, and grows into a small male filament (dwarf male plant), 

 consisting usually of a basal vegetative (attaching) cell supporting 

 one or two antheridial cells, each of the latter producing an 

 antherozoid which is set free and finally enters an oogonium. 



The fertilised oosphere, or oospore, acquires a wall which 

 becomes thickened, the contents become yellow or red, the starch 

 changes to oil, and after a resting period the outer wall bursts and 

 either grows out at once to form a filament (new plant), or (more 

 often) its contents divide into four portions which are set free as 

 motile zoospores resembling zoogonidia in form and ciliation. 

 When the zoospore germinates it often gives rise to an asexual 

 filament which produces zoogonidia, and so on for several asexual 

 generations before a sexual plant is formed ; or a sexual plant may 

 be formed at once. 



Fucus. 



399. General Characters. Two species of Fucus are 

 easily distinguished among the Brown Algae which grow 

 on the coast between the tide marks. Fucus serratus has 

 toothed margins ; while in F. vesiculosus the margin of 

 the thallus is entire, and along the middle there are con- 

 spicuous air bladders, often in pairs side by side. 



In both cases note (1) the flattened irregular attachment 

 disc, firmly fixed to rock or stone ; (2) the cylindrical lower 

 portion or " stem " formed as seen on comparison with 

 young plants by the thickening of (3) the midrib and 

 decay of (4) the thinner lateral portions or wings which are 

 distinguishable in the flattened upper part of the mature 

 plant; (5) the repeated branching of the thallus, especially 



