THE BROWN ALGA 237 



duction takes place by zoospores formed in single-celled spore 

 cases (sporangia) in large groups upon the blades. Sexual 

 reproduction is not known. 



382. The rockweeds, Fucus. The species of Fucus are very 

 numerous and widely distributed in the temperate and arctic seas. 

 They can be seen often in vast quantities attached to the rocks 

 at low tide when they are often uncovered. The short stalk is 

 attached to the rock by a disk-like holdfast. The blade is branched 

 in a forked manner, and the middle line is thickened in the form 

 of a midrib. Growth takes place in a small depression at the 

 apex of each leaf. Some species are provided with large bladder- 



Fig. 197. Fig. 198. 



Section of conceptacle of Fucus, showing oogonia Oogonium of Fucus with 



and tufts of antheridia. ripe eggs. 



like vesicles in the leaves (Fucus vesiculosus}. The sexual organs 

 are developed in small cavities called conceptades. In some 

 species the male and female organs are on different plants (dice- 

 cious), in others they are in different conceptacles of the same 

 plant (monoecious), while in some both are formed in the same 

 conceptacle (hermaphrodite). These conceptacles are grouped in 

 definite patches, their conical mouths, slightly elevated, giving 

 a punctate appearance, which is easily observed. The concep- 

 tacles are oval or flask-shaped, and numerous slender filaments 

 arise from the bottom and sides and project through the opening. 

 The egg cases are large rounded bodies attached by a stalk to the 



