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A TEXTBOOK OF BOTANY 



[Ch. X 



Order 2. Fucales: the Rockweeds or Bladder Wracks. 

 These are the common seaweeds of rocky shores, which they 

 cover densely in vast quantities. The plant body (Fig. 132, 

 308) is a branching elastic thallus, approximately a foot long, 

 buoyed up by bladders holding air and oxygen gas. The 

 reproduction is wholly sexual. In the commonest of the 

 Rockweeds, Fucus vesiculosus (Fig. 308), the tips of the 



Fig. 308. — The life history of Fucus vesiculosus, a typical Brown Alga. 



Left, a plant ; X i- Above, swollen tip showing conceptacles, and cross 

 section ; x 2. Second line, cross sections of conceptacles showing oogonia 

 and antheridia ; X 20. Third line, opened oogonium, and antheridia ; X 50. 



Lower line, egg cell surrounded by sperm cells, X 200, germinating oospore, 

 X 200, and young plant, X 25. (In part after Thuret, in part from nature.) 



fronds are enlarged and covered with small swellings, under 

 which are the hollow conceptacles. These when sectioned 

 are found lined with oogonia and antheridia, — on separate 

 fronds in this species, but in the same conceptacles in some 

 others. Each oogonium contains 8 egg cells, which, set 

 free en masse, soon separate and pass singly outside the frond, 



