458 



General Botany 



lengths of several or many feet, and internally the plants show 

 distinct tissue systems. There are three distinct lines of develop- 



FiG. 278. A filamentous brown alga, Ectocarpus. On the tips of three of the branches 

 are many-celled sporangia, which develop zoospores. This alga is common along the 

 Atlantic coast, growing as an epiphyte on the coarser rockweeds. 



ment in this group: the filamentous forms {Ectocarpus), the 

 highly branched rockweeds (Fucus), and the large stalked 

 forms with flat blades (Laminaria). About 1000 species are 

 known. 



The filamentous forms. There are a large number of branch- 

 ing, filamentous forms that are not very different from some of 

 the green algae. These reproduce by swimming spores, and by 

 zygotes formed from swimming gametes. The swimming spores 

 and the gametes of the brown algae differ from those of all other 

 groups in having two cilia laterally placed. 



The rockweeds. A second group are the rockweeds or bladder 

 wracks (Fucus), which cover the rocks between tide levels. 

 These are thick, leathery, highly branched plants with internal 

 air sacs at intervals and with reproductive structures in the 



