230 INTRODUCTION TO BOTANY 



intended to hold water enough so that there will be a suf- 

 ficient supply in times of scarcity. Such reservoirs have 

 proved so admirable as growing places for algse that these 

 plants often become a nuisance. Their presence in water for 

 domestic use is not attractive, and, besides, they may stop 

 up the water pipes. But far more serious than these objec- 

 tions is the actual pollution of the water because of their 

 presence. When they die they become the food for decay- 

 producing organisms, and often positively injurious substances 

 are generated. It has been found that by towing about in 

 such reservoirs a quantity of copper sulphate inclosed in coarse 

 sacking, minute quantities of the salt become dissolved and 

 the algae are thus killed. The solution is not strong enough 

 to render the water unwholesome for use. This treatment 

 has been an important factor in improving the water in many 

 American cities. 1 



217. The brown algae. The remaining groups of algae, 

 though almost exclusively salt-water plants, have such strik- 

 ing characteristics that brief mention of them must be made, 

 and pupils who live near the seacoast will be interested in 

 extending this study. The brown algse, or brown seaweeds, 

 are found along the shores of all the oceans. They grow 

 attached, by means of strong holdfasts, to rocks, piling, or 

 any relatively fixed support that is available. 



From high-tide mark to a little below low-water mark cer- 

 tain brown algse, known as rockweeds (Fucus and Ascophyllum) 

 (fig. 180), often form dense coatings upon rocks. At low tide 

 these rockweeds hang loosely over the exposed rocks, and 

 exhibit the characteristic dark olive-green color. 



The brown algse sometimes become detached and are car- 

 ried hundreds or even thousands of miles from their original 



1 See " A Method of Destroying or Preventing the Growth of Algse 

 and Certain 'Pathogenic Bacteria in Water Supplies" and "Copper as 

 an Algicide and Disinfectant in Water Supplies," Bulletin 64 (1904) and 

 Bulletin 76 (1905), respectively, Bureau of Plant Industry, U.S. Dept. Agr. ; 

 also Whipple, Microscopy of Drinking Water, chap, xii, John Wiley & Sons, 

 New York, 1906. 



