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 alge 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 alge 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. 
217. The brown alge. The remaining groups of alge, 
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 alge, 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 alge, known as rockweeds (Mucus 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 algze sometimes become detached and are car- 
ried hundreds or even thousands of miles from their original 
1See ‘\A Method of Destroying or Preventing the Growth of Algze 
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, 
