1911-1912.] Elimination of A Igce in Lochs and Ponds. 427 



entirely disappeared." Three days after receiving this letter a 

 sample of the water from the loch was forwarded to me : this 

 was beautifully clear, and contained some very lively water-fleas. 



It may be interesting now to learn what effect the solution 

 had on the flowering-plants, and especially on the fish, in the 

 loch. The yellow water-lily {Nym^phcea {Nwphar) lutea) and 

 the buckbean or marsh trefoil {Menyanthes trifoliata) are both 

 plentiful in the loch, especially the former, but the most 

 conspicuous aquatic is the amphibious persicaria (Folygoniim 

 am^phibium). For a number of years this plant has been 

 increasing, and now covers a considerable space here and there 

 around the sides of the loch. The contrast between its 

 appearance when growing under the two conditions of land 

 and water is very striking. In the soil it is about a foot in 

 height, with sub-sessile, lanceolate, hairy leaves ; as an aquatic 

 its stems are from two to three feet high, with long-stalked, 

 floating, oblong, smooth leaves. In summer, with its bright 

 rose-coloured flowers, it is a rather handsome object. The 

 copper-sulphate bath to which it and the other aquatic plants 

 were subjected had evidently no injurious effect upon them. 

 As regards the fish, however, a different tale has to be told, 

 for the perch suffered severely. They congregated round the 

 sides of the loch, and in the shallow water received the full 

 force of the solution. No fewer than 530 were picked up 

 dead, and it is believed that at least 600 must have perished. 

 Strange to say, the pike and eels, of which there are con- 

 siderable numbers in the loch, appear not to have suffered in 

 the slightest. They retired to the depths, where they were 

 comparatively safe, although several eels swimming near the 

 surface were observed to be in evident distress. The material 

 lessening of the numbers of both pike and eels, it may be 

 added, would not have been deplored. 



There is now little doubt that a weaker solution of the 

 ^opper-sulphate would in this case have killed off the Alga 

 equally well. The strength, as already said, was that recom- 

 mended after trials in the Washington Laboratory of Plant 

 Physiology, but it has since been ascertained that " the con- 

 centration necessary to kill Algse in the laboratory is from 

 five to twenty times as great as that necessary to destroy the 

 same species in its natural habitat." This is an interesting 



