988 Injury to Water-cress Beds by Algal, [march, 



The Small Stinging Nettle (U. Mr ens L.) is much smaller 

 than the last species, attaining only 1-2 ft., or rather over, 

 in height, while it is an annual. It is also smooth except for 

 the stinging hairs. The stem is branched; the leaves are 

 stalked, 1-2 in. in length, ovate-oblong, pointed, coarsely 

 toothed, and placed opposite in pairs on the stem. The 

 panicles or spikes of small green flowers are bisexual, J to 

 1 in. long, and spring in pairs from the axils of the leaves. 

 Wherever it occurs in arable land this nettle may be success- 

 fully combated by thorough and regular hoeing to prevent 

 seeding for a year or two. 



During January the Board received specimens of an alga 

 which was causing great loss in some Hampshire water-cress 



beds, the alga having the appearance of 

 Injury to Water- cress a "black cobweb" at the base of the 

 Beds by Algae. plants, which degenerated and died off. 



The "black cobweb," as it was 

 described, was identified as a fresh-water alga known as 

 Oscillatoria irrigua, Kutzing, which with allied species may 

 prove injurious when occurring in quantities in water-cress 

 beds. 



According to Bull. No. 64 of the Bureau of Plant Industry, 

 U.S. Department of Agriculture, these algae may be eradi- 

 cated by the use of copper sulphate, one part to fifty million 

 parts of water, or 1 lb. of the sulphate to 5,000,000 gallons 

 of water. An estimate of the amount of water in the beds 

 should be made, and then the copper sulphate, enclosed in 

 a piece of sacking, may be slowly dragged through the 

 water until dissolved. 



In the Bulletin referred to, it is stated that "the success 

 of the copper treatment for eradicating algae from cress beds 

 has been thoroughly demonstrated, and there is no reason 

 why growers should have trouble from this cause in the 

 future. The strength of the solution used for killing the 

 alg-ae is so very much weaker than that which might affect 

 the cress that there is no possible danger of injuring the 

 latter if the solution is used by anyone capable of observing 

 ordinary care. The question of how long a treatment is 



