80 Mr, Thompson on an Alga which colours Bally drain Lake. 



were in sheltered spots in the midst of a flotilla of Aphanizo- 

 menon, which was still abundant in a few creeks. 



Sept. 25. — With the desire that my friend Mr. "W. H. Har- 

 vey, now in Dublin, should have the opportunity of investi- 

 gating both plants in a recent state, I visited the lake in the 

 hope of procuring them. The water looked perfectly clear as 

 on the 23rd, but going into the boat I at once saw on looking 

 down into the lake where the sun struck upon it, that the spi- 

 ral alga was still there, though very sparingly ; I brought some 

 up in phials. The water was slightly agitated today and con- 

 sequently none of the Aphanizomenon was obtained. 



I have been thus particular in noting the observations on 

 this plant just as they were made from time to time, chiefly 

 to show that the species undergoes no change whatever 

 either in size or otherwise from its first appearance as a co- 

 louring matter until about three months afterwards, when de- 

 composition ensues and it is utterly dissolved. In consequence 

 of the great changes that some species of Algce do really un- 

 dergo, and the conjectures of botanists that others which have 

 not been investigated are likewise subject to them, I took 

 much interest in attending to this point during the time that 

 this alga plays such a prominent part in the waters of the lake. 



On examining this plant when first obtained, I could only 

 say that it did not belong to any British genus with which I 

 was acquainted, and no further attention was then given to it. 

 M. Morren, Professor of Botany in the University of Liege, 

 and well known to have successfully studied the freshwater 

 Alga of Flanders, on his visit to Dubhn in the following 

 month (Aug. 1838) was shown some sketches of the plant 

 which I had communicated to Miss Ball — a lady who has given 

 much attention to the Alga of Ireland, and made therein 

 some interesting discoveries — and he referred them to the ge- 

 nus Anabaina of Bory St. Vincent. With the genus Spharo- 

 plea, Ag., in its last or free state, my species would however 

 as well agree as with Anabaina. Except in the specific dif- 

 ference of being much more minute and more regularly spiral, 

 it resembles the Spharoplea crispa, Berk.* in this state, but 



* The specific characters of this species are — " Threads ei'ect, short, green, 

 mucous, crisp, simple, at first with articulations as broad as long, filled with 

 minute distinct granules, then with parallel rings, which at length become 



