ifc Mr. J. Ralfs on the British Desmidiese. 



mon. Most plentiful in the autumn. Ban try. Miss Hutchins ; Appin, 

 Capt. Carmichael. Trentishoe, Devonshire ; Penzance and Dolgelley, 

 J. R.; Cheshunt, Mr. Hassall; Chiltington Common near Pulbo- 

 rough, Sussex ; and in the peat bog at Fisher's Castle near Tun- 

 bridge Wells, Mr. Jenner. 



Filaments elongated, very gelatinous, of a pale translucent 

 green, not fragile. Under the microscope the joints are generally 

 about equal in length and breadth, and the endochrome forms a 

 single irregular patch. The joints are not constricted, but at one 

 end they have on each margin a minute bidentate projection : as 

 the similar ones of the next joint are at its adjoining extremity, 

 these projections occur only near the alternate dissepiments. 

 When, however, the joint is elongated, preparatory to the forma- 

 tion of two joints, the endochrome is divided into two portions, 

 and then these processes are present at both ends, the next joint 

 undergoing a similar change. These projections are extremely 

 minute, and can only be detected by employing the highest power 

 of the microscope, and even then are liable to be overlooked if 

 not carefully sought for. 



I examined many specimens of this plant in 1841, but did not 

 perceive these curious projections until the following year. I 

 believe they are really formed by a grooved rim round the end of 

 the joint, because however the filament may be moved they are 

 equally apparent ; whereas if they were processes, as in Didymo- 

 prium, they would be sometimes either entirely concealed or ren- 

 dered less apparent. 



The filaments have a very broad mucous sheath, which from 

 its great breadth and absence of colour is not easily discerned j 

 it is more evident when a specimen is dried on talc or glass, as 

 the margins are then generally perceptible. When gathered the 

 filaments are very distinct, frequently parallel and subdistant 

 even to the naked eye : this depends on the great breadth of their 

 mucous sheaths, which prevent the coloured filaments coming into 

 contact. By this character G. mucosum may in general be known 

 even without the aid of a microscope. The same circumstance 

 occurs in young plants of G. dissiliens, but is less remarkable, as 

 its mucous sheath is not more than half as broad. 



Under a low power of the microscope G. mucosum has consi- 

 derable resemblance to G. dissiliens^ with which it is probably not 

 unfrequently confounded. But they may always be distinguished 

 even without the aid of the microscope. The G. dissiliens is ex- 

 tremely fragile, and will break into pieces if a small portion be 

 placed on the hand and the finger gently passed over it; this plant, 

 on the contrary, will not break if it be taken out of the water and 

 allowed to hang down in long strings. It always has a clear 

 translucent appearance ; the G. dissiliens, except when very young, 



