154 Mr. J. Ralfs on the British Desmidiese. 



opposite sides of it, have an opening towards it, accompany it in 

 its movements, and always retain the same relative position. 



The fronds of the Staurasti'um in an end view had sometimes 

 three and sometimes four rays ; in conjugating a four-rayed va- 

 riety would often unite with one having three rays, and occa- 

 sionally a frond might be seen having four rays on one segment 

 and three on the other. These facts are another proof that the 

 number of rays on a segment cannot constitute a generic di- 

 stinction in these plants. 



It is probable that, under favom'able circumstances, conjugation 

 takes place in all the Desmidiece. It is not uncommon in various 

 species of Closterium ; I have seen it in this geiuis and in Tetme- 

 morus, and Meneghini mentions its occurrence in Desmidium^. 

 That the sporangia are not more frequently detected may be 

 partly owing to their minuteness, but I beheve that once formed 

 they descend to the bottom of the pool and become mixed with 

 the mud. It is well known that in the Conjugates, when all the 

 sporangia are formed, the plant sinks to the bottom ; and I may 

 remark in support of the opinion I have advanced, that in a small 

 pool at Dolgelley after a shower I could not obtain a single spe- 

 cimen of the Staurastrum in a conjugated state, although the day 

 before the sporangia were abundant. In a few days the Stauras- 

 trum was again plentiful, but I no longer met with any conju- 

 gated specimens, although I frequently sought for them. 



Plate X. fig. 5, S. mucronatiim : a, front view ; h, end view. Fig. 6. 

 a, fronds conjugated ; h, endochrome uniting between the fronds in order to 

 form the sporangium; c, ^and c, different stages of the sporangium ;/, per- 

 fect state. 



6. S. muricatiim, Breb. Fronds muricated ; end view triangular 



with convex sides. Mgh. /. c. p. 226. Binatella muricata, Breb. 



Alg. Fal. p. GQ (1835). Desmidium apiculosum, Ehr. Infus. p. 142 ; 



Pritch. Infus. p. 184. 

 /3. Fronds furnished with numerous short spines. Xanthidhim del- 



toideiim, Corda, Observ. Microscopiques sur les Animalcules de 



Carlsbad, p. 29. pi. 5. figs. 38, 39. 



In small pools. Cheshunt, Mr. Hassall ; Weston Bogs, Hants ; 

 Rackham Common near Fulborough ; PUtdown Common near Uck- 

 field ; Mayfield and Heathfield, Sussex, Mr. Jenner ; Dolgelley and 

 Penzance. 



/3. Sussex, Mr. Jenner ; Dolgelley. 



Fronds deeply constricted in the middle; segments broader 

 than long, transversely elliptic, but frequently both are in close 

 apposition for their entu'e breadth, and by their union form a 



* Speaking of the joints he says, " demum Diatomatum more secedant, 

 hasque siraul e latere copulare in speciebus nonnulUs detexit cl. Br^bisson." 

 —Menegh. I. c. p. 203. 



