of the Dcsmidieae and Diatomeai. 7 



Conjugation is far more rai'cly met with in the Diatomaceae 

 than in the Desniidiesc. It appears that this pi-occss occurs here 

 only at particular epochs, diffcrinj^ accordinj^ to the seasons, 

 ha|)peniii<:: sinuiltaneously in all individuals, and quickly com- 

 pleted. Fre(piently as indications of conjugation having taken 

 place have been met with (the occurrence of individuals of the 

 same species, of remarkable diversity of size, side by side, in 

 free Diatomese, ex. gr. Pinnularia viridis, Surirella bifrons, Stau- 

 rosiyma lacustre, all the year round ; besides the occurrence of 

 shorter or longer rows of cells of about double the diameter in 

 the bands, of the forms remaining connected by the lateral sur- 

 faces, ex. gr. Melusira, Cudosim), — yet it has seldom happened 

 that they have been met with in the moment of eonjugiition. 

 Since the classic researches of Thwaites* upon this subject, the 

 knowledge of it has on the whole been but little advanced by 

 the observations of Focke (conjugation of Surirella-\), GriffithJ 

 (conjugation oi Navicula), W. Smith § and Carter|| (conjugation 

 of Cocconeis, Cymbella, Amphora). The following cases have 

 been observed : 



Formation of a single conjugation- cell, dividing veiy soon 

 after its origin : in Himantidium pectorale^, Cymbella Kiltzin- 

 giana^'^-, Cocconeis Pediculus-ff, Cocconeis PlacentulaXX, Gom- 

 phonema lanceolahtm^^, ^chizonema Grcvillii\\\\, Orthosira ori- 

 chalcea*<\*^, 0. Dickiei'^*^^^, remarkable from the repeated throw- 

 ing-off of the coats of the conjugation-cell, the cracked halves of 

 which clothed the conical ends of the conjugation-cell in shape 

 of funnels; Orthosira rai-ians-ff-f, Surirella bifrons%X+> ^"tl a 

 Navicula^^^, not specifically determined. Here belongs also the 

 only conjugation of a Diatomacean that I have seen, that of 



* Ann. Nat. Hist. xix. p. 200; ser. 2, i. p. 161. Summed up iu Al. 

 Brauu's ' Verjiingung,' p. 305 (Translation, p. 285). 



t Physiologisch. Stmlien, ii. p. 39. pi. 5. figs. 19-22(1854). 



X Ann. Nat. Hist. ser. 2. xvi. p. 90. 



§ British Diatomacea;, ii. p. li^ pi. a-e (1856). 



II Ann. Nat. Hist. ser. 2. xvii. pi. 1 (1856). 



IT Thwaites, Ann Nat. Hist, xix.; Smith, /. c. pi. d. f. 280. i.-iii. (Smith's 

 figures, agreeing exactly with Thwaites's, are evidently not mere copies of 

 them, but new drawings from the same preparations.) 



** Thwaites, Ann. Nat. Uist. ser. 2. i. pi. 2. figs. 1-5. Smith, ])1. a. 

 fig. 47. i. ii. iii. 



tt Carter, /. c. pi. 1. fig. 2 j. XX Smith, pi. b. fig. 32. 



§§ Smith, pi. D. fig. 214. i. jjlj Smith, pi. e. fig. 364. 



1:1T Thwaites, Ann. Nat. Hist. ser. 2. i. pi. 11. fig. b. (as O. crenulata) ; 

 Smith, pi. E. fig. 337. 



*** Thwaites, /. c. pi. 12. figs. 1-7- 



t+t Thwaites, /. c. pi. 11. fig. A. 



XXX Focke, Physiol. Studien, ii. pi. 5. figs. 19-22; i)l. 6. fig. 42. (Bre- 

 men, IS54.) 



§H J- "^^'- Griffith, .\nn. Nat. Hist. ser. 2. xvi. p. 90. 



