Mr. G. H. K. Thwaites on Conjugation in the Diatomacese. 343 



XXXIV. — On Conjugation in the Diatomacese. By G. H. K. 

 Thwaites, Lecturer on Botany and Vegetable Physiology at 

 the Bristol Medical School. 



[With a Plate.] 



To the Editors of the Annals of Natural History. 



GENTLEMEN, 2 Kingsdown Parade, Bristol, Oct. 6, 1847. 



I am now enabled to make some interesting additions to my 

 previous papers on Conjugation in the Diatomacea, the result of 

 a very attentive examination of the species therein referred to, as 

 well as of others which it has since been my good fortune to 

 detect with conjugated frustules. 



In my last communication I stated that the sporangia of the 

 four species there named bore a close resemblance to the frustules 

 of Cocconema, and that the sporangia of Cocconema lanceolatum 

 differed apparently in no respect but in size from the frustules of 

 that species. At that time I suspected, but had not evidence 

 sufficient to prove it with certainty, that in the other three spe- 

 cies adverted to the sporangia became eventually similar in form 

 to their parent frustules, and that the Cocconema-like appearance 

 was merely that of their immature condition : such I can now 

 state positively to be the fact as respects the seven species which 

 have occurred to me with mature sporangia, as I have succeeded 

 in tracing the change to the form of the frustule so satisfactorily 

 as to leave not the shadow of a doubt in my own mind as to the 

 fact ; and not only can I venture to state thus much, but also can 

 with equal certainty affirm that the sporangia, like the frustules, 

 undergo fissiparous division ; and that in the two species of Gom- 

 phonema mentioned in this paper, they become stalked like the 

 latter, so as to make their mutual resemblance still more striking. 

 This circumstance lessens the probability of frustules and their 

 sporangia being described as distinct species, especially as it is 

 also the fact that, although sporangia far exceed in size their 

 identical parent frustules, they may not be at all or but very 

 slightly larger than ordinary frustules of the same species do 

 sometimes become. It seems to me likely, however, that Kiitz- 

 ing's iC Epit hernia Vertagus" is the sporangium of Eunotia tur- 

 gida } Ehr. 



It will be seen by the accompanying Plate, that in most of the 

 species there figured the conjugated frustules become divided into 

 two separate halves to allow of the escape of their contained endo- 

 chrome ; in Gomphonema minutissimum and Fragilaria pectinalis, 

 however, the endochrome escapes by a slit at one end of the frus- 

 tule. Fragilaria pectinalis differs too, it will be observed, from 



