SMITH, ON BRITISH DIATOMACE^E. 161 



the result of their conjugation. This mode is seen in 

 Epithemia, Cocconema, Gomphonema, Encyonema, and Col- 

 letonema. 



" II. From the conjugation of two parent frustules we have 

 a single sporangium. This occurs in Hhnantidium. 



"III. The valves of a single frustule separate ; the contents^ 

 set free^ rapidly increase in bulk^ and finally become con- 

 densed into a single sporangium. This may be seen in Coc- 

 coneis, CijcIoteUa, Melosira, Orthosira, and Schizonema. 



In Melosira nummvIouJes, M. Borrerii, and M. subflexilis, 

 the second valve of the conjugating frustule is rarely found 

 united to the mucus surrounding the sporangium^ the con- 

 jugation taking place only in the last frustule of the fila- 

 ment ; but in Melosira varians and Orthosira orichalcea, 

 conjugation taking place throughout the entire filament^ both 

 valves are usually found adherent to the sporangium or its 

 surrounding mucus. 



" IV. From a single frustule, as in the last method, two 

 sporangia are produced in the process of conjugation. This 

 takes place in Achnanthes and Rhabdonema. 



" On the whole, the facts at present within our knowledge 

 seem fully to warrant the conclusions that the conjugated 

 state of the Diatomaceae is the first step in the reproductive 

 process of these organisms, and that the sporangial products 

 of this condition become the parents of numerous young 

 frustules, destined to renew the cycle of phenomena which 

 accompanies the life and growth of the species from which 

 the sporangia have themselves originated." 



In the next section Professor Smith enters upon the 

 question of the natiu-e of the Diatomaceae, and successfully, 

 we think, vindicates their claims to be regarded as members 

 of the vegetable kingdom. Although he does not refer to 

 any experiments, he states that the Diatomaceae give out 

 oxygen from their tissues, and take up carbonic acid gas. 

 If this has been proved by experiment, we should regard it 

 as conclusive of their vegetable nature, as the performance of 

 these frmctions seem to be the most universal condition of 

 the existence of plants of which we have any knowledge. 



In the concluding sections Mr. Smith treats of the deter- 

 mination of species, and of the distriljution and uses of the 

 Diatomaceae. We can only repeat that this A^olume fully 

 maintains the reputation of the first, and express our convic- 

 tion that the whole work is one of the most important contri- 

 butions ever made to microscopic natural history in this 

 country. 



