395 



ALGM SILICAT/E. 



Fam. XX. DIATOMACEJE. 



Char. Algge silicious. 



The DiatomacecB form a group of natural productions not 

 less distinct and remarkable than the previous one of Dcsmi- 

 decB, Avith which it may be contrasted. 



The Dcsmidece. are Alga of figured and peculiar forms, 

 binary composition, vivid green colour, destitute of silex in 

 their constitution, and therefore subject to considerable change 

 of form in di'ying. 



The DiatomacecB are productions possessing also, to a cer- 

 tain extent, figured forms, but are not binary, of excessive 

 fragility, of a dark brown colour, Avhich changes somewhat to 

 green on drying, of a metallic lustre and silicious composi- 

 tion, and therefore unalterable in diying, and indestructible 

 by fire. 



The silex which enters so abundantly into the composition 

 of these plants is disposed throughout the interior of the 

 frustules in plates, which are often arranged in series, which 

 when the frustules are examined microscopically, are seen to 

 impart certain lines or mai'kings to them, which contribute 

 much to their beauty, and which are of importance in the 

 discrimination of genera and species. 



It is thus seen how very little connection tliis group and 

 family of productions has with the undoubted xUga which 

 have hitherto been described ; indeed, the claims which they 

 possess to be regarded as subjects of the vegetable kingdom 

 arc by no means so conclusive as those which have been 

 advanced in favour of the vegctability of the Dcsmidca. 



