ALG7E SILICAT/E.

Fam. XX. DIATOMACEiE.

Char. Algas silicious.

The Diatomacece form a group of natural productions notless distinct and remarkable than the previous one of Desmi-dece, with which it may be contrasted.

The Desmidece are Alga of figured and peculiar forms,binary composition, vivid green colour, destitute of silex intheir constitution, and therefore subject to considerable changeof form in drying.

The Diatomacea are productions possessing also, to a cer-tain extent, figured forms, but are not binary, of excessivefragility, of a dark brown colour, which changes somewhat togreen on drying, of a metallic lustre and silicious composi-tion, and therefore unalterable in drying, and indestructibleby fire.

The silex which enters so abundantly into the compositionof these plants is disposed throughout the interior of thefrustules in plates, which are often arranged in series, whichwhen the frustules are examined microscopically, are seen toimpart certain lines or markings to them, which contributemuch to their beauty, and which are of importance in thediscrimination of genera and species.

It is thus seen how very little connection this group andfamily of productions has with the undoubted Alga whichhave hitherto been described; indeed, the claims which theypossess to be regarded as subjects of the vegetable kingdomare by no means so conclusive as those which have beenadvanced in favour of the vegetability of the Desmidea.