Diatoms are part of the class Bacillariophyceae of the Chrysophyta phylum of microscopic unicellular plants.

This remnant, in the spore-forming unicellular plants represented by the cell wall, is then naturally dead.

The members of this order are minute unicellular plants, growing either in water or on the damp surfaces of stones, tree trunks, etc.

In these cases, certain cells of a colony of unicellular plants or of the filaments of multicellular plants enlarge greatly and thicken their wall.

In those protists ( unicellular plants and animals) which have the first rudiments of sexual reproduction the two copulating cells are at first quite equal.