Any unicellular plant, or plant forming only a plasmodium (mass of protoplasm), having reproduction only by fission (cell division), gemmation (process of budding), or cell division.

The protophytes (Protophyta) are by some botanists considered an independent branch or class of the vegetable kingdom, and made to include the lowest forms of both fungi and algae; such as, slime molds, Bacteria, the nostocs, etc.