In plant cells its presence has been demonstrated in the Thallophytes and Bryophytes.
Cell Division.With the exception of a few plants among the Thallophytes, which consist of a single multinucleate cell, Caulerpa, Vaucheria, &c., the division of the nucleus is followed by the division of the cell either at once, in uninucleate cells, or after a certain number of nuclear divisions, in multinucleate cells.
In the Thallophytes the cytoplasm may be segmented by constriction, due to the in-growth of a new cell wall from the old one, as in Spirogyra and Cladophora, or by the formation of cleavage furrows in which the new cell-wall is secreted, as occurs in the formation of the spores in many Algae and Fungi.
In all plants, except Thallophytes, leaves are present at some period of their existence.
Have been described, but, as has been said, " by far the Algae greater number of the supposed fossil Algae have no claim to be regarded as authentic records of this class of Thallophytes " (Seward, 1898).