Evolution and Classification

The organisms we call plants are assigned to a single clade; that is, a natural grouping based on the belief that they have all evolved from a common ancestor more recent than any shared with other organisms.

Among the criteria for doing this are:

* their shared use of the photosynthetic pigments chlorophyll a and chlorophyll b
* the similarities in the nucleotide sequences of both their small subunit (18S) and large subunit (28S) ribosomal RNA (rRNA) genes
* their shared cellulose cell wall.

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