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KINGDOM VIRIDIPLANTAE

In recent years the kingdom has been called Viridiplantae (Green Plants) perhaps as a means to distance itself from the old concept of plant (a multicellular photosynthetic organism).  In this sense, the term "plant" is an ecological term like "alga".  Thus, the change to Viridiplantae serves to alleviate the confusion.  Also, organisms in this group are generally accepted as being monophyletic and not related to other plant-like multicelluar organisms such as the kelps and red algae.

As with the other taxa of life, the past 25 years has seen quite an upheaval in our systematic understanding of these groups.  Prior to Copeland (1956) and Whittaker (1969), life was divided into animals and plants.  Since we had a pretty good idea of what animals were, the plant kingdom was defined by exclusion.  That is, plants were those organisms that were not animals.  With the advent of the five kingdom system, plants were defined in essentialist terms (see Margulis and Schwartz, 1982, 1988, 1998).  In the five kingdom system, plants were photosynthetic multicellular organisms that developed from an embryo.  That definition excluded the groups of green algae from the plant kingdom.  At the same time, a body of evidence from ultrastructural work (summarized by Pickett-Heaps, 1975; Mishler and Churchill, 1985; Mattox and Stewart, 1984; and Graham et al., 1991) showed that there clearly were at least two distinct groups of green plants.  One group that includes most of the green algae and praesinophytes (I call this subkingdom Chlorobionta).  One group that included the charophytes, the coleochaetalian greens, and Klebshormidium had characters like the occurrence of a phragmoplast and asymmetry of flagella in the sperm had clear affinities with the embryophytes or land plants.  The ultrastructural evidence was confirmed by the molecular phylogenetic work (McCourt, 1995; Melkonian and Surek, 1995; and Marin and Melkonian, 1999).  The embryophytes (typical land plant that include the bryophytes, the vascular cryptogams, the gymnosperms, and the flowering plants) and the phragmoplastic algae were combined into a taxon (that I call a subkingdom) called Streptobionta.

Molecular phylogeny (e.g. Soltis et al., 1999; and Duff and Nicrent, 1999) also confirmed most of the long-standing relationships between the higher taxa of embryophytes (e.g. the systems of Bold et al., 1987 and Margulis and Schwartz, 1982, 1988, 1998).  Some surprises in the recent molecular phylogenetic work on the embryophytes included:

• the monophyly of the fern-sphenophyte clade and its status as a sister group to the seed plants.  
• the fern-like status of the psilophytes.
• the apparent relationship between the ferns and flowering plants.
• the phylogenetic relationships between the flowering plant taxa.

 This system reflects all of these changes in the taxonomy of the Viridiplantae with two subkingdoms: Chlorobionta and Streptobionta.  See the cladogram adapted from Tudge (2000) and from the Tree of Life Project for the consensus view of the molecular/ultrastructural relationships between the higher taxa of the green plants.  For the sake of convenience, I have organized the green plants into five artificial groupings that are roughly based on classical taxonomic systems like Bold et al. (1987).

Click on the Night-Blooming Cereus to view images of the plant kingdom.

References for the Kingdom Viridiplantae.

This page was created and maintained by Jack R. Holt.  It was last revised: 03/20/2004