SYSTEMATIC BIOLOGY				RETURN TO TAXA OF LIFE PAGE
HOME	SYLLABUS	WEEKLY ASSIGNMENTS	J. SYSTEMATIC BIOLOGY
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 multicellular organisms such as the kelps.

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), Whittaker (1957, 1959, 1969, 1977), and Whittaker and Margulis (1978), 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).  That is, plants were those organisms that fit the following description: photosynthetic multicellular organisms that developed from an embryo, a definition that 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: the chlorobionts and the streptobionts.  

The Viridiplantae is divided unequally between the chlorobionts and streptobionts.  I consider these groups, formalized to Chlorobionta and Streptobionta, to be subkingdoms.  The Chlorobionta includes most of the green algae and all of the praesinophytes.  Thus, from a cellular, physiological, and ultrastructural standpoint, they are highly variable and defining synapomorphies are equivocal.  

The embryophytes (typical land plants that include the bryophytes, the vascular cryptogams, the gymnosperms, and the flowering plants) and the phragmoplastic algae are combined into the subkingdom Streptobionta.  The phragmoplastic algae includes the charophytes, the coleochaetalian greens, and Klebshormidium, "green algae" which have important characters in common with the embryophytes.  Some of the synapomorphies include the occurrence of a phragmoplast and asymmetry of flagella in the sperm.  The relationship that the ultrastructural evidence suggests has been confirmed by molecular phylogenetic work (McCourt 1995; Melkonian and Surek 1995; and Marin and Melkonian 1999).  

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 (summarized by Palmer et al. 2004) 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), the Tree of Life Project, and Palmer et al. (2004) for the consensus view of the molecular/ultrastructural relationships between the higher taxa of the green plants.  

Glossary for the Kingdom Viridiplantae

REFERENCES.

This page was created and maintained by Jack R. Holt and Carlos A. Iudica.  It was last revised: 04/22/2008