DIVERSITY OF LIFE

DESCRIPTION OF THE KINGDOM VIRIDIPLANTAE (CAVALIER-SMITH 1981)

EUKARYA>ARCHAEPLASTIDA>VIRIDIPLANTAE

Viridiplantae (ve-re-de-PLAN-te) is derived from two Latin roots that mean "green" (virida) and "shoot" (planta).  The reference is to these as the green plants. 

INTRODUCTION TO THE 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.  

FIGURE 1. MAJOR CLADES OF THE GREEN PLANTS.  This system reflects all of these changes in the taxonomy of the Viridiplantae with two subkingdoms: Chlorobionta and Streptobionta.  See 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. CH = Chlorobiont Clade ST = Streptobiont Clade EM = Embryophytes VP = Vascular Plants SP = Seed Plants

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 the Viridiplantae is divided unequally between the chlorobionts and streptobionts (see Figure 1).  We consider these groups, formalized to Chlorobionta and Streptobionta, to be subkingdoms.  They differ in fundamental ways which are summarized in Table 1.

THE CHLOROBIONTS (CLADE CH)

The Chlorobionta includes most of the green algae and all of the praesinophytes.  These range in form from small unicells to filaments, colonies, and complex thalloid structures.  Thus, from a cellular, physiological, and ultrastructural standpoint, they are highly variable and defining synapomorphies are equivocal.  Taxa in this group are in need of systematic revision, especially among the prasinophytes, which are not monophyletic (Proschold and Leliaert 2007).

THE BASAL STREPTOBIONTS (CLADE ST)

The base of the Streptobiont clade includes the phragmoplastic algae, which include 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).  

THE EMBRYOPHYTES (CLADE EM)

The embryophytes are the typical plants that have heteromorphic alternation of generation.  In general, these are plants that are adapted to a terrestrial existence and produce gametangia that have sterile tissue.  Margulis and Schwartz (1982, 1988) defined the Plant Kingdom in essentialist terms such that all plants develop from an embryo.  Thus, in their early systems, the Plant Kingdom (now Viridiplantae) included only the embryophytes.  Even they were forced to accept the 'Green Algae" as part of a larger Plant Kingdom (Margulis and Schwartz 1998) in the face of a growing body of evidence (see above).

The Bryophytes include the Hornworts, Liverworts, and Mosses, which are generally adapted to a terrestrial existence in most ways except the occurrence of vascular tissue.  Thus, though they may be somewhat complex in overall structure, they tend to remain small.  Also, the gametophyte is the life dominant stage and tends to be the photosynthetic form.  The sporophyte of the Bryophyte often is determinant in its growth and usually is parasitic on the gametophyte.

THE VASCULAR PLANTS (CLADE VP)

The Vascular Plants are all Embryophytes in which the sporophyte is the dominant phase of the life history.  As the name implies, the sporophytes of these plants are made up of axes that contain steles of vascular tissue (xylem and phloem).  Members of this group can grow to be true giants and live for hundreds of years.

The vascular plants in this group have a gametophyte that grows independently from the sporophyte.  The most common living Vascular Cryptogams (cryptogam means hidden gametophyte) are ferns and lycopods.  However, the diversity of this group is much greater when considering extinct taxa, which first appeared in the fossil record during the Silurian period, but exploded in abundance during the Devonian and Carboniferous periods.  These organisms showed all types of vascular steles together with microphylls and megaphylls.

THE SEED PLANTS (CLADE SP)

Plants in this clade all are heterosporous and confine the megagametophyte within the megasporangium surrounded by integuments produced by the parental plant, a complex structure called the seed.  Conversely, the microgametophytes are tiny and shed as windborne (or animal borne) pollen grains.  All seed plants have eustelic stems (except the monocot atactostele, a derivative of the eustele) and megaphylls.  The Gymnosperms include five living phyla and one extinct phylum in which the seeds are exposed (gymnosperm means naked seed) to receive the pollen directly.

Flowering Plants, the angiosperms, are the most successful members of the plants kingdom with more than 250,000 described species.  As the name implies, plants of this clade produce flowers, strobili with sterile and fertile appendages, often including both staminate and ovulate appendages.  The ovules are completely enclosed by tissue (the ovulary); so, the pollen must germinate on specialized tissue (called stigmatic) and grow a pollen tube to the ovule before fertilization can occur.  The ovulary and often other parts of the flower develop into fruit, structures that develop from a flower and contain seeds.

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.

PHYLA OF THE VIRIDIPLANTAE

INTRODUCTION TO THE DOMAIN EUKARYA

LITERATURE CITED

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By Jack R. Holt.  Last revised: 03/26/2013