Charophytes + Zygnematophytes
Sister groups to land plants
The page includes the clade of green algae that are a sister group to the land plants (embryophytes). The clade that include these green algae plus land plants has been called the Phragmoplastophyta, because they create a phragmoplast during cell division: a scaffold-like structure that forms during late cytokinesis and aids in cell plate construction and formation of a new cell wall. This algae in this clade include the Charophyceae, Coleochaetophyceae, and Zygnematophyceae (Lecointre & Guyader 2006)
Ecology & Form
These are the most closely-related algae to land plants
They are mostly freshwater or terrestrial algae
They consist of unicellular, colonial, filamentous or parenchymatous forms
Similar to land plants, they form a phragmoplast: a scaffold-like structure that forms during late cytokinesis and aids in cell plate construction and formation of a new cell wall
They exhibit branched apical growth like ancestral land plants
Life cycle
Different than land plants, these algae exhibit a haplontic life cycle (=no multicellular sporophyte generation)
The algae is haploid, and forms gametes which are created and fuse through fertilization
The resultant zygote does not go through mitosis, but instead goes through meiosis to create spores
These spores are released into water to germinate into new gametophytes
Above: Close-up of spiraling chloroplast of Spirogyra
Above: Conjugation (reproduction) of Spirogyra
Diversity
Zygnematophyceae
This group has several thousand different species in two families
Unbranched, filamentous; cell division with a phragmoplast
Sometimes called the Conjugatales, since they perform "conjugation" during reproduction
Spirogyra, called the water-silk, and Zygnema are common as freshwater pond "scum"
Evidence indicates that these are the most-closely-related algae to the land plants (Cheng et al. 2019)
Above: Zygnema
Above: Spirogyra, the water-silk
The below algae were originally thought to be the closest relatives to land plants, due to their parenchymatous and apical growth
Coleochaetales (Coleochaetales)
Parenchymatous algae, like land plants
Coleochaete is a flat, disk-like algae that lives as an epiphyte on basal portions of marsh plants
Coleochaete has sterile cells that surround the zygotes after fertilization, but unlike land plants, it has a haplontic lifecycle with meiosis taking place directly in the zygote (and not in diploid cells resulting from mitosis of the zygote)
Charophyceae (Charales)
Parenchymatous, with apical branching
Chara, also called a stonewort, is found in fresh water, particularly in limestone areas
Above: Coleochaete
Above: the stonewort, Chara
Additional Resources
The Drought Alert System of Terrestrial Plants has an Underwater Origin (In Defense of Plants, 12Mar2019)
How Did Plants Conquer Land? These Humble Algae Hold Clues (New York Times, Nov. 2019)