Every tree and flower on land grew out of a family of pond algae
Charophytes are humble freshwater green algae, yet land plants evolved from deep within their family tree. That makes oak trees and orchids, in a strict evolutionary sense, highly modified charophytes. Some of these algae still swim with flagellate sperm, a trait shared by every land plant except conifers and flowering plants.
Biologists call Charophyta paraphyletic: it includes the ancestor of land plants but, in its traditional sense, leaves the land plants out. Once the Embryophyta are placed inside it, the group becomes equivalent to Streptophyta. Its sister group is the Chlorophyta, the other great branch of green algae. The closest living relatives of land plants appear to be the Zygnematophyceae, and land plants may have come from single-celled charophytes that had already moved onto land. The name comes from the genus Chara.
All green plants, algae and land plants together, share chlorophylls a and b, cellulose cell walls, starch stored in plastids and no phycobilin pigments. Charophytes and land plants go further, sharing certain enzymes that chlorophytes lack. Their life cycle hints at the transition. Charophyte cells are haploid except for a brief single-celled diploid zygote that divides by meiosis. Land plants instead grow the zygote into a many-celled sporophyte. Among algae, only some coleochaetes hold on to the zygote, wrap it in sterile cells, coat it with sporopollenin and in one species feed it through placental transfer cells, much as land plants do.
The group is varied. Zygnematophyceae, including desmids, reproduce sexually by conjugation, two cells merging their entire contents, and flourish in acidic, nutrient-poor lakes such as Wastwater. Klebsormidium is a simple filament that breaks apart to multiply. Stoneworts, the Charales, have slender stems with whorled branchlets, often turned grey by lime deposits that hide their green. A newly found soil genus from the Czech Republic, Streptofilum, lacks a cell wall entirely and is armoured instead by layered scales.
Fossils show the lineage is old. Devonian stoneworts such as Palaeonitella cranii looked little different from some modern ones. Molecular studies are building a consensus about relationships, placing the Mesostigmatophyceae at the base of the lineage, with a recent proposal from nuclear genomes suggesting a third phylum of green plants.
Source: Charophyta
