Since colonizing land at least 475 million years ago, plants have diversified into roughly 290,000 living species.
Plants supply oxygen and are the ultimate source of most food eaten by land animals.
Green algae called charophytes are the closest relatives of land plants.
Note that land plants are not descended from modern charophytes, but share a common ancestor with modern charophytes.
Land plants share key traits only with green algae charophytes:
DNA comparisons of both nuclear and chloroplast genes.
Rose-shaped complexes for cellulose synthesis.
Peroxisome enzymes - minimize loss from photorespiration.
Structure of flagellated sperm.
Formation of a phragmoplast - allignment of cytoskeletal elements and Golgi vesicles for cell plate.
Rosette cellulose-synthesizing complexes Found only in land plants and charophycean green algae
In green algae charophytes a layer of a durable polymer called sporopollenin prevents dehydration of exposed zygotes.
The movement onto land by charophyte ancestors provided advantages: unfiltered sun, more plentiful CO2, nutrient-rich soil, and few herbivores or pathogens.
Land presented challenges: a scarcity of water and lack of structural support.
Three Clades are candidates for Plant Kingdom
A cuticle and secondary compounds evolved in many plant species. Symbiotic associations between fungi and the first land plants may have helped plants without true roots to obtain nutrients.
Four key derived traits of plants are absent in the green algae charophytes:
Alternation of generations - with multicellular, dependent embryos.
Walled spores produced in sporangia
Alternation of Generations and Multicellular Dependent Embryos
The multicellular gametophyte is haploid and produces haploid gametes by mitosis.
Fusion of the gametes gives rise to the diploid sporophyte, which produces haploid spores by meiosis.