root-located carbohydrates have been found in these plants. Photosynthesis pathway Although most of the family use the more common photosynthesis pathway, around 800 species are plants; this makes the Amaranthaceae the largest group with this photosynthesis pathway among the eudicots (which collectively includes about 1,600 species). Within the family, several types of photosynthesis occur, and about 17 different types of leaf anatomy are realized. Therefore, this photosynthesis pathway seems to have developed about 15 times independently during the evolution of the family. About two-thirds of the species belong to the former Chenopodiaceae. The first occurrence of photosynthesis dates from the early Miocene, about 24 million years ago, but in some groups, this pathway evolved much later, about 6 (or less) million years ago. The multiple origin of photosynthesis in the Amaranthaceae is regarded as an evolutionary response to inexorably decreasing atmospheric levels, coupled with a more recent permanent shortage in water supply as well as high temperatures. Species with higher water-use efficiency had a selective advantage and were able to spread out into arid habitats. Distribution Amaranthaceae is a widespread and cosmopolitan family from the tropics to cool temperate regions. The Amaranthaceae (sensu stricto) are predominantly tropical, whereas the former Chenopodiaceae have their centers of diversity in dry temperate and warm temperate areas. Many of the species are halophytes, tolerating salty soils, or grow in dry steppes or semi-deserts. Economic importance Some species, such as spinach (Spinacia oleracea) or forms of beet (Beta vulgaris) (beetroot, chard), are used as vegetables. Forms of Beta vulgaris include fodder beet (Mangelwurzel) and sugar beet. The seeds of Amaranthus, lamb's quarters (Chenopodium berlandieri), quinoa (Chenopodium quinoa) and kañiwa (Chenopodium pallidicaule) are edible and are used as pseudocereals. Dysphania ambrosioides (epazote) and Dysphania anthelmintica are used as medicinal herbs. Several amaranth species are also used indirectly as a source of soda ash, such as members of the genus Salicornia (see glasswort). A number of species are popular garden ornamental plants, especially species from the genera Alternanthera, Amaranthus, Celosia, and Iresine. Other species are considered weeds, e.g., redroot pigweed (Amaranthus retroflexus) and alligatorweed (Alternanthera philoxeroides), and several are problematic invasive species, particularly in North America, including Kali tragus and Bassia scoparia. Many species are known to cause pollen allergies. Systematics In the APG IV system of 2016, as in the previous Angiosperm Phylogeny Group classifications, the family is placed in the order Caryophyllales and includes the