Amaranthaceae

The Amaranthaceae family is a massive group of flowering plants. It is the most species-rich lineage within its parent order, Caryophyllales. This family includes about 165 different genera and 2,040 species. Many people know these plants through the type genus, Amaranthus. The family is also unique because it absorbed the former goosefoot family, known as Chenopodiaceae.

Most species in this family are annual or perennial herbs. Some are subshrubs, while others grow as larger shrubs. Very few species in the family function as vines or trees. Some plants are even succulents, meaning they have fleshy parts to store water. Many species feature stems with thickened nodes. A curious trait in many species is anomalous secondary growth in the perennial stems. Only the subfamily Polycnemoideae exhibits normal secondary growth. The leaves are mostly simple and alternate, though some are opposite. They never possess stipules. Leaf shapes vary greatly from flat to terete, which means cylindrical. Some species even have leaves reduced to tiny scales.

The flowers of Amaranthaceae are highly diverse. They can appear as solitary flowers or in groups called cymes, spikes, or panicles. Most flowers are perfect, meaning they are bisexual and contain both male and female parts. However, some species have unisexual flowers. The flowers typically have a perianth made of one to five tepals, which are often joined. In rare cases, they may have up to eight tepals. The stamens are usually one to five in number. These stamens are either opposite the tepals or alternate. The anthers, which hold the pollen, contain two or four pollen sacs called locules. In the tribe Carboxyloneae, the anthers have special vesicular appendages. The pollen grains are spherical and pantoporate, meaning they have many pores. Some species, like Froelichia, can have up to 250 pores per grain.

Survival in harsh environments is a key feature of this family. About 800 species utilize C4 photosynthesis. This is a specialized metabolic pathway that allows plants to make food more efficiently. In the Amaranthaceae, this pathway is the largest group among the eudicots. Scientists believe this pathway evolved about 15 times independently within the family. The first instances appeared during the early Miocene, roughly 24 million years ago. Other groups evolved it much later, perhaps only 6 million years ago. This evolution was likely a response to decreasing atmospheric levels and increasing temperatures. It also helped plants survive permanent shortages in water supply. By using water more efficiently, these plants gained a selective advantage in arid habitats.

The history of naming this family involves several important scientists. Antoine Laurent de Jussieu first published the name Amaranthaceae in 1789. He included it in his work, Genera Plantarum. Later, Étienne Pierre Ventenat published the name Chenopodiaceae in 1799. For a long time, these were treated as separate families. However, modern phylogenetic research has changed how we classify them. The APG IV system of 2016 now places them together in one large family. This broad definition is known as Amaranthaceae sensu lato. Research shows that the older name, Amaranthaceae, has priority and remains the valid scientific name.

Humans interact with Amaranthaceae in many practical ways. We consume many species as vital food sources. Spinach and various forms of beet, such as beetroot and chard, are common vegetables. We also eat seeds from Amaranthus, quinoa, and kañiwa. These are often called pseudocereals. Some plants, like Dysphania ambrosioides, are used as medicinal herbs. Other species, such as those in the genus Salicornia, are used to produce soda ash. In gardens, plants like Celosia and Iresine are popular ornamentals. However, some species can be problematic. Redroot pigweed and alligatorweed are often considered weeds. Some members of the family can also trigger pollen allergies.

Taxonomy, or the science of classification, is still evolving for this group. Molecular genetic studies have shown that old methods based on physical shapes were sometimes incorrect. Current research is dividing the former Chenopodiaceae into eight distinct subfamilies. These include Polycnemoideae, Betoideae, Camphorosmoideae, Chenopodioideae, Corispermoideae, Salicornioideae, Salsoloideae, and Suaedoideae. When looking at the entire broad family, there are approximately 10 subfamilies. These subfamilies contain about 180 genera and roughly 2,500 species. This complex system helps scientists understand the deep evolutionary history of these diverse plants.