What does exogen mean?
An exogen is a plant whose stem grows thicker through the addition of new woody tissue on the outside of the existing stem, a process known as secondary growth. This group corresponds largely to what botanists now call dicotyledons — the oaks, maples, and other familiar trees and shrubs whose annual rings record each year's growth. The term comes from Greek roots meaning "outside" and "to produce," reflecting the outward manner in which these plants build their wood. In older botanical classification, exogens were set against endogens, plants such as grasses and palms that lack this ringed, outward-growing structure. Although "exogen" survives mainly in historical texts and older reference works, it remains a vivid reminder of how nineteenth-century science organized the plant kingdom, and it still appears in discussions of tree anatomy where the distinction between ringed and non-ringed growth matters.
nounA plant that increases in thickness by adding new layers of tissue outward from within, as in most trees and shrubs; in older botanical classification, one of the exogenous plants (typically dicotyledons), contrasted with endogens.
- A plant whose stem thickens by the addition of new woody tissue formed externally to the older wood; a member of the exogenous group of plants (chiefly dicotyledons) in classical botanical taxonomy.
"The oak, like all exogens, adds a fresh ring of wood each year."
"Botanists of the nineteenth century divided flowering plants into exogens and endogens according to how their stems increased in girth."
"Each spring the exogen laid down a new layer of sapwood just beneath its bark."
Standard English plural; used when referring collectively to plants of this type.
"Most forest hardwoods are exogens, which is why their trunks show distinct annual rings."
Every ring you count on a tree stump is proof you're looking at an exogen — nature's own diary, written from the outside in.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of exogen
The word "exogen" derives from the Greek prefix "exo-," meaning "outer" or "outside," combined with the element "-gen," from Greek "genēs" or "gignesthai," meaning "produced" or "born." It entered English usage in the early nineteenth century alongside the development of systematic botany, when plants were grouped by their patterns of stem growth. Its close relative "exogenous" shares the same Greek root, as do words like "exogamy" and "exotic."
Related word forms
How exogen is actually used
"Exogen" belongs chiefly to historical or technical botanical vocabulary; modern botany has largely replaced it with more precise terms such as "dicotyledon" or descriptions of secondary growth via the vascular cambium. It is rarely encountered outside textbooks and older botanical writing.
Easily confused with exogen
An endogen grows by adding tissue from within and lacks the ringed structure of an exogen; the two are opposite categories in traditional botany.