Pronunciation key

( eksə-jen )



[exo- + -gen].

  1. Botany. A seed plant that matures through addition of successive concentric layers or rings on its outside beneath its bark and outside previous growth; a dicotyledon.
  2. Any plant of the obsolete class Exogenae including the dicotyledons.

A plant whose stem increases in thickness by successive additions on the outside of what surrounds the central pith, in some cases annually. Its structure is best seen in the bodies of trees having a central pith surrounded by as many concentric layers as the tree ages in number of years. Rays called medullary rays radiate from the central pith outward to the bark. Exogens have two seed leaves, or cotyledons and traditionally have been called dicotyledons. Their leaves, with a few exceptions, are reticulated while the flower usually has four or five parts. In all these characteristics exogens differ from endogens. The greater number of the trees in tropical climates and all of those in cold regions are exogens.


  1. Netted-veined leaf (Oak).
  2. Exogenous flower (Crowfoot).
  3. Section of a branch of three years growth. (From inside, outward, a. Medulla or pith. b. Medullary sheath. c. Medullary rays d. Circles of annual growth. e. Bark.
  4. Dicotyledonous Seed.



Parts of an Exogen
Section of a branch of three years' growth:

  • A) Medulla or pith
  • B) Medullary sheath
  • C) Circles of annual growth
  • D) Bark
  • E) Medullary Ray
  • Compare with endogen.


  • The American College Dictionary (Random House) ©1949
  • Webster's Improved Dictionary and Everyday Encyclopedia ©1956
  • Webster's New World Dictionary of the American Language (College Edition) ©1955
  • The New World Family Encyclopedia ©1955
  • Funk and Wagnalls Standard Dictionary, Comprehensive International Edition ©1976
  • Further Reading

  • Dicotyledon
  • Exogenous
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