A compound leaf is a form of leaf that looks like it is made of several smaller leaves (called 'leaflets') connected to a stem (the rachis, which is really the leaf's primary vein). Compound leaves appear in most types of plants. They evolved independently many times for various reasons, such as optimizing the amount of sunlight collected by the leaf for photosynthesis, or as a reaction to leaf-eating insects.

There are several distinct forms of compound leaves:

Ternately compound leaves come in threes, such as poison ivy (Toxicodendron radicans). (Remember: Leaves of three, let it be.)

                .-.  
               /   |
              /   /
              \__/
              /  .------.
     --------(--(        )
              \__`------'
              /  \
              \   \
               \   |
                `--'

Palmately or fan-compound leaves, where the leaflets are all attatched to the end of the stem. Examples include the Horsechestnuts (Aesculus), the common houseplant Heptapleurum (aka 'Scheflerra'), Lupines (Lupinus), and Palmettos.

           _
          / \    .-.
          \  \  /  /
           `-.\/.-'__
     ---------|(_____)
           ,-'/\`-.
          /  /  \  \
          \_/    \__)

Pinnately-compound leaves, where the leaflets are attatched in two rows on each side of the stem; with a leaflet from each side attaching at a point. In many plants, all of the leaflets are formed inside the leaf bud. Pinnately compound leaves are quite common; examples include the Black Locust (Robinia pseudoacacia) and many other members of the family Fabaceae, Ash (Fraxinus), Walnut (Juglans).

Each pinnately-compound leaf in a given plant species usually has an odd number of leaflets or an even number of leaflets. Even-pinnate (or "paripinnate") leaves have an even number of leaflets:

   
       __    __    __    __    __
      (  )  (  )  (  )  (  )  (  )
_______\/____\/____\/____\/____\/
       /\    /\    /\    /\    /\
      (__)  (__)  (__)  (__)  (__)    

Odd-pinnate leaves have a terminal leaflet on the end of the rachis:

   
       __    __    __    __    __
      (  )  (  )  (  )  (  )  (  ) __
_______\/____\/____\/____\/____\/_/  \
       /\    /\    /\    /\    /\ \__/
      (__)  (__)  (__)  (__)  (__)    

In some odd-pinnate compound leaves, such as those of the Red Baneberry Actaea rubra, the two rows of leaflets do not match up perfectly. These are sometimes called alternately-compound leaves:

   
       __    __    __    __    __
      (  )  (  )  (  )  (  )  (  ) __
_______\/____\/____\/____\/____\/_/  \
     /\    /\    /\    /\    /\   \__/
    (__)  (__)  (__)  (__)  (__)    
 

In Bipinnately-compound leaves, the leaflets are attached to auxilliary rachi which are connected to the main rachis. Examples include Mimosaceae and the Kentucky Coffeetree (Gymnocladus dioicus):

   
           
        O.O      O.O      O.O      O.O      O.O      O.O
       O./`O    O./`O    O./`O    O./`O    O./`O    O./`O
      O./`O    O./`O    O./`O    O./`O    O./`O    O./`O
     O./`O    O./`O    O./`O    O./`O    O./`O    O./`O
      /`O      /`O      /`O      /`O      /`O      /`O
---------------------------------------------------
      \.O      \.O      \.O      \.O      \.O      \.O
     O'\.O    O'\.O    O'\.O    O'\.O    O'\.O    O'\.O
      O'\.O    O'\.O    O'\.O    O'\.O    O'\.O    O'\.O
       O'\.O    O'\.O    O'\.O    O'\.O    O'\.O    O'\.O
        O'O      O'O      O'O      O'O      O'O      O'O

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