page_title: Leaf
section_title: Hickey system
hierarchical_section_title: Leaf / Terminology / Venation / Classification / Hickey system
caption_attribution_description: Adiantum cunninghamii - underside of leave showing reflexed marginal lobes containing sori
context_page_description: A leaf is the principal lateral appendage of the vascular plant stem, usually borne above ground and specialized for photosynthesis. The leaves and stem together form the shoot. Leaves are collectively referred to as foliage, as in "autumn foliage". In most leaves, the primary photosynthetic tissue, the palisade mesophyll, is located on the upper side of the blade or lamina of the leaf but in some species, including the mature foliage of Eucalyptus, palisade mesophyll is present on both sides and the leaves are said to be isobilateral. Most leaves are flattened and have distinct upper and lower surfaces that differ in color, hairiness, the number of stomata, the amount and structure of epicuticular wax and other features. Leaves are mostly green in color due to the presence of a compound called chlorophyll that is essential for photosynthesis as it absorbs light energy from the sun. A leaf with white patches or edges is called a variegated leaf.
  Leaves can have many different shapes, sizes, and textures. The broad, flat leaves with complex venation of flowering plants are known as megaphylls and the species that bear them, the majority, as broad-leaved or megaphyllous plants.
context_section_description: 1. Pinnate (feather-veined, reticulate, pinnate-netted, penniribbed, penninerved, or penniveined)
  The veins arise pinnately (feather like) from a single primary vein (mid-vein) and subdivide into secondary veinlets, known as higher order veins. These, in turn, form a complicated network. This type of venation is typical for (but by no means limited to) "dicotyledons" (non monocotyledon angiosperms). E.g., Ostrya. There are three subtypes of pinnate venation:
   Craspedodromous (Greek: kraspedon - edge, dromos - running)
   The major veins reach to the margin of the leaf.
   Camptodromous
   Major veins extend close to the margin, but bend before they intersect with the margin.
   Hyphodromous
   All secondary veins are absent, rudimentary or concealed
   These in turn have a number of further subtypes such as eucamptodromous, where secondary veins curve near the margin without joining adjacent secondary veins.
  2. Parallelodromous (parallel-veined, parallel-ribbed, parallel-nerved, penniparallel, striate)
  Two or more primary veins originating beside each other at the leaf base, and running parallel to each other to the apex and then converging there.  Commissural veins (small veins) connect the major parallel veins. Typical for most monocotyledons, such as grasses. The additional terms marginal (primary veins reach the margin), and reticulate (primary veins do not reach the margin) are also used.
  3. Campylodromous (campylos - curve)
  Several primary veins or branches originating at or close to a single point and running in recurved arches, then converging at apex. E.g. Maianthemum .
  4. Acrodromous
  Two or more primary or well developed secondary veins in convergent arches towards apex, without basal recurvature as in Campylodromous. May be basal or suprabasal depending on origin, and perfect or imperfect depending on whether they reach to 2/3 of the way to the apex. E.g., Miconia (basal type), Endlicheria (suprabasal type).
  5. Actinodromous
  Three or more primary veins diverging radially from a single point. E.g., Arcangelisia (basal type), Givotia (suprabasal type).
  6. Palinactodromous
  Primary veins with one or more points of secondary dichotomous branching beyond the primary divergence, either closely or more distantly spaced. E.g., Platanus.
  Types 4–6 may similarly be subclassified as basal (primaries joined at the base of the blade) or suprabasal (diverging above the blade base), and perfect or imperfect, but also flabellate.
  At about the same time, Melville (1976) described a system applicable to all Angiosperms and using Latin and English terminology. Melville also had six divisions, based on the order in which veins develop.
  Arbuscular (arbuscularis)
  Branching repeatedly by regular dichotomy to give rise to a three dimensional bush-like structure consisting of linear segment (2 subclasses)
  Flabellate (flabellatus)
  Primary veins straight or only slightly curved, diverging from the base in a fan-like manner (4 subclasses)
  Palmate (palmatus)
  Curved primary veins (3 subclasses)
  Pinnate (pinnatus)
  Single primary vein, the midrib, along which straight or arching secondary veins are arranged at more or less regular intervals (6 subclasses)
  Collimate (collimatus)
  Numerous longitudinally parallel primary veins arising from a transverse meristem (5 subclasses)
  Conglutinate (conglutinatus)
  Derived from fused pinnate leaflets (3 subclasses)
  A modified form of the Hickey system was later incorporated into the Smithsonian classification (1999) which proposed seven main types of venation, based on the architecture of the primary veins, adding Flabellate as an additional main type. Further classification was then made on the basis of secondary veins, with 12 further types, such as;
  Brochidodromous
  Closed form in which the secondaries are joined together in a series of prominent arches, as in Hildegardia.
  Craspedodromous
  Open form with secondaries terminating at the margin, in toothed leaves, as in Celtis.
  Eucamptodromous
  Intermediate form with upturned secondaries that gradually diminish apically but inside the margin, and connected by intermediat
caption: Flabellate venation, Adiantum cunninghamii
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