Photograph of a witch hazel leaf.

Angiosperm leaf architecture

Section contents

Angiosperms (flowering plants)

  1. Flowers
  2. Life cycle
  3. Pollination
  4. Fruits
  5. Fruit & seed dispersal
  6. Leaf architecture ←
  7. Overview of angiosperm phylogeny
    Related pages/background reading

    Feature image: Leaf of witch hazel (Hamamelis) showing reticulate venation. Credit: E.J. Hermsen (DEAL).

    Topics covered on this page:

    Introduction

    Angiosperm leaves are perhaps the most common and important macrofossils in the angiosperm paleobotanical record. Leaves not only provide information about fossil angiosperm diversity and affinities, but they can also be used in other types of analyses, like estimating paleoclimatic parameters. Nevertheless, leaf architecture (leaf morphology and venation patterns) is not often documented in detail in descriptions of modern plants. Leaves are often described based on their phyllotaxy (arrangement on the stem), whether they are simple or compound, size, major venation pattern (e.g., pinnate or palmate, type of lateral veins), type of margin (edge of the leaf blade), and type of indumentum (type and density of hairs, if present). Other unusual features, like stipules (paired structures at the base of the petiole where the leaf attaches to the stem), nectaries, etc., may also be noted. However, the finer details of the venation are often not described and many other characteristics may be left out.

    2-Panel image showing white oak leaves. Panel 1: Simplified line illustration of white oak showing few details. Panel 2: Cleared and stained leaf of white oak showing full venation pattern.
    Leaves of white oak. Contrast between an illustration of a white oak (Quercus alba) leaf and its actual venation. Left: Simplified illustration of the characters of white oak showing a pinnately lobed leaf with pinnate venation. Right: Cleared and stained leaf of white oak showing both pinnate venation as well as the minor veins of the leaf. Credits: Quercus alba illustration from Britton and Brown (1913) (via USDA-NRCS PLANTS Database, public domain); cleared Quercus alba leaf (NCLC145) from the The National Cleared Leaf Collection-Hickey (NCLC-H, image copyright Yale Peabody Museum of Natural History, used for educational purposes following terms of use).

    There are a few reasons that certain details of leaf architecture tend to be overlooked in descriptions of modern plants. For living plants, botanists often have access to the whole plant, or at least a specimen with more than one organ. The purpose of most plant descriptions is not to provide a catalogue of every possible observable feature on a plant, but rather to provide a summary of the characteristic features that can readily be noted, particularly in the field. Describing leaf architecture is time consuming and may not add many generally useful differentiating characters to a whole-plant description. Thus, many studies and classifications of leaf architecture grow out of the work of angiosperm paleobotanists, who have no other choice but than to try to identify plants based on their leaves, which are often preserved as isolated organs in the fossil record.

    In order to study the architecture of modern leaves for comparison to fossil leaves, paleobotanists may visit herbaria (collections of dried, pressed plants) to examine the leaves of modern plants. They may also make leaf clearings. A leaf clearing is a leaf that has been processed using chemicals to remove the pigments; afterwards, it is stained. Staining the leaf brings out the venation pattern, since the vascular tissues appear darker than the surrounding mesophyll when stained. Stained leaves are then mounted, or placed in a medium—a natural resin like Canada balsam (a conifer resin), a synthetic resin, an oil like cedar oil, or another substance—on a large glass slide or between transparent plastic sheets in order to preserve them and keep them from drying out. Clearing, staining, and mounting leaves allows them to be studied, photographed, and stored for long periods of time.

    2-Panel figure from a published paper by Jud et al. Panels 1 & 2: Leaves of Sarcomphalus saeri from herbarium specimens. Panel 3: Cleared and stained leaf of jujuba. All three leaves have three major veins and show the minor venation.
    Modern leaves showing venation. Leaves of modern members of the buckthorn family (Rhamnaceae). The left and center leaves (G and H) are not cleared, the right leaf (I) has been cleared and stained. A strap used to secure herbarium specimens can also be seen on the left leaf. All three leaves have three primary veins and acrodromous venation (explained below). Left and center: Leaves of Sarcomphalus saeri (US 2045934, US 3554997). Right: Jujuba (Ziziphus jujuba) leaf (NCLC-H 1791). Credit: Fig. 3G–I from Jud et al. (2017) PLoS ONE (CC BY 4.0). Image modified from original, figure caption adapted from original.
    Many standard resources—including glossaries and dictionaries of descriptive botanical terms, textbooks, and two editions of the Manual of Leaf Architecture—exist that provide definitions of terms used in describing the characteristics of angiosperm leaf architecture, such as attachment, size, shape, apex type, base type, margin characteristics, indumentum, etc. For this reason, this page will focus on a few special issues related to paleobotany: the identification of simple versus compound leaves in the fossil record, the basics of leaf venation in reticulate-veined leaves, an overview of parallel-veined leaves, and the basic leaf margin types. The last category, leaf margin type, is particularly important to paleoclimate reconstruction using fossil leaves.

    Simple vs. compound leaves

    Simple leaves are leaves in which the leaf blade (lamina) is not divided. Compound leaves are leaves that are divided into leaflets that resemble simple leaves (to read more and view diagrams of simple and compound leaves, visit the DEAL Leaf Structure & Evolution page). Determining whether a leaf is simple or compound is one of the most fundamental and (theoretically) straightforward steps in describing an angiosperm leaf. In living plants, confusion is most likely to occur in the case of pinnately compound leaves. A pinnately compound leaf has a rachis (central axis) bearing lateral leaflets. A pinnately compound leaf may thus look similar to a branch bearing simple leaves.

    Diagrams of odd-pinnate and even-pinnate leaves. Odd-pinnate leaves end with a leaflet, whereas even-pinnate leaves do not.
    Pinnately compound leaves. Diagrams of odd-pinnate (imparipinnate) and even-pinnate (paripinnate) compound leaves. Note that odd-pinnate leaves terminate in a leaflet, whereas even-pinnate leaves do not. Credit: E.J. Hermsen (DEAL).

    Usually, the difference between a leaf and a branch is obvious in living plants. Stems are often woody and may have leaves arranged in a spiral or several rows, whereas leaves are not woody and often have leaflets in only two rows. Furthermore, the whole leaf structure is relatively flat (planar), which is often not the case for leafy stems. When the difference between a branch bearing leaves and a compound leaf is not clear, looking for the buds can help:

    • Axillary buds: Angiosperms have axillary branching, where branches develop from buds that form in the axils of the leaves. An axil is the angle formed between the upper side of the leaf and the stem to which it is attached (read more about it on the DEAL Branching page). In a compound leaf, a bud will occur where the leaf attaches to the stem, but buds will not occur where the leaflets attach to the rachis.
    • Terminal (apical) bud: In angiosperms, a stem elongates from a terminal bud, which is a growing point (apical meristem and associated structures) at the stem tip. This terminal bud may be protected by bud scales or by a series of young, developing foliage leaves. Compound leaves lack a terminal bud. Pinnately compound leaves of angiosperms may end in one of two ways:
      • Odd-pinnate or imparipinnate leaves have a terminal leaflet.
      • Even-pinnate or paripinnate leaves have no terminal leaflet, and the end of the rachis truncates with a pair of leaflets.
    Image
    Simple vs. compound leaves. The diagram above shows an angiosperm branch with simple leaves compared to an angiosperm branch with odd-pinnate (imparipinnate) compound leaves. Note that axillary buds occur in the axils of the leaves, whether simple or compound. Axillary buds are not found in the axils of the individual leaflets on a compound leaf. Also note that the branch (stem) has a terminal bud, whereas the rachis (central axis) of a compound leaf does not. Credit: E.J. Hermsen (DEAL).
    2-Panel photographic figure showing shagbark hickory. Panel 1: Tip of stem with attached leaf petioles showing apical and axillary buds. Panel 2. Detail of portion of leaf rachis and leaflet bases showing lack of terminal and axillary buds.
    Branch vs. leaf. Comparison of a branch with attached leaves (left) and a compound leaf with leaflets (right) of shagbark hickory (Carya ovata). Left: Note that the branch ends in a bud, and each leaf has an axillary bud where it attaches to the stem. Right: Portion of an odd-pinnate compound leaf (imparipinnate leaf) showing the base of the terminal leaflet and the central rachis with attached lateral leaflets; note that there are no buds where the leaflets attach to the rachis. Credits: Shagbark hickory (Plant Image Library, via Wikimedia Commons, CC BY-SA 2.0); Carya ovata leaf (Kristel Schoonderwoerd, via Wikimedia Commons, CC BY-SA 2.0).
    2-Panel figure showing photograph of leaves of carob. Panel 1: Branch with three even-pinnate leaves. Panel 2: Detail of leaf apex showing paired leaves.
    Even-pinnate (paripinnate) leaves. Leaves of carob (Ceratonia siliqua). Left: Herbarium specimen of part of a branch bearing even-pinnate (paripinnate) compound leaves; axillary buds are present in the axils of the leaves, although difficult to see in this image. Right: Detail of one of the compound leaves showing the pair of leaflets at the apex and the lack of terminal and axillary buds on the rachis. Credit: Ceratonia siliqua, LOB119571 (California State University, Long Beach, via GBIF, CC BY-NC 4.0).

    Unfortunately, things are not so simple for paleobotanists. It may be difficult to tell whether a fossil angiosperm leaf is simple or compound because many of the above clues may be missing in fossil leaves. Many fossil structures are flattened when they are preserved. Fossil compound leaves are often not preserved as complete structures; instead, leaflets may be preserved in isolation. Buds—either axillary or terminal—are also typically not preserved, because fossil angiosperm leaves are rarely found attached to branches.

    Paleobotanists may thus have to rely on luck (for example, preservation of a branch with leaves or of a complete compound leaf) or careful comparison to leaves on related modern plants to determine whether a fossilized leaf-like structure is more likely to be a simple leaf or a leaflet from a compound leaf.

    Image of mummified zelkova leaves showing a specimen of simple leaves on a branch and a single simple leaf.
    Fossil simple leaves. Mummified leaves of zelkova (Zelkova zelkovifolia, Pliocene, Frankfurt am Main, Germany). Left (c): Several simple leaves on branches; Right (d): A single simple leaf. Scale bars = 1 cm. Credit: Portion of Fig 5 from Denk et al. (2019) Palaeobiodiversity and Palaeoenvironments (CC BY 4.0). Image modified from original, figure caption adapted from original.
    2-Panel photographic figure showing fossil sumac leaves. Panel 1: Pinnately compound leaf. Panel 2: Images of four isolated leaflets.
    Fossil compound leaf and leaflets. Fossil leaf (left) and four isolated leaflets (right) of sumac (Rhus stellariaefolia) from the Eocene Florissant flora of Colorado, U.S.A. Note that the leaflets look similar to simple leaves when preserved in isolation. Credit: Rhus stellariaefolia compound leaf (Division of Paleobotany, Yale Peabody Museum, via GBIF, CC0 1.0/Public Domain Dedication); Rhus stellariaefolia leaflets, Morphotype 19 from Allen et al. (2019), via PaleorXiv (CC BY 4.0). Images modified from originals.

    Important terminology note!

    The venation and margin terms described below on this page can be applied to simple angiosperm leaves or to the leaflets of compound angiosperm leaves. Only the word "leaf" is used in defining terms below for the sake of simplicity, but the word "leaflet" can be substituted when applicable.

    Reticulate venation patterns

    Complex schemes have been developed to characterize and determine the relationships of fossil leaves with reticulate venation on the basis of their architecture—in other words, structure, or features like shape, venation, and other characteristics. Most reticulately veined leaves are leaves of dicots. "Dicot" is a historical category in angiosperm classification. A dicot is an angiosperm that has an embryo with two cotyledons, or seed leaves. Today, dicots are recognized as being a paraphyletic group that includes plants found in the basal angiosperms, Chloranthales, magnoliids, Ceratophyllales, and eudicots. Thus, dicots are essentially angiosperms that are not monocots. (For more on angiosperm classification, see the overview of angiosperm phylogeny.)

    Diagram of angiosperm relationships showing dicot groups. Dicots include Amborellales, Nymphaeales, Austrobaileyales, Chloranthales, Magnoliids, Ceratophyllales, and Eudicots.
    Dicots. Tree of angiosperm relationships based on the Angiosperm Phylogeny Group IV (APG IV) classification. All the groups that are considered dicots (plants with two cotyledons or seed leaves) are shown in the shaded boxes. Note that most dicots also have reticulate leaf venation. Credit: E.J. Hermsen (DEAL).
    Dicots tend to have leaves with reticulate (net-like) venation, although reticulate venation is also found in some monocot leaves (such as the leaf of the yam, Dioscorea, figured below on this page). Perhaps the best known systems of descriptive terminology for dicot leaves are those developed in the 1970's by Leo Hickey (Hickey 1973) and, later, by Hickey and Jack Wolfe (Hickey and Wolfe 1975). More recently, such schemes have been revised, updated, and expanded in two editions of the Manual of Leaf Architecture (1999, 2009; see citations at the bottom of this page). These manuals and similar schemes for describing leaf architecture provide guidance for describing leaves based on a large number of features falling roughly into the broad categories of organization, shape, size, venation pattern, and margin characteristics.
    Photograph of a magnolia leaf that has turned yellow in the fall. The reticulate venation pattern is clearly visible on the leaf surface.
    Reticulate venation. This leaf of magnolia (Magnolia sp.) has reticulate venation. The major venation pattern is pinnate, the secondary venation pattern is brochidodromous. Credit: E.J. Hermsen (DEAL).

    Describing leaf venation patterns is one of the most important and potentially challenging parts of describing many dicot leaves. Veins are sorted into categories by thickness. The thickest veins are the primary veins, the next thickest the secondary veins, the next thickest the tertiary veins, and so on. The number of vein orders a leaf has as well as the organization of each order of veins is important to describing and potentially identifying an angiosperm leaf. The brief overview below mostly follows the Manual of Leaf Architecture, although in simplified form.

    Primary venation

    Primary veins are the thickest and most prominent vein or veins in the leaf. Secondary veins are the next-thickest veins. The primary veins or the primary and secondary veins together determine the major venation pattern of the leaf, which can broadly be divided into pinnate or palmate.

    Pinnate venation

    If one primary vein is present and it divides the leaf blade from base to apex, then the primary vein is a midvein. If secondary veins (major veins that are slightly thinner than the midvein) depart from either side of the midvein, then the overall major venation pattern is called pinnate (remember, "pinnate" refers to a feather-like organization). Thus, the term pinnate encompasses the pattern of both primary and secondary veins together.

    2-Panel figure. Panel 1: Cleared birch leaf with pinnate venation and straight secondary veins. Panel 2: Fossil birch leaf with pinnate venation and straight secondary veins.
    Pinnate venation in birch (Betula). The birch leaves above have pinnate venation; there is a single midvein dividing each leaf in half, with secondary veins given off to its the sides. Secondary venation is craspedodromoous (explained below). Left: Modern cleared leaf of sweet birch (Betula lenta). Right. Fossil leaf of Betula leopoldae (Eocene, Allenby Formation, British Columbia, Canada). Credits: Betula lenta (NCLC5415-1) from the National Cleared Leaf Collection (NCLC-H, image copyright Yale Peabody Museum of Natural History, used for educational purposes following terms of use); Betula leopoldae (Linda S. Klise, Yale Peabody Museum of Natural History, CC0 1.0/Public Domain Dedication as indicated in the associated GBIF record). Images modified from originals.
    Palmate venation

    If at least three primary veins emerge from the base of the leaf or slightly above the leaf base, the primary venation pattern is palmate (remember, palmate is organized like the fingers radiating from the palm of a hand). Note that some sources distinguish between palmate venation and ternate or trinerved venation; in that case, leaves with ternate or trinerved venation have three primary veins radiating at or just above the leaf base, whereas leaves with palmate venation have at least four primary veins.

    Palmate venation is divided into a number of different categories depending on the course of the primary veins. Note that the terms for categories of palmate venation end with "-dromous"; this ending derives from the Greek word for course or running. Thus, these terms describe the course of the veins. Some examples of palmate venation include:

    • Actinodromous (Greek, aktis = ray): The primary veins radiate from one point and proceed on a relatively straight course toward the leaf margin.
    • Palinactinodromous (Greek, palin + aktis = again ray): Like actinodromous, except the lateral primary veins branch above the leaf base.
    • Acrodromous (Greek, ákros = end or peak): The major veins arise from one point; the lateral primary veins and/or thick secondary veins curve outward and then come together near the leaf tip.
    • Campylodromous (Greek, kampylos = bent or curved): The lateral primary veins are recurved (the veins curve toward the leaf base then turn upward), coming together near the leaf tip. Note that the lateral primary veins may branch above the leaf base.
            2-Panel photographic figure. Panel 1: Palmately-veined leaf of Japanese maple, with major veins radiating from a single point. Panel 2: Leaf of western sycamore, with major veins radiating from the base of the leaf and lateral veins branching.
            Palmate venation: Left: Japanese maple (Acer palmatum) leaf with actinodromous venation; note that the major veins radiate from the base of the leaf blade. Right: Western sycamore (Platanus racemosa) leaf with palinactinodromous venation; three primary veins radiate from the base of the leaf blade, with the two lateral primary veins branching (the arrowhead indicates a branching vein). Note: These are cleared leaves; the dark areas at the edges are caused by mounting medium that has crystallized. Images modified from originals. Credits: Acer palmatum (NCLC1671) and Platanus racemosa (NCLC6143). Specimens from the The National Cleared Leaf Collection-Hickey (NCLC-H, images copyright Yale Peabody Museum of Natural History, used for educational purposes following terms of use). Images modified from originals.
            2-Panel photographic figure. Panel 1: Leaf of paliurus with three major veins emerging from the leaf base, the lateral veins curving outward and converging at the apex. Panel 2: Leaf of yam with major veins emerging from the base, the lateral veins curving toward the leaf base before converging toward the leaf apex.
            Palmate venation. Left: Cleared leaf of Paliurus ramosissimus showing acrodromous venation (this leaf can be considered either ternate or palmate); three primary veins emerge from the leaf base; the two lateral veins arc outward, then come together near the leaf apex. Right: Cleared leaf of a yam (Dioscorea amazonum) with campylodromous venation, in which the major lateral veins are recurved (bent backwards). Credits: Paliurus ramosissimus (NCLC4779) and Dioscorea amazonum (NCLC6759). Specimens from the The National Cleared Leaf Collection-Hickey (NCLC-H, images copyright Yale Peabody Museum of Natural History, used for educational purposes following terms of use). Images modified from originals.
            Secondary venation

            Secondary veins are veins that are narrower than the primary veins, but thicker than the tertiary veins that fill in the lamina (blade) of the leaf (see below). The most prominent secondary veins are often those branching off the sides of the primary vein(s); they are called major or costal (Latin costa = rib) secondary veins. These secondary veins are categorized based on their course (straight or curved) and how they end (for example, looping to join other secondary veins, ending at the leaf margin, or thinning out as they approach the leaf margin).

            The terms for categories of major secondary venation end with "-dromous"; as noted above, this ending derives from the Greek word for course or running. Thus, these terms describe the course of the major secondary veins. Examples of major secondary venation types include:

            • Craspedodromous (Greek, kraspedon = edge or border): Secondary veins are relatively straight and end at the leaf margin without looping.
            • Semicraspedodromous: Each secondary vein divides. One half of the division curves upward and connects with the secondary vein above it. The other half of the division travels to the leaf margin, often ending in tooth.
            • Brochidodromous (Greek, brochos = loop): Each secondary vein curves and fuses with the secondary vein immediately above it.
            • Eucamptodromous (Greek, eu + kamptos = true curved): Secondary veins curve upward but do not connect with one another to form loops. The secondary veins diminish in thickness as they near the leaf margin.
                  Drawing of four leaves showing different patterns of secondary venation: craspedodromous, semicraspedodromous, brochidodromous, and eucamptodromous.
                  Secondary venation patterns. The drawings above show four patterns of major secondary venation; all are illustrated on pinnately veined, simple leaves. Venation types: Craspedodromous, semicraspedodromous, brochidodromous, and eucamptodromous. Credit: Drawings from Hickey & Wolfe (1975) Annals of the Missouri Botanical Garden 62: fig. 3 (via Biodiversity Heritage Library, CC BY-NC-SA 3.0). Drawings modified from originals.
                  4-Panel photographic figure of cleared leaves. Panel 1: Elm leaf, craspedodromous venation. Panel 2: Lamnonia leaf, semicraspedodromous venation. Panel 3: Coffee leaf, brochidodromous venation. Panel 4: Silky dogwood leaf, eucamptodromous venation.
                  Secondary venation patterns. The cleared leaves and leaflet above show four major secondary venation patterns. From left to right these are craspedodromous (leaf, September elm, Ulmus serotina), semicraspedodromous (leaflet, Lamanonia iomeniosa), brochidodromous (leaf, coffee, Coffea arabica), and eucamptodromous (leaf, silky dogwood, Cornus amomum). Credit: Ulmus serotina (NCLC 0255), Lamanonia iomeniosa (NCLC 0019), Coffea arabica (NCLC 4623), and Cornus amomum (NCLC 0162). Specimens from the The National Cleared Leaf Collection-Hickey (NCLC-H, images copyright Yale Peabody Museum of Natural History, used for educational purposes following terms of use). Images modified from originals.
                  Tertiary venation

                  Tertiary veins are thinner than secondary veins, and they are part of the network of veins that fill in the spaces between major veins in the leaf lamina (blade). Tertiary veins that occur between the major secondary veins are known as intercostal (Latin inter + costa = between rib) veins. Three major categories of intercostal tertiary venation are recognized:

                  • Percurrent: Tertiary veins run between secondary veins without branching or fusing with one another. Percurrent tertiary veins are not necessarily straight and may be curved or have angles in them.
                  • Ramified: Tertiary veins branch but do not anastomose (fuse or join).
                  • Reticulate: Tertiary veins fuse with one another to form a network (reticulum) between the major veins.
                  2-Panel photographic figure. Panel 1:
                  Percurrent intercostal tertiary venation. These cleared leaves each have a single primary vein (midvein) with lateral secondary veins. The intercostal tertiary veins are thinner veins that occur between the secondary veins. Left: Major secondary venation is eucamptodromous to brochidodromous, intercostal tertiary venation is percurrent. Right: Major secondary venation is eucamptodromous (curving upward without looping), tertiary venation is percurrent. Credits: Phaeanthus cumingi (NCLC0390) and Shorea lissophylla (NCLC4640). Specimens from the The National Cleared Leaf Collection-Hickey (NCLC-H, images copyright Yale Peabody Museum of Natural History, used for educational purposes following terms of use). Note: The dark areas at the edges of the images are caused by mounting medium that has crystallized. Images modified from originals.
                  2-Panel photographic figure of cleared leaves. Panel 1: Leaf of sumac showing ramified intercostal tertiary venation. Panel 2: Image of Turpinia leaf showing reticulate tertiary venation.
                  Ramified and reticulate intercostal tertiary venation. Left: Cleared leaflet of sumac (Rhus sylvestris) showing ramified intercostal tertiary venation in which the tertiary veins branch but do not fuse; major secondary venation is cladodromous (Greek klados = branch), a pattern where the secondary veins branch as they approach the leaf margin but do not form loops. Right: Cleared leaflet of Turpinia glaberrima showing reticulate tertiary intercostal venation; major secondary venation is brochidodromous. Credits: Rhus sylvestris (NCLC6539) and Shorea lissophylla (NCLC4484). Specimens from the The National Cleared Leaf Collection-Hickey (NCLC-H, images copyright Yale Peabody Museum of Natural History, used for educational purposes following terms of use). Note: The dark areas at the edges of the right image are caused by mounting medium that has crystallized. Images modified from originals.
                  Black and white image of a portion of a cleared leaf of hog plum labeled to illustrate features of the secondary and tertiary venation.
                  Reticulate intercostal tertiary venation. Cleared leaflet of hog plum (Spondias mombin) showing details of secondary and tertiary venation. This leaflet has pinnate venation, and the midvein is the vertical vein on the left-hand side of the image. The intercostal tertiary venation (label D) is reticulate. Major secondary venation (label B) is brochidodroums with intersecondary veins, or weak secondary veins that occur between the major secondary veins (label C). This image also shows epimedial tertiary veins, or tertiary veins that connect to the primary veins (label A). Credit: Spondias mombin, Hopkins 1454, NY, Fig. 7 from Mitchell and Daly (2015) PhytoKeys (CC BY 4.0). Figure caption adapted from original.
                  Higher-order veins & areoles

                  Leaves may have vein orders above tertiary venation, including quaternary veins (fourth-order), quinternary veins (fifth-order), and even higher orders. Like tertiary veins, these veins fill in the spaces between the major veins of the leaf. In many angiosperm leaves, the higher-order veins anastomose (fuse) and form enclosed regions known as areoles. The areoles may or may not have branching veins within them. Branching veins with an areole that do not fuse to any other veins are known as freely ending veinlets (FEVs).

                  2-Panel figure of chinquapin leaves. Panel 1: Detail of cleared leaf with secondary and tertiary veins labeled. Panel 2. Detail of fossil leaf with secondary and tertiary veins labeled.
                  Higher-order venation. Detail of pinnately veined leaves of chinquapin (Castanopsis) showing the higher-order venation between the major secondary and the tertiary veins. Secondary venation is craspedodromous, tertiary venation is percurrent. Note that the small higher-order veins forms areoles (enclosed areas). Left: Modern cleared leaf of Castanopsis orthacantha. Right: Fossil leaf of Castaneophyllum patagonicum (Eocene, Laguna del Hunco, Chubut, Argentina). Credits: Castanopsis orthacantha (as Castanopsis concolor, NCLC0627) from the National Cleared Leaf Collection (NCLC-H, images copyright Yale Peabody Museum of Natural History, used for educational purposes following terms of use); Castaneophyllum patagonicum, LH13-1109 (20160605 092534) from Wilf et al. (2019) on Figshare (CC BY 4.0).
                  2-Panel figure showing black and white images of higher-order leaf venation, areoles, and freely ending veinlets. Image from Hickey and Wolfe (1975).
                  Areoles and freely ending veinlets. Details of leaves showing areoles and freely ending veinlets inside. Leaves are Magnolia angatensis (left, C) and Exbucklandia populnea (right, D). Credit. Fig. 4C, D from Hickey and Wolfe (1975) Annals of the Missouri Botanical Garden (CC BY-SA-NC 3.0). Image modified from original, figure caption adapted from original.
                  Black and white photomicrograph showing freely ending veinlets in a cleared leaflet of hog plum.
                  Freely ending veinlets (FEVs). Cleared leaflet of a species of hog plum (Spondias admirabilis) showing a detail of the freely ending veinlets. The ends of some of the veinlets are inicated by the arrows labeled "A." Credit: Spondias admirabilis, Farney et al. 3957, NY, Fig. 8 from Mitchell and Daly (2015) PhytoKeys (CC BY 4.0). Figure caption adapted from original.

                  Parallel venation patterns

                  Monocots are angiosperms with embryos that have one cotyledon (seed leaf). Examples of monocots are grasses, palms, bananas, and gingers. Monocot leaves often have parallel or modified parallel venation. In a parallel-veined leaves, the major veins run parallel to one another and converge at the tip of the leaf. The major parallel veins are often connected by finer, less conspicuous veins that run between them. Monocot leaves often do not have a distinct stalk (petiole) and are often linear or strap-like in form. Leaves of plants like grasses (family Poaceae), sedges (family Cyperaceae), and daylilies (Hemerocallis fulva) fit the typical monocot pattern.

                  Two types of parallel or modified-parallel monocot leaf venation are included in the Manual of Leaf Architecture: parallelodromous and campylodromous. In parallelodromous venation, at least two (but often many) primary veins emerge from the base of the leaf, coming together near the leaf tip. In campylodromous venation, venation is palmate and the lateral primary veins are recurved (curve toward the leaf base before turning upward), coming together at the leaf tip. Note that the definition of campylodromous—specifically, whether the primary veins emerge from one point or next to one another from the leaf base—differs slightly among sources; leaves fitting both definitions are shown on this page (see the Dioscorea amazonum leaf above and the Maianthemum dilatatum leaf below).

                  Some monocot leaves have other modifications of the parallel pattern. In fan-shaped leaves like those of the palmetto (Sabal, a type of palm), the major veins radiate either from the base of the leaf or from a short midrib. Thus, the venation may be described as palmate and parallel. In monocots like bananas (Musa) and gingers (Zingiber), each leaf has a large midvein with many lateral veins to either side. The lateral veins are parallel to one another. This type of venation pattern is called penni-parallel.

                  2-Panel figure of color images of monocot leaves. Panel 1: Portions of fossil cattail leaves showing parallel major veins. Panel 2: Detail of corn leaf showing parallel major veins.
                  Parallel venation. Left: Fossil leaves of cattail (Typha latissima) from the Miocene of France. Right: Detail of a leaf of corn (Zea mays). Credit: Typha latissima (Jocelyn Falconnet, via GBIF, CC BY 4.0); underside of a corn leaf (Cbaile19, via Wikimedia Commons, CC0 1.0/Public Domain Dedication). Images modified from originals.
                  2-Panel figure showing photographs of leaves of Maianthemum from herbarium sheets. Panel 1: Leaf with parallelodromous venation. Panel 2. Leaf with campylodromous venation.
                  Parallelodromous and campylodromous venation. Left: Parallelodromous venation in a leaf of false solomon's seal (Maianthemum racemosum); note that many veins emerge from the base of the leaf and run parallel to one another, coming together near the tip. Right: Campylodromous venation in two-leaved solomon's seal (Maianthemum dilatatum); many veins emerge from the base of the leaf and follow a recurved course, coming together near the leaf tip. Credits: Maianthemum racemosum, NEON B53765 (Arizona State University, via GBIF, CC0/public domain dedication); Maianthemum dilatatum, ALA H1147650 (University of Alaska Museum, via Morphbank, CC BY-NC-SA 3.0 US). Images modified from originals.
                  Color photograph of the base of a palmetto leaf showing veins radiating from the petiole. The leaf blade is plicate (folded).
                  Palmate & parallel venation. A leaf of dwarf palmetto (Sabal minor) with parallel veins radiating from the base in a palmate pattern. The leaf is also plicate (folded). Credit: Sabal minor (Susan Fawcett, GBIF/iNaturalist, CC BY-NC 4.0).
                  2-Panel figure of color photographs of banana leaves. Panel 1: Whole leaf showing midvein and parallel lateral veins. Panel 2. Detail of midvein and parallel lateral veins.
                  Penni-parallel venation. Banana leaves (Musa) showing penni-parallel venation. Left: Flowering banana (Musa ornata) leaf. Right: Detail of a banana (Musa acuminata) leaf showing midvein and parallel lateral veins. Credit: Musa ornata (nicolefitz, via GBIF/iNaturalist, CC BY-NC 4.0); Musa acuminata (3268zauber, Wikimedia Commons, CC BY-SA 3.0). Images modified from originals.

                  Leaf margins

                  The leaf margin is the edge of the leaf. Leaf margins can come in a variety of types. An entire margin is smooth and uninterrupted.

                  Margins that are not entire may have teeth, lobes, or both teeth and lobes. A toothed margin has teeth—essentially, it has an edge that resembles the blade of a saw. Teeth may be large or small, pointed or rounded, and have different shapes. The upper side of the tooth—meaning the side of the tooth facing the apex of the leaf—is know as the distal flank. The lower side—meaning the side of the tooth facing the base of the leaf—is known as the proximal flank. The valleys between teeth are called sinuses.

                  Lobes are larger than teeth and are divided by deeper sinuses. Usually, the difference between teeth and lobes is clear. However, large teeth and shallow lobes can be difficult to distinguish. The 2009 edition of the Manual of Leaf Architecture defines a lobe based on the depth of the adjoining sinus on the upper (apical or distal) side of the lobe. The depth of the sinus must be at least 25% the total length of the projection for a projection to be classified as a lobe. (The length of the projection is measured from the tip of projection to the midvein of the leaf.) Other sources, however, may use a different cut-off.

                  3-Panel photographic image of fossil leaves. Left: Leaf with entire (completely smooth) margin. Center: Birch family leaf with toothed margin. Right: Leaf of an extinct sycamore with 5 lobes.
                  Leaf margins. From left to right: Leaf margin entire (Schoepfia republicensis, Eocene, Repbublic flora, U.S.A.), toothed (Betulaceae/beech family, Eocene, Florissant flora, U.S.A.), and lobed (Macginitiea gracilis, Eocene, Republic flora, U.S.A.). Credits: Schoepfia republicensis (Kevmin, via Wikimedia Commons, CC BY-SA 4.0); extinct Betulaceae (National Park Service, Public Domain); Macginitiea gracilis (Kevmin, Wikimedia Commons, CC BY-SA 4.0). Images modified from originals.
                  2-Panel figure showing leaves from herbarium specimens. Panel 1: Bigtooth aspen with apical direction, basal direction, and tooth parts (distal and proximal flanks, apex, and sinus) labeled. Panel 2: Leaf of purple passionflower with 3 lobes with apical and basal directions labeled, lobes and sinus labeled, and measurement lines shown for measuring sinus depth.
                  Teeth and lobes. Left: Toothed, unlobed leaf of bigtooth aspen (Populus grandidentata) with parts of the tooth labeled; teeth are not compound (discussed below). Right: Three-lobed leaf of purple passionflower (Passiflora incarnata) showing how to measure lobe length and sinus depth. Note that both lobe length and sinus depth are measured parallel to the midline of the lobe, with lobe length extending from lobe tip to the leaf midvein. The red lines extend perpendicularly (each at a right angle) from the from the midline of the lobe. Credits: Populus grandidentata, EKY31234100652195 (Eastern Kentucky University herbarium, via GBIF, CC BY-NC 3.0); Passiflora incarnata, NY4209876 (New York Botanical Garden, via GBIF, CC BY 4.0). Images modified from originals.

                  Leaf teeth come in a variety of shapes. In modern plants, these are usually defined using a series of terms that describe a combination of tooth size and tooth tip shape (for example, rounded or pointed). Three basic types are serrate (teeth sharp, tooth apex pointed apically), dentate (teeth sharp, tooth apex pointed outward), or crenate (teeth with rounded apices) to describe the leaf margin.

                  Additionally, teeth may be compound. Compound teeth are teeth that have more than one projection (point), a large projection and one or more smaller projections that are usually on its distal flank. On a leaf with compound teeth, the primary teeth are the largest teeth, which are typically fed by the most prominent veins. Smaller teeth—sometimes called secondary teeth—are the smaller points on the flanks of the primary teeth.

                  Image of a leaf tip showing a margin with compound teeth, examples of primary and secondary teeth labeled.
                  Compound teeth. Leaf apex of European white elm (Ulmus laevis) showing compound teeth. Credit: Ulmus laevis, US03413312 (Smithsonian Institution, NMNH, Botany, via GBIF, CC0/public domain dedication).

                  In paleobotanical studies that use the physiognomy (essentially, architecture and/or dimensions) of woody dicot leaves to estimate paleoclimate, leaf margin type may be one of the leaf characters scored. In these studies, leaves are divided into untoothed and toothed. The categories are as follows:

                  • Untoothed leaves: Leaves with entire margins and leaves with lobed margins and no teeth.
                  • Toothed leaves: Leaves with teeth, whether unlobed or lobed.

                    The reason for this division is because leaf teeth are an important climate indicator. Woody dicots with untoothed leaf margins make up a higher percentage of species in warmer-climate floras and a lower percentage in cooler-climate floras.

                    2-Panel figure with photgraphs of sassafras leaves. Panel 1. Three-lobed leaf of sassafras without teeth on the margin. Panel 2. Three-lobed leaf of hops that has a margin with teeth.
                    Untoothed and toothed, lobed leaves. Left: A three-lobed leaf of sassafras (Sassafras albidum) without teeth (the tip of the left leaf lobe is broken off). Right: A three-lobed leaf of hops (Humulus lupulus) with teeth. Credits: Sassafras albidum, NCU00314272 (University of North Carolina at Chapel Hill Herbarium, via GBIF, CC BY-NC 3.0); Humulus lupulus, NY3883733 (New York Botanical Garden, via GBIF, CC BY 4.0).

                    Selected references & further reading

                    Note: Free full text is made available by the publisher for items marked with a green asterisk.

                    Academic articles & book chapters

                    * Allen, S.E., A.J. Lowe, D.J. Peppe, and H.W. Meyer. 2020. Paleoclimate and paleoecology of the uppermost Eocene Florissant flora (Central Colorado, USA), ver. 2. PaleorXiv. [Preprint] https://doi.org/10.31233/osf.io/xpm26

                    Allen, S.E., A.J. Lowe, D.J. Peppe, and H.W. Meyer. 2020. Paleoclimate and paleoecology of the latest Eocene Florissant flora (Central Colorado, USA). Palaeogeography, Palaeoclimatology, Palaeoecology. https://doi.org/10.1016/j.palaeo.2020.109678

                    * Angiosperm Phylogeny Group [APG] IV. 2016. An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG IV. Botanical Journal of the Linnean Society 181: 1–20. https://doi.org/10.1111/boj.12385

                    * García-Gutiérrez, E., F. Ortega-Escalona, and G. Angeles. 2020. A novel, rapid technique for clearing leaf tissues. Applications in Plant Sciences 8: e11391. https://doi.org/10.1002/aps3.11391

                    Hickey, L.J. 1973. Classification of the architecture of dicotyledonous leaves. American Journal of Botany 60: 17–33. https://doi.org/10.1002/j.1537-2197.1973.tb10192.x

                    * Hickey, L.J., and J.A. Wolfe. 1975. The bases of angiosperm phylogeny: Vegetative morphology. Annals of the Missouri Botanical Garden 62: 538–589. https://doi.org/10.2307/2395267

                    * Jud, N.A., M.A. Gandolfo, A. Iglesias, and P. Wilf. 2017. Flowering after disaster: Early Danian buckthorn (Rhamnaceae) flowers and leaves from Patagonia. PLoS ONE 12(5): e0176164. https://doi.org/10.1371/journal.pone.0176164

                    * Kvaček, Z., V. Teodoridis, and T. Denk. 2019. The Pliocene flora of Frankfurt am Main, Germany: taxonomy, palaeoenvironments and biogeographic affinities. Palaediversity and Palaeoenvironments. https://doi.org/10.1007/s12549-019-00391-6

                    * Mitchell, J.D., and D.C. Daly. 2015. A revision of Spondias L. (Anacardiaceae) in the Neotropics. PhytoKeys 55: 1–92. https://doi.org/10.3897/phytokeys.55.8489

                    * Royer, D.L., P. Wilf, D.A. Janesko, E.A. Kowalski, and D.L. Dilcher. 2005. Correlations of climate and plant ecology to leaf size and shape: potential proxies for the fossil record. American Journal of Botany 92: 1141–1151. https://doi.org/10.3732/ajb.92.7.1141

                    * Vasco, A., M. Thadeo, M. Conover, and D.C. Daly. 2014. Preparation of samples for leaf architecture studies, a method for mounting cleared leaves. Applications in Plant Sciences 2: 1400038. https://doi.org/10.3732/apps.1400038

                    Wilf, P., K.C. Nixon, M.A. Gandolfo, and N.R. Cúneo. 2019. Eocene Fagaceae from Patagonia and Gondwanan legacy in Asian rainforests. Science 364: eaaw5139. https://doi.org/10.1126/science.aaw5139

                    Books, textbooks & manuals

                    * Ellis, B., D.C. Daley, L.J. Hickey, K.R. Johnson, J.D. Mitchell, P. Wilf, and S.L. Wing. 2009. Manual of Leaf Architecture. Comstock Publishing Associates, Cornell University Press, Ithaca, New York. Free PDF on ResearchGate (shared with Creative Commons Attribution-NonCommercial license, CC BY-NC 4.0): https://www.researchgate.net/publication/270216727_Manual_of_Leaf_Architecture

                    Evert, R.F., and S.E. Eichhorn. 2013. Raven Biology of Plants, 8th ed. W.H. Freeman and Co., New York, New York. 

                    Harris, J.G., and M.W. Harris. 2001. Plant Identification Terminology, an Illustrated Glossary, 2nd ed. Spring Lake Publishing, Spring Lake, Utah.

                    * Leaf Architecture Working Group (A. Ash, B. Ellis, L.J. Hickey, K. Johnson, P. Wilf, and S. Wing). 1999. Manual of Leaf Architecture: Morphological description and categorization of dicotyledonous and net-veined monocotyledonous angiosperms. Smithsonian Institution, Washington, DC. Free PDF from P. Wilf, Pennsylvania State University: https://personal.ems.psu.edu/~pdw3/1999_MLA.pdf

                    Simpson, M.G. 2010. Plant Systematics, 2nd ed. Academic Press, Burlington, Massachusetts.

                    Websites

                    * CLAMP leaf character state definitions and scoring. CLAMP online. http://clamp.ibcas.ac.cn/CLAMP_Definitions.html

                    * Cleared Leaf Image Database (J. Weitz, A. Bucksch, A. Das, C. Price): http://www.clearedleavesdb.org/

                    * Fisher, J. 2007. Leaf or branch. Fairchild Tropical Botanic Garden Virtual Herbarium: http://www.virtualherbarium.org/gardenviews/LeafOrBranch.html

                    * The National Cleared Leaf Collection, Yale Peabody Museum and Smithsonian National Museum of Natural History (L.J. Hickey and S. Hu). https://collections.peabody.yale.edu/pb/nclc/

                    * Trees of Wisconsin. Herbarium, Cofrin Center for Biodiversity, University of Wisconsin-Green Bay. Page with images of simple and compound leaves, showing differences. Link

                    * Wilf, P., K.C. Nixon, M.A. Gandolfo, and N.R. Cúneo. 2019. Image archive for: Eocene Fagaceae from Patagonia and Gondwanan legacy in Asian rainforests. Figshare. https://doi.org/10.6084/m9.figshare.7480007.v1

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                    Page first online: 24 August 2021; last updated: 28 September 2021.