What’s in a leaf
As the primary means by which a tree harnesses and translates solar energy into growth, defense, repair, and reproduction, leaves are fundamental to a plant’s ability to compete and thrive in various habitats. While there is certainly wide variation across species in leaf morphology, several patterns emerge in trees of the northeast, indicating convergent evolution as plants have developed adaptations to confront similar challenges. Here in Vermont, all trees must weather our long, harsh winters (growing seasons can range from 150 days down in the Champlain Valley to less than 100 in the Northeast Kingdom). While a bird might migrate south or a mammal hibernate underground, trees have to stand up against the cold temperatures, frozen soil, and dry air. Most of our flowering trees and shrubs (ANGIOSPERMS) take the easy way out and just lose their leaves each fall (akin to migrating) while conifers (GYMNOSPERMS) – except larches and tamaracks – have narrow, waxy needles that remain dormant on the branch throughout the winter (like hibernation). These needles have lifespans between 2 and 40 years. On this page you’ll learn about the different features of leaves (e.g. the blade, margin, and venation) and how to interpret the ecological significance of variation in leaf shape both within and across species.
Leaf type: Needles and Simple + Compound Leaves
Conifer needles are typically small, narrow, waxy, and darker green. Needles borne singly off the branch are short, typically around 1″, and can either be flattened in cross section (e.g. hemlock, yew, fir) or diamond shaped (spruce). As with spruce and hemlock, the branches can be covered in small knobby projections (STERIGMATA) that hold the needles. The leaves of the cedars (red and white) are small overlapping scales. On redcedars, the terminal leaves on lower branches or often modified into sharp, awl-shaped needles that deter herbivores. On white cedar, the awl-shaped needles are on the branch and occasionally on new growth. The pines have a FASCICLE, bundles of 2-5 needles enclosed at the base in a papery sheath. Tamaracks and larches have 12+ needles that all emerge from a small spur branch.
The leaf blade of angiosperms is broad, flattened, and significantly larger than with needles. The leaves can be simple or compound, with numerous leaflets. Small and medium sized leaves are more typically simple. Leaflets and simple leaves can look very similar, but there a couple key differences between simple and compound leaves:
- A simple leaf has a bud at the base of the petiole, leaflets lack a bud
- Leaflets typically drop with the entire leaf in the fall rather than falling off individually (though this isn’t always true as in ashes, sumacs, and to a lesser extent walnuts and black locust).
- Leaflets lie on a single plane, while the arrangement of simple leaves spiral up a branch or are perpendicular from one node to the next.
Advantage of compound leaves
Compound leaves act as a cheap, deciduous branch. There is also more turbulence as wind blows over a compound leaf, which can knock dust, fungal spores, and small insects off the surface. The more convoluted surface also makes it more difficult for insects and pathogens to spread from one part of the leaf to another.
For broad-leaves, the categories of leaf (and leaflet) shape are based on length-to-width ratio and where the widest point is. There can be some variety of leaf shape within a species, and you might find both elliptical and ovoid leaves on a single plant. It’s also good to remember that there grades between the different categories, so get a general sense for the variation, but don’t expect each species to have textbook worth leaf shapes.
Terminology for shape of the blade
- ORBICULAR: Round
- OBTUSE: bluntly tipped
- ELLIPTICAL: Widest in the middle
- OVOID: Widest at base (egg shaped)
- OBOVOID: Widest at tip (but also egg shaped)
- DELTOID: Triangular shape
- LANCEOLATE: Pointed at both ends, significantly longer than wide
- PALMATE: Hand shaped
- ODD PINNATE: Compound with paired leaflets and a single terminal leaflet
- EVEN PINNATE: Compound with paired leaflets and 2 terminal leaflets
- TRIFOLATE/TERNATE: Compound with 3 leaflets
- BIPINNATE: Compound leaf with compound leaflets
Terminology for the leaf apex:
- CORDATE: Heart shaped
- ACUMINATE: Long tapering point
Terminology for the leaf base:
- CORDATE: Heart shaped
- ACUMINATE: Long tapering point
The margin of a leaf is where both the highest rates of photosynthesis and evapotranspiration occur. The perimeter can also function in heat dissipation (as in the plates of a radiator or the sail of a spinosaurus). A leaf’s shape, or rather the blade’s area to perimeter ratio, then, is a function of its environment. Different shapes have different ratios, and as you can see in the table below, the rounder a leaf, the larger the ratio of area to perimeter is. The ratio drops significantly when the margins are concave (the equivalent of a leaf having teeth or lobes).
|Circle||31.8 sq in||20 in||1.6 : 1|
|Octagon||30.2 sq in||20 in||1.51 : 1|
|Square||25 sq in||20 in||1.25 : 1|
|Triangle||19.25 sq in||20 in||.96 : 1|
|Rectangle (2″x8″)||16 sq in||20 in||.8 : 1|
(with equilateral triangles as points)
|15.5 sq in||20 in||.78 : 1|
(with equilateral triangles as points)
|12 sq in||20 in||.6 : 1|
Because the margin of a leaf is where the most water is lost, having a higher ratio of leaf surface area to perimeter is useful in conserving water in dry environments. In Vermont, trees are limited more by our shorter growing seasons than water availability so trees can afford to lose a little moisture at the benefit of increasing photosynthetic rates. And indeed, all of our native trees have toothed and/or lobed margins. If you drove south towards warmer (and often dryer) habitats, you would see CONGENERIC species with relatively higher ratios. Compare, for example, our native oaks like northern red oak or bur oak with their deeply cleft lobes to the nearly entire leaves of the much more southern live oaks. As you go south you even counter trees that have entire margins, like catalpa and osage orange. Caution is advised with congeneric comparisons and conspecific comparisons are likely more accurate. For example, red maple leaves across their range show a strong correlation with mean annual temperature: those in colder habitats are toothier and have deeper lobes. California black oak leaf shape, on the other hand was not linked to temperature but instead to high wind and ample sunlight contributed to smaller teeth at higher elevations (link).
Leaf margin is also impacted by a leaf’s location on the crown of the tree. Sun leaves tend to have lower area:perimeter ratios than shade leaves. When exposed to full sunlight, sun scald and overheating can be a bigger concern than water loss. Adding extra perimeter to a leaf by having deeper cut lobes or teeth increases the leaf’s ability to dissipate heat. To counterbalance the potential for water loss, sun leaves tend to have much thicker cuticles (link).
There’s a spectrum from a perfectly smooth (entire) margin to the extreme of a compound leaf, and there’s a term for nearly every gradation between the two ends (I don’t include all types below):
- ENTIRE: smooth
- TOOTHED: having any sort of small projection
- CRENATE: wavy teeth
- DENTATE: symmetrical teeth
- SERRATE: teeth that point forward
- DOUBLY-SERRATE: teeth with teeth
- LOBED: indented less than half way to midrib
Terminology for the leaf margin:
- DOUBLY SERRATE:
Anatomy of a leaf
Generally, we can break down leaves into 2 broad categories: NEEDLES (found on conifers) and BROAD-LEAVES (found on flowering plants). The needles of conifers can have several different shapes (see below), but in general they have a thicker waxy cuticle and are darker green, evergreen, and stiffer. Broad-leaves, on the other hand, are deciduous leaves with large, round, and thin blades (see diagram below for a typical leaf cross section). Regardless of whether you’re looking at a needle or a broad-leaf, they share a common ancestor and many internal anatomical features. We can group the internal parts of a leaf by the various functions each leaf must perform:
|Function for Leaf||Structure in Leaf (Tissue)|
|1. Photosynthesis||Palisade mesophyll (ground tissue)|
|2. Gas exchange||Stomata (epidermis), spongy mesophyll (ground tissue)|
|3. Nutrient, water, and sugar transport||Xylem, phloem (vascular tissue)|
|4. Protection from water loss||Cuticle (epidermis)|
|5. Defense||Trichomes, glands, etc. (epidermis), resin pits (ground tissue)|
1. Photosynthesis in the Mesophyll
While some photosynthesis occurs in the bark of woody plants or on the PERICARP of fruits, the vast majority of photosynthesis occurs in the MESOPHYLL layer of a leaf. Epidermal cells are typically non-photosynthetic and light passes right through these. Beneath these cells is a row (or several rows) of vertically oriented photosynthetic cells densely packed with chloroplasts. Cells in this PALISADE MESOPHYLL utilize incoming light to construct sugars, but they’re not 100% efficient at capturing light and some of it wiggles right through. In leaves that grow in full sun, there are more layers of palisade mesophyll to harness as much of this light as possible. In shadier environments, a leaf may have just be a single layer of palisade mesophyll (as in the image below).
2. Gas exchange via stomata & spongy mesophyll
The epidermis of a leaf is pocked by tiny openings, called STOMA (pl. stomata). Each stoma is flanked by a pair of GUARD CELLS that can be opened or closed by pumping water into or out of these cells’ vacuoles. As water enters the guard cell, it goes turgid, forming a C-shape; the pair of cells together form an aperture. As water is drawn out of the vacuoles (e.g. at night or during a drought), the pore closes. Guard cells, curiously, contain chlorophyll (other epidermal cells do not), and it’s exactly not clear why. Possibly these light-sensitive pigments help the guard cells figure out when to open and when to close.
CO2 enters the leaf through the stomata and concentrates in the porous spaces of the SPONGY MESOPHYLL. From here it diffuses into the palisade cells where it is used to run the Calvin Cycle. Water also evaporates out of the stomata; up to 95% of all water absorbed through the roots evaporates from the leaves. EVAPOTRANSPIRATION might seem like wasteful management of water, but it serves a couple essential functions. First, as the water evaporates, it slowly draws more water up the trunk through the xylem. This water contains minerals extracted from the soil that are essential for growth. While the water evaporates, the nutrients remain in the leaf. Second, when water evaporates it draws heat away from the leaf; leaf temperatures in the summer are often cooler than the air. In the winter, when the ground freezes and the roots cannot replace water that would be lost through evapotranspiration, broad-leaf plants avoid desiccation by dropping their leaves. Conifers embed their stomata in grooves on the underside of needles, hiding them away from the dry, cool air.
3. Nutrient, water, and sugar transport in vascular bundles
Leaves need to move materials both into and out of the leaf. Diffusion across cell membranes can move materials, albeit slowly, and is only useful for moving materials less than a half dozen cells away. This is why non-vascular plants (e.g. mosses, liverworts) are short and grow in moist environments. Vascular tissue is responsible for the vast amount of movement of water, nutrients, sugars, and other compounds throughout the plant (see more on transport here). Roots pipe water, minerals, and stored energy up the trunk through XYLEM, while sugars and metabolites are piped back out of the leaf through the PHLOEM. In broadleaves, the strands of xylem are on top of the phloem in cross section (aphids and other insects with sucking mouth parts attack the phloem on the underside of the leaf). The vascular tissue is wrapped in a BUNDLE SHEATH. The bundle sheath is a ring of a single layer of photosynthetic cells that provide structural support to the leaf (particularly important since spongy mesophyll makes a leaf structurally weaker). The different arrangements of vascular tissue into a network of veins can be diagnostic for identifying trees.
4. Protection from water loss
Epidermal cells secrete CUTIN on the outer surface of the leaf to prevent water from escaping the leaf. The CUTICLE primarily functions in water retention, and its thickness is variable within a leaf or tree and across species. It tends to be thicker on the ADAXIAL surface where it is exposed to sunlight than on the ABAXIAL surface of the leaf. Likewise, on a single tree, leaves growing at the top of the canopy exposed to full sun (SUN LEAVES) will have thicker cuticles than those growing in the shade (SHADE LEAVES). So to is this mirrored across species, where trees that grow in drier and/or sunnier habitats (e.g. pines, oaks, aspens) tend to have thicker cuticles than their wetland or understory counterparts.
5. Defense with trichomes, glands, etc.
As plants are fixed in place, they have developed many different mechanisms by which they can protect themselves from predators, UV radiations, and temperature fluctuations. The epidermis of a leaf is directly exposed to these threats and has various means of addressing these challenges. TRICHOMES can densely coat a leaf, protecting the leaf from UV radiation by scattering light or may provide a physical barrier to deter herbivorous insects. Trichomes may also be irritants (as in the case of nettles). Glands on leaves may contain noxious metabolites that deter herbivores. Extrafloral nectaries on cherry leaves attract beneficial insects that control mite populations.
- Leaf Snap (tree ID app)
- Tree Finder by May Theilgaard Watts
- Virginia Tech Dendro page (link)
- Graphic of leaf shapes (link)
- Road dust induced increase of leaf temperature (link)
- Leaf shape and venation pattern alter the support investments within leaf lamina in temperate species: a neglected source of leaf physiological differentiation? (link)
- Sensitivity of leaf size and shape to climate within Acer rubrum and Quercus kelloggii (link)
- How strong is intracanopy leaf plasticity in temperate deciduous trees? (link)