Tree Anatomy: 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:

1. A simple leaf has a bud at the base of the petiole, leaflets lack a bud
2. 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).
3. 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.

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.

• 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

• CORDATE: Heart shaped
• ACUMINATE: Long tapering point
• ACUMINATE
• ACUTE
• CUSPIDATE
• EMARGINATE
• MUCRONATE
• OBCORDATE
• OBTUSE
• TRUNCATE

• 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).

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

• ENTIRE:
• CILIATE:
• CRENATE:
• DENTATE:
• DENTICULATE
• LOBATE
• SERRATE:
• DOUBLY SERRATE:
• SERRULTE:
• SINUATE:
• SPINY:
• UNDULATE:

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: