Leaves: External

Although for some it might be interesting to know the terminology associated with leaf shape ( and especially helpful if you are trying to key them out), most of us would rather not be expected to assign a name to a form...Why this page exists is to encourage you to think through why such shapes exist.

At the same time, this is not to say we can predict a functional association with every form...some shapes may be historical relicts or even mutations..however, there appears to be some logic for many of these permutations. Consider a few possibilities given below, then work through some of your own:

1. Drip tips: when the leaf tip points sharply at the end is thought to be an adaptation for rapid movement of water off of a leaf; in wet climates this may prevent water standing on the leaf encouraging microbial/fungal growth as well as leaching of nutrients out of the leaf.

2. Cooling: thin, larger leaves may move more readily when a breeze comes by: with the movement, the boundary layer is disturbed encouraging more rapid transpirational losses and thus cooling. Reverse logic states that smaller, thicker leaves may be do better in drier climates ( less surface area to volume and more structural strength.

3. Lobing: the more lobed the lead ( edges scooped in), the more light can reach the leaves below. This is important when you consider that in a forest, with light absorption occurring rapidly through layers of vegetation, there might reach a point where there might not be enough light to support the respirational needs of a leaf ( photosynthesis<respiration).

In some cases, the petioles of leaves move bend towards the light, with the leaf tracking the sun to maximizes light interception. Which leaf shape would best support this?

4. Sun vs shade leaves: on the same plant, leaves in the shade tend to be thinner( only 1-2 layers of palisade parenchyma) but broader, while those in the sun will have more layers of palisade, be thicker and sometimes not as wide.

5. Leaves in windy areas vs stable air masses: what would predict here?

Obviously these are only a few of the critical considerations... can you come up with more?

****Next, check out the graph on this next page ( don't copy it necessarily) taken from: http://www.mobot.org/MOBOT/education/tropics/page3.html Predict what leaf shpae would be found in each of the the climactic zones. This will be a mini-test question.... explain your prediction

lanceolate

length greater than width, broadest at the base, narrowing to the apex, lance-shaped

Fraxinus pennsylvanica

ovate

more or less rounded at both ends and broadest below the middle, egg-shaped

Dogwood tree

elliptical

ellipse-shaped, narrow at each end, widest at the middle

Cotoneaster dammeri

cordate

more or less rounded at both ends and broadest below the middle, egg-shaped

Cercis canadensis or red bud

elliptical

ellipse-shaped, narrow at each end, widest at the middle

Nyssa sylvatica; black gum tree

oblong

longer than wide, sides parallel, rectangular with rounded corners

linear

parallel sides, long and very narrow

Hemerocallis or day lilies

auriculate

having auricle(s) -- ear-shaped lobe(s) at the base of leaves or petals

Arabis caucasica

obovate

inversely ovate, broader above the middle than below

Myrica pensylvanica

oblanceolate

inversely lanceolate, broader above the middle and tapering to the base

Ilex glabra or holly

spatulate

oblong with the basal end narrowed and the apical end rounded, spoon-shaped

Myrica pensylvanica

needle

slender leaf of many conifers

Pinus sylvestris

palmately lobed

three or more lobes radiating fan-like from a common base

Acer saccharum or sugar maple

pinnately lobed

segments or lobes along each side of a common axis

Quercus rubra or red oak

awl-like

tapering from the base to a sharp point

Additionally there are four basic kinds of leaves themselves: simple, compound, peltate, and perfoliate:

1. Simple leaves-have a flat, undivided blade that is supported by a stalk called a petiole. The petiole is typically supported by collenchyma and sclerenchyma fibers. Cercis canadensis or the red bud

2. Compound leaves-have blades divided into leaflets that form in one plane and lack axillary buds. Each compound leaf has a single bud at the base of its petiole. There are two kinds of compound leaves: pinnately compound and palmately compound leaves.

Pinnately form in pairs along a central stalklike rachis, and palmately attach at the same point.

3. Peltate leaves-have petioles that attach to the

middle of the blade.

4. Perfoliate leaves-are sessile leaves that surround and are pierced by the stems.