Leaves

Gathering Plants
Youíll see more leaves on your field trips than any other plant parts, so understanding them will enhance your outdoor experiences. Leaves are solar panels par excellence: Only seven cells thick, they harness sunlight more efficiently than anything we've invented, forming the food chain's first link. Leaves can be oval, elliptical, sword-like, lance-shaped, arrow-shaped, or anything in between. They may be hairy or bald, tasty, bitter, acrid, or toxic. Their different shapes and forms reflect complex adaptive answers to environmental challenges: They must contend harsh weather, hungry vegetarians, and other plants competing for sunlight, while exposing the largest possible surface area to the sun.

So why donít plants with the largest leaves always win out? In the tropics, where the climate is comparatively uniform, they often do. But there are no benefits to exposing leaves to shade, so small leaves sometimes work better. Further from the equator, where weather varies more, large leaves are a liability. Storm winds can tear them to shreds. So leaves divide partially into lobes, or separate completely into compound leaves.

Palmate-compound Leaf
Pinnate-compound Leaf
Double-pinnate Leaf
Palmate-compound Leaf
Pinnate-compound Leaf
Twice-compound Leaf
Leaf configuration is basic to identifying plants. We call paired leaves opposite. Unpaired leaves are alternate. Leaves that encircle a stem are whorled. Donít look at the newest growth to determine leaf configuration, because the stem parts that would separate alternate leaves may have had insufficient time to develop. Look at older growth.

A few plants will try to fool you by having opposite leaves in some places, and alternate leaves in others. So look at a few parts of the plant. Always examine many samples in nature, to get a good understanding of whatís going on.

Basal Rosette
Double-pinnate Leaf
Basal Rosette
Double-Toothed Leaf
Another important leaf formation is the basal rosette, where all the leaves emerge from the ground at a common point. Many plants take this form in the cold weather, when growing tall would expose them to the elements, there are no taller plants competing for sunlight, and spreading in a circle captures the most sun. One of my first magazine articles was about cold-weather plants and basal rosettes. The editor consistently changed the spelling of "basal" to "basil," leaving the readers wondering where to find the recipe for spaghetti sauce.

Teeth are another feature that help you identify plantsósome leaves have serrated edges. If the serrations are serrated, the leaves are double-toothed. Double toothed leaves and twice or thrice-compound leaves are again themes of repetition on different scales, as described above.

The leavesí attachment to the rest of the plant provides further distinctions. Some leafstalks are long, others are shorter. Some, like winged sumac's, have flared edges. We call these stalks winged.

Some leaves have no leafstalks The leaves that partially surround the plant's stem clasp it. When the leaf base surrounds the stem, it looks like the stem is perforating the leaf

The above distinctions are for broad leaves, which include all leaves that arenít needles. Needles, which are narrow, come in different shapes and configurations too, and weíll discuss them as we cover the species that have needles.