As we walk our trails at Hilton Pond Center during most of the year, we're constantly reminded the background color of the Carolina Piedmont is green. In fact, except for polar, desert, and high mountain regions of the world, green is the predominant color of terrestrial habitats. All this green, of course, comes courtesy of vegetation that surrounds us and--more specifically--from chlorophyll that is the lifeblood of diverse flora from lowly mosses to climbing vines to towering trees. Most green plants have leaves (or leaf-like structures) that expose chlorophyll to sunlight and enable the pigment to perform photosynthesis--an elegant chemical process that combines carbon dioxide and water while giving off oxygen and making simple sugars plants use as food. A few plant species can photosynthesize through stems and trunks, but for the most part it is the leaf of the plant that contains chloroplasts. It's interesting to consider that despite their basic similarities, there are almost as many different sizes and shapes of leaves as there are plants--a pretty important concept when students making leaf collections for biology class are basing success on an ability to identify trees solely by leaf configuration.

Despite their diversity, there are two basic leaf types--"simple" and "compound"--and simple leaves can be grouped further into those that are "entire" or more or less "lobed." Foliage of Flowering Dogwood (above left) is a good example of a simple, entire leaf, while that of White Oak (above right) is simple and lobed. Although simple leaves are interesting, we're more fascinated by compound leaves and ways in which they are adaptive for plants that bear them.

A leaf typically is attached to its stem by a petioles, sometimes known as a "leaf stalk." In a simple leaf, the petiole--variable in length among species--gives rise to the leaf blade; this blade is most often symmetrical and flattened and sticks out on either side of the mid vein, as in the simple, lobed Sweetgum leaf above.

In compound leaves, however, the blade is dissected into leaflets, so the petiole gives rise to "petiolules" that in turn hold the blades of the leaflets. If we look at the evolutionary history of leaves, it seems likely the first ones were simple and entire and that they sooner or later developed lobes. Eventually, some leaves became so deeply lobed the remaining portions of the blade were separated into what are now leaflets. In a simple, lobed leaf the main central vein has lateral extensions to each side that reach the rest of the blade; in a compound leaf such as Shagbark Hickory (above) the central vein has become a "rachis" and side veins have toughened to become petiolules. These transport water and nutrients to--and sugar from--multiple outlying leaflets.

If simple leaves are sufficient for plants that have them, one might wonder how compound leaves would be advantageous. That's something we've pondered quite a bit and have considered a few hypotheses. Most of them boil down to the premise that "there's more than one way to skin a cat"--or, in this case, to capture sunlight for photosynthesis.

It might seem logical that if a leaf is the main way a plant presents its chlorophyll for photosynthesis, the bigger the leaf the better. A really big blade would have lots of surface area for gas exchange and transpiration of water, and it would be like a big solar panel collecting sunlight. But an over-sized simple leaf also would have disadvantages--especially in areas subject to cold weather. It takes lots of energy to make big leaves, and since most temperate trees are deciduous, that would mean devoting a huge amount of resources to making huge new leaves each spring. For example, at Hilton Pond Center this week we found a fresh new Black Walnut compound leaf (above) that was 19 inches by 9 inches at its longest and widest points. If instead of being dissected into 11 leaflets it had been a simple, entire leaf blade it would have been enormous and would require a major effort to produce. (It's interesting that on our hummingbird expeditions to Central America we've observed many trees and shrubs with giant leaves, perhaps because their foliage is evergreen and doesn't need to be renewed in entirety each year.)

We also suspect big simple leaves might provide so much green mass that herbivores would find them irresistible, meaning it might be better to have many smaller leaves--or compound leaves with parts of the blade "missing." And think of what a summer storm would do to a huge but relatively delicate leaf blade when wind gusts got really strong. (In fact, Dr. Steven Vogel tested leaves in wind tunnels and discovered compound leaves typically had much less stressful fluttering and eventual tearing than did simple leaves.)

Perhaps the best rationale for compound leaves comes to mind when we look at a compound-leafed plant from sun's-eye-view. In our photo of a young Winged Sumac shrub (above), we can see its compound leaflets branch off the main trunk not directly beneath each other but from different angles. This means the topmost newer leaves are not directly blocking the sunlight from the older leaves below. And--more important--since the leaves are compound rather than a big, entire blade the amount of shade they cast is diminished even more. Lots of simple leaves branching alternately from a central trunk might have the same effect, so compound leaves are likely just a different way of solving the shading problem.

Although it seems the majority of plants have simple leaf configurations, it's surprising how many examples of compound leaves can be found when one starts looking. Below is a sampling of compound-leaved flora we photographed with little effort at Hilton Pond Center, and there were quite a few more specimens for which we didn't use our camera. How many examples of compound leaves can you find in your own backyard?

In plants with "palmately compound" leaves the leaflets radiate from the end of the petiole, creating a hand-like configuration. One example of this is five-leaflet Virginia Creeper (above). Many folks confuse this native vine with Poison Ivy, which has only three leaflets.

"Pinnately compound" leaves exhibit wide variety, but in all the leaflets are arranged along the mid-vein. A few are "even pinnate" without a terminal leaflet, while most are "odd pinnate" and bear a leaflet at the end of the rachis--as in the native Trumpet Creeper vine (above).