First of all, leaves A and B have pretty much the same structure, right?
Wrong. Leaf A is the pinnately compound leaf of Hercules-Club, Zanthoxylum clava-herculis, bearing seven leaflets, while "Leaf" B is actually a stem from a Privet Bush, Ligustrum ovalifolium, bearing about 20 simple (undivided) leaves. I know that each of the Privet's green things is a leaf because at the base of each leaf's petiole there is a bud harboring nextspring's leaf, and you'd never find a bud at the base of a leaflet, which is just a part of a leaf.
Now for something easier: Leaf C is just a normal simple leaf("simple" is the proper way to refer to a leaf that is not compound, or separated into distinct leaflets). It's the 5-lobed leaf of the Sweetgum tree, Liquidambar styraciflua.
Leaf D is another compound leaf, but it's compound in a special kind. When compound leaves just have three leaflets, there's a special name for it. They are trifoliate leaves. This is the trifoliate leaf of a weed called Tick-Trefoil, Desmodium sp.
Leaf E has five leaflets, but notice that its leaflets are arranged much different from the seven leaflets on Leaf A. In Leaf A, the leaflets arise from a rachis,which is the continuation of the leaf's petiole -- the stem-like thing attaching the leaf to the twig. However, in Leaf E there is no rachis. The five leaflets all come together in one place. They are held like the fingers, or digits, on a hand. Therefore, we say that this special kind of compound leaf where all the leaflets arise in one spot are digitally compound. Leaf E is the digitally compound leaf of a Dewberry, Rubus sp.
Leaf F is the trifoliate compound leaf of the herb known in both English and Latin as Oxalis.
Leaf G is a compound leaf in which the lower leaflet sections are themselves divided into leaflets. Leaves with leaflets that are themselves divided are referred to as being bipinnately compound. There are even tripinnately compound leaves, but I couldn't find any during my walk. Leaf G is the bipinnately compound leaf of the Pepper-Vine, Ampelopsis arborea.
Even simple leaves can have lots of interesting ways of being. The above leaves, also picked up right around my home, show a variety of leaf margins.
Leaf-margin A is the crenate margin of the Swamp Cottonwood tree, Populus heterophylla.
Leaf-margin B is the spiny-toothed margin of the American Holly, Ilex opaca.
Leaf-margin C is the entire margin of a Greenbriar, Smilaxsp.
Leaf-margin D is the shallowly lobed margin of an immature leaf of the Black Oak, Quercus velutina.
Leaf-margin E is the dentate margin of a wild grapevine, Vitissp.
Leaf-margin F is the doubly toothed or twice serratedmargin of the Eastern Hophornbeam, Ostrya virginiana.
And the real neat thing about leaf variability is that here we've just scratched the surface. We've only considered the normal things leaves do, and there's a whole world out there of other configurations, margin types, sizes, shapes and unexpected permutations, and each manner of being has its own special terms describing it! For example, can you figure out what's going on at the right?
Well, this is a single pinnately compound leaf, about life size, of one of the most common plants right outside my door, a kind of wild vetch, in the bean family. Notice that the leaf's midrib extends beyond the top of the leaf and divides into three slender appendages. These appendages are tendrils. If you watched the tendrils for several hours you'd realize that each tendril is twisting around veeeeeeryslowly, so slowly you can't see it happening. When a tendril touches something, it wraps itself around it and hangs on. This enables the vetch plant to support itself as it grows upward, hanging on to other things around it, usually other plants.
Finally, not only are leaves put together in a real rainbow of configurations, but also they are positioned upon stems and twigs in certain ways. In fact, when you are confronted with a new leaf, a good way to begin your thinking about them is to ask yourself two questions:
At first glance, the leaves and stems in the photo above are pretty similar, especially since both are vines (that's Supple-Jack, Berchemia scandens, on the left, and Honeysuckle, Lonicera japonica, on the right). However, notice that Supple-Jackon the left has alternate leaves, while Honeysuckle on the right has opposite ones. Can you see it?
Noticing this detail can be very helpful in identification. For instance, in most of eastern North America tree identification is helped when you remember the term "Madcap horse." This term helps you remember eastern North American trees with opposite leaves. "M" in madcap is for maples, "a" is for ash, "d" is for dogwood, "cap" is for Caprifoliaceae (the plant family in which you find viburnums), and "horse" is for the Horsechestnut Family, which has the buckeyes in it.
Well, it's kind of a clumsy trick, but when you're trying to remember, it's pretty neat.
At the right you see Mullein, Verbascum thapsus, a common roadside weed in much of North America. The plant in the picture is at the end of its first year of growth. "First year," because Mullein is a biennial -- it lives for two years. During its first year it forms the rosette you see, storing energy in the form of carbohydrates in its taproot and leaves. At the beginning of its second year of growth a stem emerges from the center of the rosette, grows very fast, and when summer comes the stem holds the plant's flowers high, where pollinators can find them easier. The fruits also will be held higher, where birds will find them easier, and disseminate them more. Thus an advantage to being a biennial is that during the plant's second year of growth the stem, drawing upon the energy stored in its taproot and rosette leaves, can overtop surrounding fellow weeds who are not biennials and have no stored energy to tap.
A plant living for only one year is called an annual. A plant living for more than three years is called a perennial.