The Science of Botanical Art

Leaves

By Dick Rauh

Originally appeared in The Botanical Artist, Number 35

 

I get a bit emotional when I talk about leaves. As far as I can figure out they are the basis of life, as we know it. Plants are what are known as autotrophs. What that boils down to is their ability to create food (sugars) from carbon dioxide and water, using the energy of the sun, a process known as photosynthesis. Certain plants can perform this function using sulphur compounds in place of water, but they don’t have as much to do with our food chain as the green plants I am thinking about. The rest of us are known as heterotrophs, either eating plants, or eating the animals that eat plants, or both. %But it all gets back to our dependence on this autotrophic function of vegetation and although various organs of the plant - stems and roots for example - do occasionally photosynthesize, the leaf is its primary source. Following the rules of economy that drive the natural world, the leaf has evolved into an ideal structure for this. A maximum surface area without unneeded thickness, pivoted for the most part on a flexible stem, allows for the most exposure to the energy-giving rays of the sun with a minimum expenditure of energy. Stomates, (small controlled openings) embedded in the surface of the leaf allow for the intake of carbon dioxide, and the release of oxygen (which is a byproduct of photosynthesis and another reason we have to be grateful to the leaf). To say this in another way, if it wasn’t for the leaf we wouldn’t be here. I don’t know how much this knowledge will advance your ability to paint leaves; I just want you to show them a little more respect in the process. 

Because leaves are so widespread in the plant kingdom, a vast vocabulary has grown up describing in minute detail their various aspects. Part of this is due to the need for scientists to identify characteristics in new species, and since leaves are so evident, distinctive and diverse (often very subtly so) they have become a valuable tool in the descriptions of new species. And guess what? If leaves are so important to our scientific friends for purposes of identification, they become important aids for us, in botanical art, for pretty much the same purpose (unless you are somewhat like me, and enjoy drawing and painting leaves for their own beauty). 

There are many aspects of leaves; arrangement on the stem, how they are attached, their form and shape, their texture and margins, each of which has its own set of terms. So it will probably be most meaningful if I discuss them separately, which means this article may spread itself over a couple of volumes. Most glossaries of botanical terms (i.e. Harris, J.G. and Harris, M., Plant Identification Terminology; or Hickey, M. & King, C., The Cambridge Illustrated Glossary of Botanical Terms) have sections devoted to the terms relating specifically to leaves. If you are curious after what will be a fairly general approach on my part, be my guest (figure out the difference between apiculate and mucronulate, for example) but for the most part as artists it’s the observation and accurate rendering of the details that are important. Let the taxonomists apply the terms. 

As usual it is the telling details that I want to point out, so that your drawing of a specific leaf will convince your audience of its rightness. First lets look at how leaves are arranged on a stem. Some are arranged on a spiral, with one leaf at a node, and this is called an alternate arrangement. The spiral is prevalent in many living patterns and this one is no exception, following the number series discovered by Fibonacci. Although this is a fascinating phenomenon, it’s the look produced by the variations in the Fibonacci numbers that is important to us. For example a beech tree has what is known as a 1,2 arrangement. What this means is that when you start with one leaf and find the next leaf directly above it, you will have created one spiral through one leaf to get there and you count the final leaf. In effect a 1,2 arrangement produces a rather flat and slightly zigzag branching pattern There is a whole discipline known as phyllotaxy that studies this phenomenon. Suffice it to say that this is yet another example of the order that exists in nature. 

Another arrangement finds two leaves attached at one node, an arrangement known as opposite. This comes in two variations however, and is something we as artists need to observe. There are plants where the opposite leaves more or less fall in two ranks, a condition known as opposite distichous, and again the appearance of the branch is rather flat and spreading, like dogwood. Other opposite arrangements have the succeeding opposite leaves at a 90 degree angle from the preceding ones (four ranks) a condition known as opposite decussate, and typical of the arrangement of leaves in the mint family. Another arrangement has more than two leaves at a node, known as whorled and typical of the leaves on sweet woodruff, and some Anemones. Some leaves, like the needles of pines, are gathered together in a group - a condition known as fascicled. For us it is not so much the names of the arrangement that count, but the close and accurate observation of the way it occurs in the plant we are illustrating and its final rendering. 

Look closely at how the leaf is attached to the stem, and we have another series of words that we will replace with images. A ‘typical’ leaf is made up of two parts, the blade or lamina, and the leaf stalk that is called a petiole. There is usually a swelling at the base of the petiole where the leaf is attached to the stem, and this varies depending on the species, but look for it. A ‘typical’ leaf is therefore petiolate. Sometimes in addition to the petiole and the swelling (known as a pulvinus), there will be leaf like appendages at the base of the petiole. These are known as stipules, and they are present in rose leaves, and very prominent in peas, and in many other genera. A leaf with stipules is ‘stipulate’ –big surprise, huh? But what about leaves that are missing stalks? Happens. In fact this is one of the characters that distinguish monocots and dicots. Dicots can go either way, but monocots are rarely petiolate. Anything missing a stem; leaves, flowers, or fruits are known as sessile. Now we have a progression that is worth noting, and obviously denoting in our art. Sometimes these sessile leaves don’t just meet the stem but wholly or partly surround the stem, called clasping - check out some lilies on this. Some leaves carry this further, not only surrounding the stem but with blade tissue that extends all around (perfoliate), often on plants that use the cup like extensions to hold water in dry or epiphytic conditions. A fairly common variation on this, especially in monocots, is where the blade completely sheathes the stem, covering the succeeding node before it eventually splits open out into a leaf. This is a manifestation of the single cotyledon (embryonic leaf) growing-pattern of the class with new growth forming on the inside.

  • Euonymus alata - the petiolate leaves are arranged in an opposite distichous manner on the stem. The leaves are elliptic with acute apices and bases
  • Euphorbia robbiae shows alternate arrangement of leaves on the stem- one leaf at a node spiralling up the stem in what appears to be a 3/5 Fibonacci sequence.
  • Anemone florist’s hybrid - whorled leaves (more than two leaves at the node) circle the stem
  • Rosa multiflora:The rose leaf is an imparapinnate compound leaf (terms we will discuss in the next issue), which has small leaf like extensions on either side of the petiole where it meets the stem. These are called stipules, and the leaf that bears them, either simple or compound is called a stipulate leaf.
  • Sedum spectabile shows leaf blades attached directly to the stem, wilhout petioles. This is called sessile, and the term can be applied to any organ attached to a stem without a stalk, for example a flower or fruit. In this species as in many plants that have adapted to xeric conditions the leaves have become succulent, and are heavily cutinized to provide storage for water with minimum evaporation