The Science of Botanical Art

More Leaves 

By Dick Rauh  

Originally appeared in The Botanical Artist, Number 36

 

Leaves come in basically two forms; simple or compound depending on whether the leaf is an entity on its own, or is made of a number of leaflets. Simple leaves are either ‘entire’, which in this case means unlobed, or lobed. Lobed simple leaves with the lobes branching out from a single point at the base of the lamina, like maple and sweet gum, are called palmate. Lobes arranged vertically along the midrib, like a feather, are called pinnate - characteristic of most oaks and holly. The leaflets of a compound leaf are similarly arranged; palmately compound, like Virginia creeper or poison ivy, or parapinnate (or imparapinnate). Imparapinnate means the leaves are arranged along the midrib, ending in a single leaflet at the apex for an odd number. Roses, hickory and ash have imparapinnate leaves. Much less common are compound pinnate leaves with even pairs – two leaves at the apex. Honey locust is the only example that comes to mind. The question often comes up whether a seemingly compound leaf is actually a group of simple leaves (winged euonymus always gives me pause, for example). Look for the swelling at the base of the petiole for confirmation. Getting technical, the main stalk of a compound leaf is know as a rachis, and the individual stalks to the leaflets are petiolules, so don’t say I didn’t tell you. 

The basic shape of a leaf is another way to describe it, along with the shape of its apex and base. A scientist wanting to describe his leaf in words (a requirement of the World Botanical Congress) refers to a geometric table that uses variation on basic shapes; circle, triangle, square based on the relationship of height to width. A circle with a height to width ratio of 2 to1 or 3 to 2 is considered an ellipse for example. 

Keeping in mind that we are dealing with natural forms here, one never achieves the perfection of a mathematical figure, but there are leaves where the sides are more or less parallel, and these are considered oblong in this context. An egg-shaped leaf is ovate, if the wider part of the egg is lower on the leaf. If it’s the other way round, with the wider part of the egg nearest the apex it’s obovate. The use of the ‘ob’ prefix also goes for other shapes (like the triangle or deltoid), which appear upsidedown in their natural state. Long and narrow leaves are ‘linear’ and leaves that take on other familiar forms bear those names, ‘cordate’ or ‘cordiform’ is heartshaped, ‘reniform’ is kidney-shaped and ‘lanceolate’ lance-shaped. A peltate leaf is round with the petiole attached to the lower surface, as in nasturtiums. 

The ellipse is probably the most common leaf shape. Where the variations occur is in the way the apex and base vary. If the apex comes to an angle from about 30 to 50 degrees it’s ‘acute’, a narrower angle is acuminate and so forth. This insistence on a vocabulary of extreme subtlety is important if you are describing a leaf in words, because so often the difference between species comes down to very minor but vital differences in such things. 

For us, as artists it is our eye that we have to rely on. The questions we need to ask are about features that indeed have names and if we are following a text provided by a scientist/client they become important. Here are some to think about. What kind of a point does the leaf come to? Do the apex and the base seem similar? What is the relationship of an extended point (if it exists) to the rest of the leaf? Is the line that forms the angle straight, or curved in? If there are points or lobes, at the base of the leaf are they pointing down, (sagittate) or out (hastate)? Are they even or uneven (oblique)? 

Another variation that it behooves us to depict is the texture, or quality of our leaves. Some are waxy, heavily cuticled and have a hard, leathery appearance, perhaps with a high shine. Some are softer or papery, with the secondary ribs quite subtle, or raised and very obvious in others. Sometimes the area enclosed by the ribs is flat, others have highly rounded surfaces. Most leaves have a local color, and whatever form we need to show, rarely vary from very light to dark. Some leaves display mottling, or vivid surface patterns, remember that these are indeed on the surface and should never destroy the basic form of the leaf. 

Another aspect of leaves that cries out for our attention is the margin. Some are smooth (entire), but most have some minor lobing, not to be confused with the major palmate or pinnate lobing of leaf forms. If the lobes are rounded the margin is crenate, if the lobes are pointed and more or less point out from the midrib they are ‘dentate’, if the pointed lobes aim towards the apex they are serrate. In typical botanic doubletalk, smaller variations of these basic margins are crenulate, denticulate or serrulate. Again this is to make the written descriptions more specific, for us it is only important to see and depict the proper relationship of margin to leaf. 

There are certain adaptations of leaves that bear a word. In the compound leaves of some legumes that are climbing plants, some of the leaflets have evolved into tendrils, with a loss of blade tissue in order to enhance the clasping function of the remains. In carnivorous plants it is the leaves that have evolved into the insect catching organs. In Venus flytrap the ‘traps’ are leaves equipped with three trigger hairs and a barb-like margin at one point, that interlaces when the trap snaps shut. The leaves of the sundews are paddle shaped and covered on their top surface with glandular hairs that exude a very sticky substance. When the unsuspecting prey is fighting to free itself, it triggers the leaf to fold in and digest the insect. 

The most beautiful leaf variation in this category is the special leaf of the pitcher plant. Here the leaf is formed into a slender chalice, the upper part folded over to form a canopy. Inside the vase, sides are covered with downward pointed hairs, leading to a pool of digestive juices. In some species, towards the place where the deadly fluid awaits its next meal the walls of the leaf are translucent, giving the hapless insect the false hope he is reaching freedom. When it comes to storage organs the bulb is the only one formed of leaves. If you look closely at tulip or lily bulbs you will see a flat plate-like layer between the bulb and the roots. This is stem tissue, to provide reproductive function, but it is the leaf that provides the storage. 

  • Asclepias tuberosa - The leaves here would be considered linear or lancelate, with acute apices.
  • Alliaria peltata, early spring Note the kidney shape of the leaves (reniform) and the margin with small rounded lobes (crenate)
  • Nasturtium leaves. This is an example of peltate leaves- the petiole is attached on the back (dorsal) surface and the leaf is round