The Science of Botanical Art

Leaves Redux

By Dick Rauh

Originally appeared in The Botanical Artist - Volume 15, Issue 4

 

One of our members asked if I would do another piece about leaves. After reviewing the couple of articles I wrote a few years ago, I find myself hard-pressed to come up with enough new information to fill a page. There are a few areas that I didn’t talk much about and there are a couple of new sources that are worth looking at, but I am not sure how much this information is going to shape your attitude and artist’s eye. I am inspired by a new book* on the architecture of leaves, which attempts to clarify and codify the process of describing leaf form, and a recent article in the New York Times Science section** which stresses the importance of leaves in the evolutionary history of plants.

In 1790, the German poet Goethe (who in his own right was an outstanding observer of the natural world) wrote an essay entitled “The Morphology of Plants.” In this he proposed that the origin of all floral organs were leaves, and stated that, “from first to last, the plant is nothing but a leaf.” Now we are finding out that his thoughts were prescient. Evolutionary biologists are discovering, first with experiments on the small cabbage family plant Arabidopsis, and now in work in the poppy family where the petal form arose independently, that the four organs that make up the flower, calyx, corolla, androecium and gynoecium, all were derived at some point from leaves. I have been teaching this in my classes for 15 years now, and it is rewarding to find that more and more molecular studies seem to affirm this theory.

Here is the thinking, not only with petals, and sepals that most obviously display the possibility of leaf derivation, but also with stamens and pistils. Common genes, or ones that have proven to be strikingly similar, appear to shape the process of petal and sepal formation in the flowers that have been studied so far. It seems, for example, the genes that create petals are controlled by a toolbox of other genes that can affect the properties that distinguish different families. Apparently the strategy of creating flower variety parallels a similar strategy in the animal world, where tool-kits of different controlling genes work on appendage-building genes creating all the variety of legs from insects to mammals.

Now that I write this I find leaves playing significant roles that are not usually thought of. Even in lower vascular, seedless plants, it is the leaf that carries the organs, the archegonia and antheridia, which in turn carry the male and female gametes (the egg and the sperm). With mosses, it is the leaves at the apex of the shoot that bear these reproductive organs, and with ferns it is the leaf-like prothallis or gametophyte that performs the task. In the cycad genus Cycas, a flush of leaves bears the sporangia, or spore cases, which in these cases bear the egg and pollen. Because cycads are dimorphic (having separate plants that carry the male and female organs) there are different appearing leaves for each. These leaves that bear the spores, with the spell-binding names of megasporaphylls and microsporaphylls can be followed through intermediate stages in the more primitive flowering species as they develop into the pistils and stamens of the flowers we paint today.

What scientific thought is more and more focused on, as a possible source of the common ancestor of flowering plants, is the group of seed-forming but flowerless plants we call the gymnosperms (a name that is currently under siege, but I use for its convenient familiarity). This group includes the cycads as well as conifers and gingko. The great mystery that botanists have never successfully resolved – to find where flowers came from – appears to be getting closer to solution, because of these genetic investigations of the adaptations of the leaf into various other floral parts. It is interesting, too, that some leaves have gone another step or two in adapting. Not just into sepals and pistils, but leaves have become the spines and areoles in the cactus family, tendrils in some vining legumes, the traps of insectivorous species, and the storage organs of onions and other bulb forming monocots.

The new book Manual of Leaf Architecture is aimed at scientists describing leaf characters and at paleobiologists trying to make sense out of leaf fossils. Well documented with drawings and photographs it presents a criteria of terms that is hoped will clarify and standardize the process. Most of the categories are already familiar to us, and these can apply to us as artists observing and rendering leaves. This regimen of character description that they suggest a scientist should follow, are also the ones we, as artists, should be asking ourselves as we draw leaves. To paraphrase: how are leaves attached, arranged on the stem, organized into simple or compound forms? When present, what do the lobes look like, and how about the margins? Are they smooth or toothed in some way? And what about the surface texture – Smooth? Shiny? Hairy? Rough? %While the book doesn’t stress this, often the surface quality of leaves varies from the topside to the underside, and this is worth noting. The terms here are also something to keep in mind, because if you start talking to a scientist about the top or back of a leaf you may not be getting through. Think of a leaf and its relation to the stem. The surface of the leaf blade that faces the axis is called adaxial, and is usually the upper surface. The surface facing away from the axis is abaxial, usually the underside or back of the leaf. This thinking also applies to the axil of a leaf, which describes the angle between the adaxial side of a leaf and the stem. This accounts for the axillary bud, which nestles in that angle and is present in most leaves if you look closely enough. I have often wished that this distinction was made with more than a single letter, but that’s the way it is, and we need to live with it and not get confused.

There is one position on a leaf that the book uses frequently, mostly to give scientists a specific spot from which to take measurements.

This is what they call the insertion point, the place where the petiole meets the blade. I have never thought much about that particular place, but it might be an important reference position when we are drawing. I have tended to use the leaf stalk, and its extension, the midrib of the blade, as a helpful guide in planning the rendering of a leaf, especially one with extreme foreshortening. Attention to that insertion point, useful in ascertaining the relation of blade size to stalk, might add another strength to accurate drawing, as we ask where it appears on that guide line, visible or not from our perspective.

In all of this, the descriptive words that are so important to scientists are only secondary for us. The most important thing I can stress about this architecture, is that we pay as close attention to the appearance of the leaves we draw, as we do the flowers or fruit that accompany them, and give the leaf the respect Goethe believed it deserves.

*Manual of Leaf Architecture Ellis,Daly, Hickey, Johnson, Mitchell, Wilf and Wing; Comstock Publishing Associates

** “Where Did All The Flowers Come From?” Carl Zimmer, New York Times September 8, 2008

  • Adaxial view of Polygonatum biflorum, Solomon’s Seal, watercolor, ©Dick Rauh 2002