The Science of Botanical Art

Conifers

By Dick Rauh

Originally appeared in The Botanical Artist - Volume 17, Issue 1

 

Conifers are a fascinating group of plants. They arrived earlier on the scene than flowering plants and belong to a larger collection of primitive flora that includes cycads, ginkgos and gnetums. These are archegoniates that have developed seeds in place of spores, as the agent for reproduction. What makes them different and more primitive than flowering plants is that they carry their seeds uncovered, hence the name gymnosperms that, I am told, roughly translates into ‘naked seeds.’

Notice I slipped an unfamiliar term into the mix, and some of you may be aware of this (remember, all my students?). It is worth going into a bit. An archegoniate is a plant that bears a female organ called an archegonium in its gametophytic stage. This organ which holds the unfertilized egg and produces hormones to attract the male reproductive cell, is present in mosses, ferns and other members of the gymnosperms, but has been eliminated in the life cycle of the angiosperms.

The male version of the archegonia is called the antheridium, a tiny organ that holds the sperm in containers we call pollen. In conifers another advancement has taken place. Instead of the flagellate sperm that characterizes the earlier plants, conifers have evolved the pollen tube, a delivery system for the sperm built into the male gametophyte, although the cycads and ginkgos still rely on the swimming unit.

Excuse the digression, but the progression of solutions to environmental and natural problems that this represents is one of those wonders that I am constantly brought short by. Improve on a spore, dependent on the wind to find a place ideal in moisture and temperature in order to be fertilized and germinate and so invent a seed that encloses a fertilized embryo and food to support it, but just let it lie on the surface of a scale. Figure out that water, necessary for the flagellate sperm to reach its egg, is not a practical method, when you are thrust in the air, so you substitute pollen. Here the special reproductive process of cell division provides not only sperm but an enclosing cell that grows a tube to bring the sperm to the egg. Flowering plants have taken these technological developments even further, and thus have become even more successful than their predecessors.

Conifers may be small in number as far as species are concerned with only some 600 to 630 named extant, but they make up for that in huge populations in areas where they dominate, primarily in what is called the boreal forest. Most have made successful adaptations to the cold, and that accounts both for the shape of the trees (conical with drooping branches to handle snow) and the type of foliage – highly cutinized needles with stomata on the lower, protected side and an evergreen habit, in most cases, to minimize the expense of annual leaf replacement.

But what is it that we as botanical artists should be looking for as we paint conifer details, and leave the depiction of the forests to the landscape artists? Conifers are monoecious or sometimes dioecious, which means that they produce both staminate and seed cones, usually on the same tree, but sometimes on different ones. There is a major structural difference between the way these cones differ. The male cones are simple, the scales holding the microsporangia that are the conifer equivalent to the pollen sac, on the side facing away from the axis (abaxial).

The female cones on the other hand are compound, the ovule- bearing scale having a bract on its abaxial side and the megasporangium that consists usually of two egg-bearing ovules on the side facing the axis (adaxial). In most cases this bract is minimal, and will not be obvious in a painting. On the other hand check out the cones of a Douglas fir, where the bracts are well exerted and make a great addition to the character of the strobilis. These ovules, that become the seeds when fertilized, mostly have wings, like the samaras of the elm and are just lying there on the scale, not fused or protected by any other structures the way flowering plants hold their ovules and seeds within the ovary. If the cone you are painting still bears the seeds, they can be released by a vigorous shaking.

The male cones bearing pollen tend to be smaller, more abundant but highly evanescent, appearing in the spring, releasing gales of pollen and then disintegrating. The ovule bearing cones are the dominant reproductive structures and vary in shape as they mature, in some cases taking years to do so. In the genus Pinus, the pines, these are usually woody and their shapes and sizes when mature are a strong identifying character. Besides being a very visible representation of the interlocking spirals that bespeak Fibonacci’s sequence, the scales bear certain features that we need, as artists, to observe and render.

When the cones are immature and the scales lie tight against one another the area of each scale that is exposed is known as the umbo. When the cones open to release their seeds this shape forms a distinct area on the upper part of the scale, often thickening  and growing a bump or in some cases a spike or hook-like protuberance. This character is an important defining structure in the  identification of the many species of this genus. A painting of a pine branch, showing male, immature and mature seed cones as  well as needles and scale leaves would be a tour-de-force and this does happen on the same tree, depending on the species.

Pine needles are held in clusters or fascicles on short shoots or spurs, and the number of needles to a fascicle is another defining character. Although there can be any number from one to eight, the most common native species have 2, 3 or 5 in a bundle. The needles of other common genera in the division are notably smaller, and except for larches grow singly. Larches, which are deciduous, bear their short needles in bundles of 10 to 40 on short shoots similar to pines, and the true cedars – the genus Cedrus, also carry their evergreen needles in bunches on spurs. These prove a challenge to draw, but like so many difficult problems can be doubly rewarding when successful.

As usual I have gone on without covering half of what I intended- like a general look at the taxonomy and the major differences between some of the common genera, so it looks like I have enough material for yet another article. So see you next time and if  you have any specific questions on this fascinating group please let me know.

Copyrighted by the author. Not to be downloaded, copied, reposted or republished elsewhere.

  • Gray Pine Cone and Needles, Pinus sabiniana, Graphite Pencil, (C) Suzanne Olive