The Science of Botanical Art

Trees 

By Dick Rauh

Originally appeared in The Botanical Artist - Volume 14, Issue 2

 

Although trees are one of my favorite drawing subjects, botanically, trees are a little difficult to talk about. The concept of ‘tree’ refers to one of a number of options under the general category of habit. Habit is a term that describes the appearance of a plant; basically, how does it look as it grows. There are two choices here. A plant can either be woody or herbaceous.

Herbaceous plants confine growth to apical meristems that direct growth upwards or laterally, or in the case of root meristems, downward. Meristems are the plant equivalent of the stem cells we have been hearing so much controversy about in the animal world. They are small, actively dividing non-vacuolated cells that have the ability to differentiate into any tissue type depending primarily on where they happen to end up (Location, location, location!). For example, the cells formed at the center of a shoot apical meristem become the vascular tissue, those cells systems in a plant devoted to the movement of water and nutrients. The cells formed at the outermost area become the epidermal tissue, and those in between, the body or cortical tissue.

Meristems are the only actively dividing cells in a plant Once cells formed by the meristems have reached their final size and type, they no longer divide. Most of these cells are alive, with a full compliment of nuclei and organelles, some with special functioning. Some tissue systems are made up of dead cells. One of these is the xylem, that part of the vascular system that ends up as vessels, a series of open tubes that bring the water up from the roots, and in trees it is these vessels that form the heartwood.

Woody plants add another dimension by also growing in girth. This in turn is dependent on a special type of meristem. This exists in a one-cell thick cylinder that surrounds the stem called the cambium layer. The cambium layer initiates xylem cells to the inside and phloem cells (the other cell type that makes up the vascular tissue) to the outside. Outside of these, another cambium ring (the cork cambium) initiates bark, the distinguishing feature of woody plants. The whole process of girth expansion in plants is given the name ‘secondary growth’.

Actually there are only about three types of woody habits: trees, shrubs and woody vines that are called lianas. Here is where it gets a bit messy. The term tree includes the gamut from the smallest birches to the mammoth redwoods, generally tall, perennial, erect plants. Most have a single stem, but there are a number of large trees that have multiple trunks. But what about the smaller many stemmed plants? When do these become shrubs and when are they trees? This problem becomes confounded by the fact that certain species will produce trees under one growing condition and shrubs under another. If it strictly a question of height where do we draw the line? Thirteen feet? Fifteen feet? Be my guest. When it gets to this questionable group even the taxonomists disagree.

For the moment let’s look at those local plants that we have no trouble calling trees. We can divide them into deciduous or evergreen; meaning whether they hold their leaves season round, or shed them after the growing season. Most coniferous trees are evergreen, although larches shed their needles. Large-leaved deciduous trees are flowering plants whereas all coniferous trees belong the more primitive gymnosperm group. Although a few families, Fagaceae and Betulaceae, for example, concentrate almost exclusively on deciduous trees, most families do have some trees within their habit forms, even though these are often tropical when they occur. Solanaceae, the nightshades and Fabaceae the peas, all have genera that have tree species.

There are also some examples of forms that we call trees that do not fit our botanical definition and remain herbaceous in spite of a tree appearance. Palms and bananas do not grow in girth, have no cambium layer, nor do they have bark. It is an accumulation of leaf bases that provide the strength for these plants to grow tall and have acquired the title trees, even when they are just great big herbs.

Over the years there has been a discipline of botany that has concerned itself wholly with this particular habit and its domain is primarily environmental. It is called dendrology. Description of tree architecture has been one of the facets of this discipline. If you need to discuss populations and other ecological issues you first need to define your terms, and it is these basics that most interest us.

The dendrologists divide tree architecture into four categories. The first with a central stem and a conical crown is called ‘excurrent’, and it includes the trees of northern or alpine environments, primarily conifers and some deciduous trees such as sweetgum and tuliptree.

When the lateral branches grow as fast or faster than the terminal shoot, the result is known as ‘decurrent’ or ‘deliquescent’. This with all its variants is the type we associate most readily with our concept of ‘tree’. Oaks, maples, elms are all decurrent in form, as are the majority of our northern deciduous trees and even some temperate conifers.

The third category is ‘palmlike’, with an unbranched trunk and leaves in a top rosette, which describes the palms and cycads. These are treelike but not truly within the woody category.

The fourth really finds me wondering what they were thinking about. It is called ‘yucca-like’, with either a basal rosette of long spiky leaves and a central flowering stalk, or an irregularly branched trunk as in the treelike yuccas and cactus. How or why this last comes to be included in the category of trees I cannot fathom. Here again is an example of why trees are difficult to discuss from a purely scientific angle.

Most important for us is the effect of the environment. Like all living organisms their growth habit is influenced both by the inside and the outside, their genetic makeup and the physical conditions they face. The typical architecture of a particular species, the diagrams you are apt to see in the guide books for example, are very much subject to where and how the tree grows. A tree is much more apt to conform to the expected shape when it is growing alone in the open. Trees living in a woods or forest, competing with their fellows for sunlight and moisture are most likely to develop longer trunks and smaller crowns than their genetically designed ideals. Fighting for space, or against a strong prevailing wind for example, can further distort the genetic blueprint (often producing a subject that is far more intriguing for us as artists). This variation is less liable to happen to the conifers where the genetic imprint is so strong that the external climactic conditions appear to have less effect on changing the original architecture.

To know what you are drawing, what to expect from the subject in a scientific way, is always for me the most important beginning. You want to feel the skeleton inside the mass of foliage, the balance of trunk and branches that makes the tree sturdy and logical, able in the most extreme conditions to remain erect and stable. And in drawing a tree, speaking for a minute as an artist and not a scientist, one almost has to sacrifice the detail to depict the whole. You cannot draw every leaf, every square of bark with the precision and accuracy you give to your flower portraits. Sacrificing some detail to show the mass and form of the tree seems to work best for me.

  • Tilia tomentosa, silver linden in Prospect Park, Brooklyn, NY, ©Dick Rauh
  • Cocos nucifera, coconut palm, Brooklyn Botanic Gardens, ©Dick Rauh
  • Cedrus libani,Cedar of Lebanon, Queens, NY, ©Dick Rauh