The Science of Botanical Art

A New Hierarchy?

Dick Rauh 

Originally appeared in The Botanical Artist - Volume 12, Issue 1

 

I wrote a piece about hierarchy some months back. It was the way I learned it only fifteen years before. It turns out to have been obsolete even then. I followed the teachings of Arthur Cronquist, one of the world’s leading taxonomist of the late 20th century. I knew some of his assumptions were under attack, but I needed some basis to teach people who were not going to be scientists. It gave them some idea of how the plants they knew fit into a larger picture. I am talking about the ranks with the special Latin suffixes; division (phyla), class (opsida), subclass (idae), order (ales), family (aceae). Order and below are still largely accepted, but when we get to the “higher” groupings, they now appear to be out of the picture. 

Plant systematics, the science dealing with this aspect of taxonomy, is still in such a state of flux that no single authority is widely accepted. The argument boils down to just how much subjectivity or objectivity counts, or if you will, whose subjectivity you accept. Scientists such as Cronquist, with his years of study in the field, felt qualified to make decisions based on his own experience. He confirmed this by reference to similar-thinking taxonomists, and included the most recent discoveries that backed up his point of view. 

Understand there has never been universal agreement on these categories. The picture I always got from the ‘old’ hierarchy was one of a growing circle of plants, with a particular species in the center. Next would come a larger circle embracing all the plants in a genus, then a larger circle for family, order, subclass, class, division and finally kingdom. This concept has given way to the image of a tree of many branches, describing, not only the groupings of plants but their place in evolution. The subjectivity of the individual scientist, no matter how qualified, is giving way to a new approach that is more objective. 

It uses all of the known facts about a plant: its morphology, anatomy and fruit type, for example, and now with increasing weight, its molecular make-up. These are compared to closely related plants using various computer softwares. The results are diagrams called ‘trees’ that give us a more complete picture of plant groups and their relationships. These groups are called ‘clades’. 

The determining factor in creating an acceptable clade is whether or not the clade is monophyletic. This means that all the plants in the group have a common recent ancestor. This translates into a common trait, or combination of traits, that exists in all the species in the clade. Many of the ranks that we are familiar with fit this definition. A family is a clade, a genus is a clade. It’s just when we try to fit some of the old higher ranks into this rule we run into trouble. 

For example Cronquist named the subclass Hamamelidae to include families he felt fit into a natural grouping. These were mostly deciduous woody plants of the temperate zone. The problem arises when we put this to the new test, and find no common ancestor. No clade, no subclass, and the included orders are dispersed to their nearer relatives. Dilleniidae, another of Cronquist’s subclasses, always seemed to me to be rather a catch-all, and when the latest thinking is applied I seem to be borne out. The orders that once comprised Dilleniidae are scattered throughout the tree. The heathers, once close to the mallows and mustards, are now near to the top of the tree and the asters, for example. New names appear to distinguish clades, such as rosids and asterids but need definition and acceptance. 

We now hear of something named eudicots (or real dicots). The old concept of the two classes of the angiosperms has been rethought, based largely on the shape of the pollen. What were once thought of as primitive dicots, Magnloiaceae, Lauraceae, fall into a clade called Magnolids that are on the same branch as the monocots. The Eudicots comprise a clade that includes the majority of dicots, all of whom have similar pollen. 

I am not sure how much of this is vital to our ability to draw and paint flowers, but it has been nagging at the back of my mind that you must be aware, at least, of what is going on in this very vital science of botany.