I noticed last year as I was writing my book, Kingdoms of Life Connected, that plant systematics textbooks were not using phyla, except for the bryophytes in some cases. What’s going on here? It is nothing less than a new view of the plant kingdom and of the classification of life.
Our traditional view was set by Linnaeus, back in the mid 1700’s. His system was based on the assumption that all life had been created simultaneously and that it was unchanging. He based his work on the appearance of the organisms and placed them in the hierarchical categories that we still (sometimes) use – kingdom, phylum, class, order, family, genus, and species.
Now botanists look at the lineages for each group of plants. For example ferns can be seen as members of the plant kingdom (embryophytes), vascular plants (tracheophytes), true-leaf plants (euphyllophytes), the fern lineage (monilophytes), and finally the branch of the leptosporangiate ferns, sometimes called the true ferns. This is a much richer view than a simple box labeled “ferns.”
What Linnaeus missed is the history of each organism. Darwin bought forward the important idea that organisms have histories. Some share a recent common ancestor, others do not. The history of an organism doesn’t show up in a row of boxes. Instead it must be displayed as a branching diagram that shows which organisms are more closely related.
Getting back to the plant kingdom and how we introduce it in the elementary classroom – usually the plant kingdom was broken into phyla (or divisions if you prefer the traditional name). Most of children’s literature – the small body of learning resources that actually address the plant kingdom – use phyla, whereas college texts and professional botany writings have largely discarded that category for plants.
Why have most of the phyla names been dropped? They didn’t work with the new view. The horsetails were previously placed in their own phylum, but they are embedded in the monilophytes, the fern lineage. So are whisk ferns, the psilophytes. The phylum name is even less useful for whisk ferns because it excludes their close relatives, the grape and adder’s tongue ferns.
Will phyla come back? It’s not impossible, but it may take a while for botanists to settle on what to call a phylum. Is it a branch of the ferns, the whole fern lineage, or the euphyllophytes? If children know the main branches of plant life, they are well-prepared whatever system develops. The animal kingdom, by the way, has kept its phyla, although they are now grouped into different lineages than they were a decade or so back.
Another term to discard is “seedless vascular plants.” The vascular plants have two branches, the lycophytes (club mosses and their relatives) and the euphyllophytes. The latter has two branches, the monilophytes (ferns in the broad sense) and seed plants. Club mosses shouldn’t be in the same category as the ferns, and “fern allies” mixes unrelated lineages.
For children to see the current view of the plant kingdom, they need a branching diagram that shows who is related to whom. Rows of boxes are out, phylogenies are in. A phylogeny is a branching diagram that illustrates a hypothesis about the evolution of organisms. Actually, the word applies both to the hypothesis and to its illustration. For more about current phylogenies, see my book, Kingdoms of Life Connected: A Teacher’s Guide to the Tree of Life and take a look at the charts that are available for free download from my website, http://www.bigpicturescience.biz.
Another term for a phylogeny is a Tree of Life or, informally, a family tree. The Tree of Life web project (http://tolweb.org/tree) has a great illustration in its home page. If you go to the page for the plant kingdom (aka embryophytes), you will see the extant lineages and a number of extinct ones. See http://tolweb.org/Embryophytes/20582. It is always thought-provoking to see the extant lineages in the matrix of extinct ones.
What should you do with your old plant kingdom charts? Keep them for historical perspective. Children’s literature still shows older classifications. Older charts also help children see the changing nature of science thought. Just make sure that children have working knowledge of the new, phylogenetic view of life.