Many of us involved in teaching botany feel a sense of urgency in our profession. Botany departments, botany majors, and botany curricula have gradually disappeared from most colleges and universities in the United States, and I suspect in many other parts of the world as well. Too many students are graduating with little or no understanding of the unique ways in which plants meet the challenges of survival and reproduction in the Earth’s diverse ecosystems. Biology faculty who don’t have training or experience with plants are often ill-prepared to relate to or take advantage of the unique contributions plants might make to their own teaching and research.
So if we have only a semester, or worse only a week or two, to teach the fundamentals of plant life, and to pass on the exhilaration we feel in the face of their diverse adaptations, how do we do it? If our non-botanical colleagues have been assigned to teach introductory botany, how do we help them understand the basics and develop some enthusiasm for the subject matter?
Some teachers prefer an ecological approach, emphasizing the pivotal and diverse roles of plants in the ecosystem. Others prefer an approach emphasizing applications to human technology, agriculture, nutrition or medicine. All of these approaches are useful in developing interest, but may end up being too superficial with respect to fundamental structure and function. Traditional botany texts tend to be dry and encyclopedic. Non-majors texts may be more appropriate for most of today’s audience, but they still tend to avoid a side of biology that I call the “why” questions.
One must have the “what” before the “why,” but it is the latter that gives some context or meaning to the former. The “what” is the factual material one finds in a textbook. The “why” is the explanation of the “what.” For example, textbooks typically contain a little section on the differences between monocots and dicots (or now monocots and eudicots, awkwardly ignoring magnolids, waterlilies and other basal angiosperms). We are told that dicots typically have net-veined leaves, vascular bundles arranged in a ring in the stem, and secondary (woody) growth, while monocots typically have parallel-veined leaves, vascular bundles scattered within the stem, and no secondary growth. That is the “what,” at least in a simplistic sense, but there is typically no “why” to follow it.
Monocots are the newer invention in plant architecture, having developed their unique structures and way of growth as they split from ancient dicots. Why do they have parallel veins? Why do they not have secondary growth? How do they interact differently with the world than dicots? How did their innovative structures come about? (Hint: it has to do with ancestral monocots going “underground.”)
“Why,” in scientific terms therefore, has to do with the process of adaptation. It’s the story of origins, of plants facing environmental challenges, and finding innovative ways to cope. This is what makes botany interesting. It is also a way to make sense of some fundamental features of plants that may be dismissed as obscure and unimportant, but which are loaded with both meaning and utility.
For example, let’s take everyone’s favorite: life cycles. Students already sophisticated enough to know that sperm and egg in animals are produced through the special kind of nuclear division called meiosis are truly puzzled by why that does not happen in plants. Others are surprised that plants produce sperm and egg at all. In the evolutionary story of sexual reproduction in plants, we find that the algae that gave rise to land plants did produce sperm and egg directly, as do the most ancient of land plants: mosses and liverworts. In both cases, however, the joining of sperm and egg does not result in a new plant, but rather the production of spores.
The production of spores in green algae mostly occurs within a single cell, but in land plants, a special multicellular body, technically a separate plant called a sporophyte, develops for that purpose. That plants alternate between gamete-producing plants and spore-producing plants is the “what” of plant reproduction. Students might memorize dozens of life cycle diagrams, but won’t know “why” such things exist, or why they have to bother with such tedia until the adaptive story is told.
That story has primarily to do with the fact that plants cannot move around to find mates, and that if they simply released sperm cells to go off and find an egg on their own, it would lead at best to severe inbreeding. Such a strategy works well enough in some marine invertebrates, like sea stars, where currents can help disperse the sperm cells, but on land, these tiny, fragile cells just don’t get very far. Spores do the traveling for plants, taking the place of mate selection in mobile animals. Genetic diversity in plants depends on spores from different genetic backgrounds landing close to one another, so that when they develop into gamete-producing plants, suitable mates will be next to one another.
Spores are launched best from an elevated vantage point, and so sporophytes tend to stretch upward as much as possible. Sporophytes became larger and larger in most land plants, and in fact the trees, herbs, and grasses we see today are actually the sporophyte generation of the plant life cycle. The egg- and sperm-producing “plants” (gametophytes) – the equivalent of the algal or moss colonies, are hidden within the embryonic seeds and pollen grains of these more advanced plants. Yes, it’s complicated, but if the story unfolds from the perspective of how and why this system evolved, it does make sense.
Feature Image: The sword-shaped leaves of cat tails, have parallel veins because new tissues are added at their bases, pushing them upward from their underground stem systems, lengthening each vein as the leaf lengthens. The evolution of this specialized architecture in early monocots also explains the suppression of woody tissues in favor of clonal spreading. Photo by Frederick B. Essig.