Wednesday, August 26, 2015

The evolutionary perspective in teaching botany

[This essay is modified from one that I recently posted at the Oxford University Press blog site]
Many of us involved in teaching botany feel a sense of urgency in our profession.  Botany departments, botany majors, and botany curricula have gradually shrunk or disappeared from most colleges and universities in the US, 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 or teaching assistants have been assigned to teach a beginning level segment on plants, 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.
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, This lengthens each vein as the leaf lengthens.
The corresponding suppression of woody tissues in
the underground stems occurred as the stems adapted
 for clonal spreading rather than vertical growth. 

  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 their leaves have parallel veins?  Why do they not have secondary growth?  How do they interact differently with the world than dicots, and how did their innovative structures come about? (Hint: it has to do with ancestral monocots going “underground.” ) See the caption to the right, and for a more extended exploration of these questions see my blog post: How the grass leaf got its stripes.

“Why,” in scientific terms, has to do with the process of adaptation. It’s the story of origins, of plants facing environmental challenges and evolving innovative ways to cope.  This is what makes botany interesting.  It is also a way to make sense of the fundamental features of plants, some of which may be dismissed as obscure and unimportant, but which are loaded with both meaning and utility. 

For another 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. Meiosis mixes chromosomes and reduces a double set (diploid) into a single one (haploid) in each of the resulting cells.  In animals the haploid cells combine into a diploid zygote, which develops into a new diploid individual  In algae and plants, however, it's more complicated. Bear with me, even the short version is convoluted!

In the evolutionary story of sexual reproduction in plants, we find that the algae similar to those that gave rise to land plants, and simpler land plants themselves, are haploid and do produce sperm and egg directly.  In both cases, however, the joining of sperm and egg does not result in a new plant, but rather in a short-lived diploid zygote that produces spores through meiosis. Spores are adapted for long-distance dispersal, and germinate to form new haploid plants that will eventually produce gametes.  So spores, not gametes, are produced through meiosis in plants.

A moss colony, such as this Isopterygium, is
both photosynthetic and gamete-producing (the
gametophyte generation).  It holds water within 
its spongy matrix, which sustains the  life of the
vegetative tissues and also provides a watery
pathway for sperm cells in search of eggs.
Because of this mode of reproduction, mosses
must remain small and close to the ground.  The
sporangium and its elongate stalk constitute the
sporophyte generation, a separate individual
resulting from the fertilization of the egg.
Spores will germinate to establish new 
genetically mixed moss colonies.  
The production of spores in green algae mostly occurs within individual cells, but in land plants, a small, diploid, multicellular body, technically a separate plant called a sporophyte, develops for that purpose.  The fact that plants alternate between haploid gamete-producing plants and diploid 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. 

In the early vascular plants, the diploid sporophyte
generation became the dominant part of the life cycle,
lifting spore-producing structures high into the air. 
Some of their descendants, including the ancestors 
of these giant douglas firs, evolved seeds and pollen 
grains - the more complex spore-derived vehicles that 
bear tiny egg- and sperm-producing individuals. 
Spores are launched best from an elevated vantage point, and so sporophytes tend to be stretch upward as much as possible.  In mosses, this can be only a few centimeters (see "Why are there no moss trees?") but this suffices to get spores above the low-growing moss foliage. In the land plants we call vascular plants, however, sporophytes became larger and larger, 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 it evolved, it does make sense. And it is an important story.  Understanding how plants and algae reproduce impacts both agriculture and ecology.  

Plant Life – A Brief History,” provides the adaptive perspective of plant features for students, instructors, and others interested in the biology of plants.