A list of root words to help you understand the vocabulary of plants.
a- = without; -pomo = fruit (apomixis: the asexual production of seeds)
anth- = a flower (anther: the terminal pollen sac of a stamen, inside which pollen grains with male gametes form in the flower of an angiosperm)
carp- = a fruit (carpel: The female reproductive organ of a flower, consisting of the stigma, style, and ovary)
coleo- = a sheath; -rhiza = a root (coleorhiza: the covering of the young root of the embryo of a grass seed)
di- = two (dioecious: referring to a plant species that has staminate and carpellate flowers on separate plants)
dorm- = sleep (dormancy: a condition typified by extremely low metabolic rate and a suspension of growth and development)
endo- = within (endosperm: a nutrient-rich tissue formed by the union of a sperm cell with two polar nuclei during double fertilization, which provides nourishment to the developing embryo in angiosperm seeds)
epi- = on, over (epicotyl: the embryonic axis above the point at which the cotyledons are attached)
gamet- = a wife or husband (gametophyte: the multicellular haploid form in organisms undergoing alternation of generations, which mitotically produces haploid gametes that unite and grow into the sporophyte generation)
hypo- = under (hypocotyl: the embryonic axis below the point at which the cotyledons are attached) mega- = large (megaspore: a large, haploid spore that can continue to grow to eventually produce a female gametophyte)
micro- = small (microspore: a small, haploid spore that can give rise to a haploid male gametophyte)
peri- = around; -carp = a fruit (pericarp: the thickened wall of fruit)
proto- = first; -plast = formed, molded (protoplast: the contents of a plant cell exclusive of the cell wall)
scutell- = a little shield (scutellum: a specialized type of cotyledon found in the grass family)
sporo- = a seed; -phyto = a plant (sporophyte: the multicellular diploid form in organisms undergoing alternation of generations that results from a union of gametes and that meiotically produces haploid spores that grow into the gametophyte generation)stam- = standing upright (stamen: the pollen-producing male reproductive organ of a flower, consisting of an anther and filament)
uni- = one (unisexual flower: a flower missing either stamens or carpels)
An Interview with Botanist Linda Graham
Professor Linda Graham is a time traveler. By peering through microscopes at fossils of the earliest plants and their closest living relatives, Dr. Graham looks back a half–billion years to investigate a major breakthrough in the history of life: the origin of land plants from their aquatic algal ancestor. Those early plants were the first macroscopic creatures on land. In addition to her research on the origin and early diversification of land plants, Graham and her students study how plants, especially mosses, continue to have an enormous impact on the biosphere today. Dr. Graham, who is a professor of botany and environmental studies at the University of Wisconsin–Madison, is also a gifted teacher, and the interview that follows offers some sage advice for why the study of biological diversity is an important part of your education.
What is the ancestor of land plants?
Plants originated from a particular group of green algae known as charophyceans. The evidence is very strong that these ancestors had already acquired some degree of complexity in terms of being able to branch. They also had some reproductive complexities that were probably inherited by the earliest land plants as well.
How long ago did this origin of land plants from algae occur?
The origin of land plants is still somewhat controversial in terms of timeline. The consensus among the paleobotanical community is that there were plants on the terrestrial surface at about 475 million years ago. This conclusion is based on evidence from fossil spores and other types of fossilized plant material. However, some of my colleagues have found fossil spores that they believe to have originated from land plants that lived as far back as 500 million years ago, in the mid–Cambrian period. I predict that in the future we will extend the origin of land plants back in time to at least the mid–Cambrian, perhaps even further back.
Which living plants do you think are most similar to the earliest land plants?
The molecular evidence indicates that the bryophytes—liverworts, hornworts, and mosses—are the oldest branches of the plant kingdom. And yet no one had found complete fossils of plants similar to bryophytes that were older than fossils of early vascular plants—plants with veins that transport water and nutrients—which the molecular data indicate diverged after bryophytes. That just didn't make sense. The standard explanation for the absence of early bryophyte fossils was that they just didn't preserve well. However, I knew that bryophytes produced spores and suspected that they produced other resistant materials as well. So, we wanted to test the idea that bryophytes have materials that could fossilize.
How did you do that?
We used two techniques to try to mimic the effects of degradation that would occur when a plant dies and falls into a water body and becomes partially degraded by microbial action. First, we treated living bryophyte material with an extreme technique known as acetolysis that combines high heat with strong acids. This is the same technique that the paleobotanists use to extract spores from rocks. We hypothesized that any plant material that would survive such an extreme treatment was fossilizable and should appear in a fossil record. Some of our colleagues argued that we might actually be generating resistant materials by such extreme treatment. So we added rotting techniques to our repertoire. We would leave our living bryophyte material in moist soil for months, and then retrieve the material and see what was left. Amazingly, the same types of resistant bryophyte materials that stood up to acetolysis also survived the more gentle process of rotting. And amazingly, those bits and pieces that survived rotting and acetolysis looked like some scrappy fossils of very ancient origin that people simply didn't recognize as being plant material.
And some of those fossil fragments are older than vascular plants?
Yes. Those scrappy fossils consisting of spores, tubelike structures, and bits of cellular sheets were much older than the vascular plant fossils. So we think that our work helps cement the idea that bryophyte–like plants were indeed present prior to the origin of vascular plants, as is supported by the molecular data.
How did the spread of these early land plants change the biosphere?
First, they helped produce early soils. Even the earliest plants had some organic materials that weren't easily degraded by microbes. So these materials built up as an organic layer in the soil. Second, by this photosynthetic conversion of carbon dioxide to resistant organic materials, early plants began to lower the amount of CO2 in the atmosphere. That started a trend that culminated in the lowest historic CO2 level as a result of the early woody plants of the coal swamps during the Carboniferous period. In addition, by producing organic acids, early plants probably released phosphate from the soil, and the runoff of phosphate would have stimulated growth of photosynthetic microorganisms in marine and fresh water ecosystems. Finally, early land plants established terrestrial ecosystems that eventually had sufficient organic productivity to support early land animals through food chains.
In addition to your interest in the origin of land plants, you also study the ecology of peat bogs. What are peat bogs, and why are they important?
Peat bogs are wetlands in which the dominant plant is Sphagnum, or "peat moss." This is a particularly important moss because it is an ecological engineer. The Sphagnum of peat bogs absorbs massive amounts of CO2 from the atmosphere and stores it in organic materials that are not easily broken down by microorganisms—much like the early bryophytes we just talked about. Peat bogs are very extensive across the Northern Hemisphere—far more than most people realize because the bogs are located in northern regions that are not heavily populated. Very large areas of North America, Europe, and Asia are covered by vast peat lands that store an enormous amount of carbon. By helping to regulate atmospheric CO2, which is a greenhouse gas, peat bogs function as a global thermostat. The moss helps stabilize the climate. If the temperature rises just a little bit, then that will facilitate moss growth, which will pull more CO2 out of the atmosphere and help cool the planet. If it gets too cool, the moss won't grow as much, and there is a net release of CO2 due to microbial degradation, which helps to warm the climate. So, we should be thankful for the vast peat lands that perform this thermostat function for our planet.
And how are we treating this important peat bog ecosystem?
Ecologists who study peat bogs are concerned about the disruption of peat bogs for mining or for agricultural uses such as cranberry production. By reducing the area of peat bogs, which play such an important role in regulating climate, we may be accelerating global warming.
How did you get started in science, Dr. Graham, and how did that interest turn to plants?
I had some wonderful schoolteachers when I was in elementary school, middle school, and high school—teachers who stimulated my interest in science. In particular, I had a female chemistry teacher who was a wonderful role model. She was intelligent and confident, and I got the idea from being in her class that I could be a scientist too. I have always been attracted to microscopes and being able to see the wonders of intricate structures. Plants have very interesting internal organization, and their relatives, the algae, are also very beautiful under a microscope. So, my specific interest in plants mainly came from my fascination with microscopic structure.
And how did that fascination become focused on the origin of land plants?
That was a pivotal event that occurred while studying for a botany final exam when I was an undergraduate at Washington University in St. Louis. One of the topics on the exam was the life cycles of plants, which have an alternation of multicellular haploid and diploid generations. The puzzle of how this complex life cycle originated stimulated my curiosity about the evolution of plants from their algal ancestors.
Why do you think it's important for first–year biology students to learn about the diversity of life, including plants, even if they plan to specialize in cellular or molecular biology or plan to go to medical school?
One reason we include biological diversity in our curriculum here at the University of Wisconsin is that we recognize that this may be the only point in the education of biology students that they will be exposed to the variety of organisms. And we recognize that biological diversity is important to all citizens because of its impact on human health. As students learn about prokaryotes, protists, plants, fungi, and animals, including invertebrates, they begin to see how these diverse organisms perform essential roles in ecosystems. And ultimately, our own health depends on the health of these ecosystems, which sustain humans with such services as clean water and clean air.
What other main points do you emphasize in your first–year courses?
I think that one of the most interesting aspects of biology is the linkage that occurs between the different hierarchical approaches. For example, understanding molecular processes and structures tells us a lot about processes at the organismal level and also at the ecological level. I think that what I can contribute to beginning biologists is an appreciation for integrative thinking and thinking across hierarchical levels. So in my classes, I try to point out that biological knowledge is not compartmented, but rather each topic is linked very tightly to other areas of biological inquiry. I also encourage first–year students to think about large issues and big questions—even if those big questions can't be answered by the application of a single experiment or a single set of observations. There are many questions in my personal research area, the origin of plants and extending back into the origin of life, that seem to some people to be unanswerable because they occurred so long ago that we can't perform direct observations. But I would encourage students to expect that we can answer such questions by deduction and integrative thinking. Thinking large and not focusing too much on the details of particular systems will prove useful in understanding all of biology.