Introductory Botany
Tillie
Plants can be defined as multicellular photosynthetic organisms with reproductive structures that are more complex than single cells. By this definition, algae are not considered plants because they are either unicellular or their reproductive structures are essentially unicellular, and fungi, too, are excluded because they are not photosynthetic. At least 400 million years of diversification has resulted in a wide diversity of taxonomically distinct major groups of plants. Some of the most important groups of plants found in Virginia are described below.
A plant can exist in a particular spot only if temperature, moisture, nutrient availability, and interactions with other organisms (to name just a few of these factors) permit its survival. Seldom do the factors controlling plant distribution act in isolation; elevation, for example, is correlated with length of growing season, average daily temperature, and daily extremes in temperature. On Virginia mountaintops, growing seasons are shorter, temperatures are cooler, and day-to-night temperature fluctuations are greater than what a plant would encounter on the shores of Chesapeake Bay.
Some plants have very narrow environmental tolerances and other plants are much wider in their adaptability. Some species we would consider generalists, that is they tolerate a wide variety of conditions allowing them to occupy a diversity of habitats. In terms of plant distribution within Virginia, some species occur essentially throughout the state (and well beyond), for example red maple (Acer rubrum) and jack-in-the-pulpit (Arisaema triphyllum), whereas others are much more limited in their distribution.
Some plant species widespread to the North reach their southern limit in the mountains of Virginia. Some of these plants (for example, Potentilla tridentata) probably reached Virginia from the north while retreating from the advancing Pleistocene glaciers and now persist in the mountains as relicts. Other species widespread in the Southeastern U.S. reach their northern limit in the Tidewater region, where zone 8 climatic conditions prevail.
Climate change has always been an important factor in plant distribution. When climate change brings about unfavorable conditions, plants have two options: adapt/ evolve to accommodate the new conditions or migrate (for example, by seed dispersal) to colonize new regions that match their requirements. While it seems clear that some degree of human-induced climate change is occurring or will soon occur, there is no certainty about the rate at which that change will take place or how extreme its full extent will be.
Climate change presents rare plants with especially large challenges. It remains to be seen whether rare plants have sufficient genetic diversity to adapt in place or whether they can produce enough seeds to colonize efficiently favorable sites for growth in a changing landscape.
Most commonly, we associate plant species with specific soil types or substrates, certain hydrological conditions, sun or shade, and disturbed or undisturbed conditions. The degree to which a species adheres to these parameters varies, of course, across the full spectrum. As a consequence, the patterns that emerge are complex and tend to be fuzzy.
The leaves of a plant from the coastal strand community are often succulent or tough and in-rolled. The open, windy conditions favor the wind-pollinated grasses such as sea oats (Uniola paniculata), beach grass (Ammophila breviligulata), and saltmarsh cordgrass (Spartina patens). On the beach we find searocket (Cakile edentula) and Carolina saltwort (Salsola caroliniana). In one sense, the habitat is rare in that it is limited to the near-shore environment, but geographically it is widespread and many of its plant components range all the way up and down the East Coast and well beyond.
The ecological roles of plants are many. Plants comprise the base of the food chain. The vast majority of plants are photosynthetic: via a complicated series of reactions, they use sunlight to synthesize sugar from carbon dioxide and water. These sugar molecules provide both the energy and chemical starting material for plant metabolism and growth. In essence, plants make their own food; therefore, in ecological terms, they are often characterized as autotrophic (literally, self-feeding).
In most familiar ecosystems, plants ultimately provide the food consumed by all other organisms: Herbivores eat plants, and carnivores eat the herbivores that once ate plants. And when plants die, or shed dead leaves or flakes of bark, etc., microorganisms (e.g., bacteria and fungi) derive their sustenance from these once living plants or plant parts.
Plants require more than water and carbon dioxide to survive, however. Plant roots absorb up to 14 different elements from the soil and incorporate them into the biochemistry of their cells. Other forms of life require these elements, too, and ultimately these elements enter the biosphere largely by way of plants.
Finally, it is of vital importance that plants release oxygen as a byproduct of the photosynthetic process. Ultimately, every breath we take, every breath of every animal, and every passage of water over gills, serves to deliver life-giving oxygen made and continually renewed by plants. In summary, plant photosynthesis captures energy and builds nutritious tissues that eventually feed all other organisms and in doing so plants also release oxygen upon which the efficient utilization of that food energy depends for plants and animals and human beings, too.
Plant life defines ecological communities. Forest, marsh, meadow, and bog are recognizably distinct in large part because of the nature of their plant life. The presence of certain plants, the absence of others, determine not only the overall appearance of any given habitat, plants also dictate much about the availability of food, nesting sites, and hiding places for animals. Plants create multiple niches within any given community; consider for example the different environmental aspects exploited by animals that live in a forest canopy versus those that occupy the forest floor or those that burrow into the soil and live among the roots of trees.
While plants and animals share a great many fundamental aspects of their biology, they also differ in profound ways. Perhaps one of the most obvious differences is that plants are sessile (nonmotile) because their roots anchor them. In contrast, many animals are capable of locomotion. While animals appear active, dynamic, and engaged in all manner of fascinating behaviors, plants seem static and nonchanging on first impression.
That impression, of course, is an artifact of our perception of time. Plants are dynamic, they do undergo growth and change, but their rates of change are usually too slow for impatient humans to take much notice.
Nevertheless, it is common knowledge that an apple tree will be leafless in winter, covered in flowers in early spring, leafy all summer, and bear delicious ripe apples in the fall. These seasonal transformations are profound and, further, as the years go by, that apple tree greatly increases its size, far surpassing the growth capacity of most animals. Plants are, indeed, dynamic!
Familiar terrestrial animals have a determinate body plan; after birth or hatching, they grow to a certain size and attain a stereotyped form characteristic of their particular species. All monarch butterflies are very nearly the same size and they all have six legs and four wings, etc.
Seeds are fundamental to reproduction of many plants, and reproduction is one of the general characteristics of all living things. There are certain similarities about the nature of sexual reproduction in plants and animals, but there are also some profound differences.
At its core, sexual reproduction involves processes that deliver portions of the genetic characteristics of both parents to their offspring while also subtly altering how those characteristics are combined in successive offspring. This shuffling of genetic characteristics between generations occurs essentially the same way in plants and animals and depends upon the process of meiosis leading to gamete (sperm and egg) formation coupled with the random combination of gametes to form the new generation. In animals, gamete formation follows directly from meiosis whereas in plants there are a number of intervening stages, the details of which involve pollen as the male component of sexual reproduction and structures inside ovules that function as the female counterpart. Pollen releases sperm cells that fertilize eggs located inside the ovule; from that union arises the embryo, an immature plantlet located inside the seed.
One of the subtle differences between plants and animals has to do with nitrogen metabolism. In general, animals are relatively limited in their biochemical capacity to process molecules containing nitrogen. Consequently, animals generate significant amounts of nitrogen-based waste products that must be excreted, either as ammonia, urea, or uric acid.
Plants, in contrast, are much more versatile in their biochemical repertoire concerning nitrogen compounds. In essence, plant cells can recycle nitrogen-containing compounds more or less indefinitely. Consequently, because plants consume no bulky foodstuffs the way animals do and because they generate no nitrogenous wastes, plants have no real excretory system. Nor do they have any specialized cells directly comparable to the muscles and nerves of animals. A few plants (e.g. the Venus flytrap) are capable of rapid leaf movements, but the resemblance to nerves and muscles is only rudimentary. There are no synapses, and rapid leaf movements are always driven by abrupt changes in turgor pressure (intracellular water pressure), not contraction of muscle fibers.
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