UNCG Biology 105 Study Guide, Part II
This outline of topics is designed to help students in Dr. O’Hara’s section of Biology 105 follow the main points made in the class. It is not a comprehensive presentation of all the course material; instead it is an outline that will help you keep the course structure and main points in mind. In studying for exams you should study your lecture notes, the syllabus, the poems-of-the-week, the assigned parts of the texts (including especially for this section of the course the seasonal pages of the Peabody Park website), and the species particularly described in class.
This section of the study guide (Part II) covers the second third of the course. The second exam will have 50 questions; 5 will be drawn from the material in the first third of the course, while the remaining 45 will come from material in this second third. (In other words, 10% of the exam will be cumulative.)
- Anonymous, “John Ray, Master of Arts”
- William Cullen Bryant, “A Forest Hymn”
- Tom Lehrer, “Poisoning Pigeons in the Park”
- Robert Louis Stevenson, “Summer Sun”
- John Keats, “To Autumn”
Species You Should Know
The University is destroying nearly every plant in the vicinity of the Eberhart Building this semester, so the list of species I had expected to use here (all of which were easy to study a few weeks ago) has had to be modified. In addition to the species we looked at in the first third of the course, for this middle section of the course you should learn only these additional four. Notice that the majority of plants found in the landscaped areas of the campus are not native to this region.
- Heavenly Bamboo (Nandina domestica) — exotic (native to China and Japan)
- Forsythia (Forsythia sp.) — exotic (native to Europe and Asia)
- Pecan (Carya illinoiensis) — native to the central United States, but widely planted in North Carolina
- Redbud (Cercis canadensis) — native understory tree of the oak-hickory forest, also frequently planted as an ornamental
Neighbors and Relatives: Ecology and Systematics
If I want to know about you as a person, I can follow at least two different approaches. First, I might study your relatives: your parents, grandparents, great-grandparents, siblings, children, cousins, and so on. Knowing about them will help me understand a lot about you, particularly your biological characteristics (your size and shape, the color of your hair and eyes, and a whole host of other traits), because many of your biological traits have been inherited from your ancestors and have been passed on to you, your relatives, and their descendants. That is one way to understand something about who and what you are.
Another way is more “proximate”—more close up. It is to study you in the context of your neighbors and the interactions you have with them: the people you live with, live near, buy things from, sell things to, eat with, cooperate with, compete with, are friendly to, are hostile to, and so on. If I know about all the “neighbor” interactions that you engage in every day I will also know a lot about who and what you are. (Notice that these interactions always imply the existence of a neighborhood—a space in which you live and move and have your being. Your relatives may be part of that neighborhood, or they may not.)
We can understand organisms in nature in the same two broad ways: in terms of the other organisms they are related to, and in terms of the other organisms they interact with where they live. The first type of study is the branch of natural history called systematics, and the second is ecology. (The study of close relatives is also the province of genetics, which we will talk about later in the term.)
The name “systematics” for the study of biological relatives comes from the old term “natural system” or “system of nature,” which refers to the comprehensive arrangement or pattern that is exhibited by the diversity of life. If you think of the complex diversity of life as an unknown country to be explored, you can think of “the natural system” as a map of that country that systematists (specialists in systematics) are trying to work out.
Naturalists have been surveying the diversity of life for almost 300 years, and there is still much to be discovered. Some of the early workers in this field are people you know, such as the English naturalist John Ray and the Swedish naturalist Carl von Linné (or Carolus Linnaeus). Linnaeus published the world’s first comprehensive species inventory in mid-1700s. It was called Systema Naturae (The System of Nature) and it went through many editions that got bigger and bigger as time went on. Many such surveys of particular groups of animals and plants from particular geographical areas (“Birds of Africa,” “Mosses of New Zealand,” “Butterflies of Guatemala,” etc.) continue to be published every year as knowledge of biological diversity increases.
Much research on biological diversity—systematic research on which species are related to which—is carried out in natural history museums. Museums preserve collections of specimens for research and public display, specimens collected from all over the world and from all groups of organisms. When you visit a museum to view the displays, you may not realize that the museum also houses thousands of research specimens behind the scenes.
- American Museum of Natural History — The AMNH in New York City is one of the largest public research institutions in the world.
- National Museum of Natural History — A division of the Smithsonian Institution, the National Museum in Washington, D.C., is the second largest natural history museum in the United States.
- Museum of Vertebrate Zoology, University of California at Berkeley — Many universities maintain natural history research collections and sometimes exhibits. The MVZ at Berkeley is an example, and it specializes in the study of vertebrates (the back-boned animals).
The basic unit of biological diversity is the species. We will say more about what species are later, but for the time being we can think of them as a basic “kind” of animal or plant. The individuals that make up a species can breed with one another, but not with members of other species. (We say that species are reproductively isolated from one another.) How many different species of living things are there in the world? No one knows, because so many have never been described or studied. The total may be 3 million, or perhaps as many as 30 million. Here are a few rough figures: there are about 9000 species of birds in the world, and about 400 in North Carolina; about 150 species of flowering plants have been found on our campus in Peabody Park (but there are probably twice that number here); there are hundreds of thousands of species of insects in the world, many of them completely unknown.
From the time of the early naturalists it was obvious that species could be grouped together on the basis of their characteristics, and that these groups could be combined into larger groups, and so on. This hierarchical structure was understood from the beginning to be an important aspect of “the natural system” (whatever that system might be in its totality). A series of formal levels was established beginning with Linnaeus: species, genus (plural: genera), family, order, class, phylum (plural: phyla), and kingdom. A kingdom (such as the animal kingdom) may contain several phyla; each phylum may contain several classes, and so on. You know the genus Pinus, for example, and two of its species, Pinus strobus and Pinus echinata. If you look at the technical survey page on the Peabody Park website you can see some of these groupings for plants and animals that have been found on our campus:
- Peabody Park Biological Survey — A listing of all the species so far recorded from our campus park, with pictures of many of them.
The language used by early systematists was full of genealogical metaphors: cousins, relatives, families, and so on. But for early systematists like Linnaeus this language was only metaphorical. By 1800 or so, however, the growth of geology and the increasing knowledge of fossils and the history of the earth led a number of naturalists to wonder whether this language could not be taken literally: maybe there is a pattern of true descent behind the abstract notion of “the natural system.” The fossil record showed that species have appeared and disappeared over time; maybe they also transform into other species. Maybe the natural system was a family tree of species (as opposed to just a family tree of individuals—we are all part of a family tree of individuals).
Among these early “transmutationists” in first half of the 1800s were the French naturalist Lamarck, Erasmus Darwin (a physician and Charles’ grandfather), and the Scottish author Robert Chambers. (The word “evolution” was not used in this context at that time; it had more of an embryological meaning, referring to the development or “unfolding” of an embryo.) But apart from a few authors, these notions of “transmutation” or “descent” were almost universally rejected. Many people considered them interesting speculations, but there was no mechanism known that could cause species to change or multiply. (This is the same reaction that proposals of continental drift would receive in the early 1900s: an interesting idea, but how could it possibly happen?)
The reason Charles Darwin’s book On the Origin of Species is so important is that it provided a serious answer to this question of how species could change. We will talk more about Darwin’s work later, but for now you can see that his answer to “How does it happen?” is given in the Origin’s full title: On the Origin of Species by Means of Natural Selection. The “origin of species” had been talked about by many people in the decades before Darwin, but natural selection was a new idea, and it was the mechanism he proposed for the origin of species.
The Origin of Species contains only one illustration. It is a “family tree” diagram that illustrates the concept of descent. “The natural system,” in Darwin’s interpretation, is indeed a vast family tree that connects all living things—all species—together in one system of “descent with modification.” Working out the details of that tree is exactly what systematics is now about: figuring out which species and groups of species are related to which, and how the entire “family tree of life” is put together.
Ecology of Our Eastern Forests
Ecology is the study of how organisms live together and interact in nature as “neighbors.” We will examine some basic ecological principles using the native oak-hickory forests of our region as an example, and the campus environment (Peabody Park) as a particular example. The Peabody Park website gives many further details. What we will see is that a forest (or any natural environment) is not a random jumble of plants and animals, but a community with a very distinct spatial and temporal structure. The members of that community display a great many adaptations that allow them to survive and reproduce.
The eastern United States was once covered almost entirely with forest. Human activity has extensively disturbed the original forests over the past 300 years. The particular type of forest that occurs in the North Carolina Piedmont is usually called an “oak-hickory” forest, after the two principal canopy trees found in it. (See Kricher’s pages on oak-hickory forest species.)
The different species of plants that can be found in a forest are not randomly arranged, but instead exhibit patterns. An oak-hickory forest is typically stratified, with a canopy of tall trees, an understory of smaller trees of different species, a shrub layer, and a fern/herb layer on the forest floor. Each year the forest leafs-out from the ground up, so that sunlight is available to each layer in sequence: the earliest spring flowers appear on the forest floor first, then the shrub layer produces leaves, then the understory, and finally the canopy trees in May. If the herbs “waited” until later in the season to produce their leaves there wouldn’t be enough sunlight available for them to complete their annual cycle.
The wooded section of Peabody Park is a fragment of the kind of oak-hickory forest that once covered this entire region. In addition to oaks and hickories, Tuliptree, Sweetgum, and Beech are common canopy trees in the Park woods. Dogwood and Redbud are the most common understory trees in the Park; they rarely grow to more than 30–40 feet in height. In the shrub layer, Strawberry Bush, Sweetshrub, Great Rhododendron, and Wild Azalea (all about 4–10 feet in height) occur in the Park. The fern/herb layer is most visible in spring and includes Mayapple, Trout Lily, Trillium, and many others. The Peabody Park website has a page for each season of the year, and the spring page illustrates many of these species. Most of the small herbs are known as “spring ephemerals” because they are visible only a few weeks each year in the early spring. They are perennial plants and may live for many years, but they spend most of the year underground as dormant bulbs or tubers; if you visit the woods in August you won’t even know they are there.
Gaps in a forest (caused by fire, disease, or windthrow) are filled in over a period of years. At first, the sunlight that reaches the forest floor causes lush growth of many weedy, opportunistic plants. Eventually the tree seedlings overtop and shade out these fast-growing, short-lived plants, and the canopy is restored. This is not a random process, but instead display a distinctive pattern.
The simple case of the natural repair of a gap in a forest is an example of ecological succession. When a habitat is disturbed there is a succession of different species that typically replace one another in the gap over a period of years. You can see this when an agricultural field (say) reverts to forest over the course of a century. As in a small forest gap, the first plants to come in are usually fast growing annuals that are sun-tolerant. Eventually saplings of the canopy trees will shade the ground once again, and the short-lived sun-loving species will disappear. This process can be seen clearly at an “ecotone”—a boundary between two different types of habitats (between a forest and a field, for example). The “mature” state that is arrived at after many years (sometimes centuries) of succession is known as the climax condition.
The Peabody Park fields are representative of the old agricultural habitat that has covered much of the eastern United States in the last three hundred years. The fields were used for cattle grazing 100 years ago and are now maintained in part as a golf course. In the twentieth century much agricultural land in the eastern United States was abandoned or converted to suburbs. Many of the abandoned farmlands have reverted to forest, but this “second growth” forest is often not as ecologically diverse (not as many species) as the original forest. It may someday become that diverse again, but it may take centuries for this to happen.
Some environments are naturally subject to disturbance on a regular basis. Even before humans were present, the grasslands in the American West were subject to periodic fires (a kind of natural disturbance). Coastal areas are often subject to periodic storm disturbance. Even the banks of small streams like the ones in Peabody Park are subject to regular flooding. The plants and animals that live in these areas are often adapted to disturbance and can tolerate it better than species that are adapted to a more stable habitat (like a mature forest).
The type of ecological succession just described is usually called “secondary succession” because it is the repair of an already-existing ecological community. Less visible in our area is the phenomenon of primary succession, which is the initial colonization of a completely uninhabited area. For example, a new lava field on the side of a volcano will initially be completely devoid of life, and will gradually become colonized by plants and animals over many years. You can actually see primary succession in a small way in our area: look at the moss and lichens that are gradually colonizing bare brick walls. Small plants like mosses and lichens are often the first plants to appear in an area undergoing primary succession.
Adaptation and Design
Many of the things we have described and will continue to describe are examples of adaptation—the seeming “fit” of organisms to each other and to their environments like a hand in a glove. The concept of adaptation—this observed fit—is one of the Big Ideas in natural history, just as “Truth” is a Big Idea in philosophy and “Justice” and “Freedom” are Big Ideas in political science.
Adaptation is obvious: just study nature with any care and you will see that in a great many ways things seem to fit together: camouflaged insects and the tree trunks they hide on, long-billed hummingbirds and the flowers they feed on and pollinate, and so on. The question immediately arises, “How did this state of affairs come about?” Simply saying “It has always been that way” is not a very satisfactory answer; in fact it isn’t really an answer at all.
Among the early naturalists we have discussed—Ray, Linnaeus, and others—only two explanations for the origin of adaptation could be imagined: either (1) everything fits together accidentally, or (2) nature was purposely designed to fit together. If something is “designed” that means there was a designer—a thinking agent—who did the designing. In the history Judaeo-Christian-Islamic cultures, that designer has always been understood to be God (or at least a god of some kind).
Early natural history (in the 1600s, 1700s, and early 1800s, for example) was filled with this way of thinking: the observed adaptations of organisms were seen as evidence that they had been designed by God. Notice how I am phrasing this: the act of designing was not something people could see, what people could see was the adaptive fit. They asked how the adaptive fit could have come about, and could think of no answer other than that things must have been purposely designed this way by God.
This fusion of natural history and religious thinking often goes by the name “natural theology.” People like Ray and Linnaeus belong to the tradition of natural theology just as much as they belong to the tradition of science. Natural theology was part of the world view that most people in the sciences had at that time. Charles Darwin studied natural theology as a student at Cambridge University in the early 1800s, just as everyone else there did. The 1825 William Cullen Bryant poem we read is a beautiful literary expression of the perspective of natural theology.
The weak spot in the arguments of natural theology was that only two alternatives were offered: accident and design. The complicated adaptations we observe in nature make accident seem very unlikely, so that leaves us with design as the only alternative. Later in the term we will see what happens in the 1800s: the reason Charles Darwin is such an important figure in the history of science is that he came up with a third explanation for adaptation, namely, natural selection. Natural selection is neither design nor accident (though chance does play a role in it), and it not only can account for adaptation, but also for some seeming puzzles and failures of adaptation that had been difficult for the natural theologians to understand. We will look at natural selection in more detail later in the term.
Seasonal Patterns of Adaptation: Nature in Spring
Most annual plant species “survive” the winter, in a sense, by not surviving: they complete their whole life cycle in one year and produce seeds, and it is the seeds that over-winter to the next spring. The seeds are alive of course, but they are dormant and their parent plant is gone. Perennial plants, those that live for many years, may survive the winter in several ways. Above-ground evergreen species, such as the Southern Magnolia and the pines, typically have tough, waxy, leathery leaves that resist drying out. (Dessication is often a greater hazard in the winter than cold.) Even in the winter these plants can undergo some photosynthesis because they still have all their leaves in place. (Photosynthesis is the process whereby plants manufacture their own food—sugar—from air and water by means of solar energy.) There are even some small herbs on the forest floor that follow this strategy: Spotted Wintergreen is one example in our campus woods. Although it is only about eight inches tall, it is an above-ground evergreen perennial, and not a spring ephemeral like most of the forest floor herbs.
Above-ground deciduous perennials, such as most of the native trees and shrubs in our area, store food in their roots over the winter and jettison all their fragile “solar panels” (leaves). They go into a more complete state of dormancy than the evergreens.
Some smaller perennials, such as the spring ephemerals, disappear completely underground in the winter and exist only as underground bulbs (you may be familiar with exotic garden species that do this also, such as tulips and daffodils). Many plants from all over the world that follow this same adaptive strategy are ones that we like to eat: onions, garlic, leeks, etc. The part of the plant we eat is the underground sugar storage organ of the plant. (Spring “ephemerals” you should remember are ephemeral only with respect to their above-ground flowering period, which is short. The individual plants live many years, but you can only see them above ground for a few weeks each year in the spring.)
When spring arrives and the amount of sunlight available each day increases, the evergreens are ready to go because they have their leaves all in place to catch the light. The deciduous trees and shrubs and the bulb/tuber herbs can’t make use the sunlight until they produce new leaves, however, and so they use the fuel they have stored underground to “kick start” the growth process. Once some leaves have been produced the plant can begin to manufacture new sugar on its own.
One well-known case of this latter phenomenon has been exploited by humans and other animals for centuries. The roots of Sugar Maples store food in the winter, just like the roots of many deciduous trees, but for some reason we humans find Sugar Maple sugar particularly tasty. In the early spring this sugar is dissolved from its stored areas in the roots and it begins to rise up through the tree as sap—liquid fuel to power the growth of the spring leaves and flowers. Humans have for centuries known that if you drill a small hole in the trunk of a Sugar Maple the sugary sap will leak out and you can collect it for food. Because it is very dilute, people commonly boil it until it is more concentrated: that is what genuine maple syrup is. If you boil off almost all the water you have maple sugar candy, another tasty natural treat. Check the change in your pocket for a Vermont state quarter showing maple tapping in progress in the early spring, a traditional industry in Vermont and much of New England. People didn’t discover this on their own, however: Yellow-bellied Sapsuckers, one of our local woodpecker species, feed on tree sap and are very fond of Sugar Maples also. They drill rows of small holes in the trunk and drink the sap as it leaks out. You can see sapsucker work all through Peabody Park and in many other places on campus.
Migration is another way to deal with harsh winters: just move away. Bird migration occurs world-wide, but it is most common in temperate regions. Migratory species have a breeding range and a winter range; these may overlap somewhat, or be completely separate. The extent of migration varies from species to species: some species are short-range migrants (up and down a mountain perhaps), while others like the Arctic Tern travel more than 10,000 miles from Arctic Ocean waters to Antarctic Ocean waters. (Of course not all species of birds migrate; some are resident in a given area year round.) In addition to birds, some species of bats migrate, as do some insects (the Monarch butterfly, for example), and some terrestrial mammals such as caribou.
Migratory birds that breed in our area mostly travel to Central and northern South America in the winter, but some go only as far as the Gulf Coast. Some species that winter here breed farther north in New England and Canada (or at higher elevations in the mountains). And some of our bird species are non-migratory and are here all year round.
In our Piedmont region of North Carolina, the arrival of the first species of spring migrants begins in February. April and May are the heaviest months. Arrival dates for each species are quite regular: often to the week, though not precisely to the day. Precise days of arrival are usually influenced by the movements of warm weather fronts from the south during the period of arrival.
Many small birds migrate at night. How do they find their way? Birds can use both the sun and the stars, as well as landmarks (in daytime) and perhaps magnetic fields. This can be studied by examining the nocturnal restlessness of birds held under special experimental conditions.
Although some species of mammals also migrate (some bats and northern grazing mammals like caribou), most of the mammals in our area either remain active in the winter or they hibernate. Hibernation is much more than sleep: it is a long period of physiological inactivity during which the heart and breathing rates drop very low and the body temperature also drops. Most insects also over-winter in a dormant state; some species as eggs, some as larvae or pupae, and some as adults. One of the first butterfly species you see in our area each spring is the Mourning Cloak because it over-winters in the adult stage of the life cycle: as soon as there are a few warm days, the Mourning Cloaks emerge from their hiding places and fly away.
Seasonal Patterns of Adaptation: Nature in Summer
The themes we picked out from our text for the summer were flowers and pollination, and bird behavior.
Flowers and pollination. A flower is a plant’s reproductive organ. Flowers are commonly made up of four whorls of parts. From the outside to the center these are the sepals, petals, stamens, and carpels. Not every species has all four whorls, but many species do.
The pollen is produced in the stamens. Wind pollination is typical of plant species that inhabit open areas (including the forest canopy). Wind-pollinated plants produce very small pollen grains in large quantities and their flowers are often small and inconspicuous. Since the work of dispersing the pollen is done by the wind, the plants have no “need” for conspicuous flowers.
Insect pollination also occurs in many species. Insect-pollinated plants usually have conspicuous flowers and often produce nectar, a food lure for the insects. The nectar does nothing for the plant itself in its own functioning; it is purely a pollinator attractant. The precise structure of particular species’ flowers is often an adaptation to attract particular types of pollinators, such as hummingbirds, wasps, or flies. This may be done by providing nectar, by imitating (say) a female wasp so the male wasp will land on the flower in an attempt to mate and pick up pollen incidentally, and by many other means. In addition to insects, birds and bats pollinate some species of plants. Plants and their pollinators are often excellent examples of co-adaptation: each species is tightly fit to the other. (“…dependent on each other in so complex a manner…”)
Pollination is the falling of a pollen grain onto the carpel of a flower. (Specifically, onto the stigma, the sticky upper tip of the carpel.) Once pollination has happened, the pollen grain begins to grow down into the carpel and then it discharges a sperm cell into the base of the carpel where the plant egg cells are located. The union of the sperm cell made by the pollen grain and the egg cell in the base of the carpel is the act of fertilization. Note that pollination and fertilization are two different things.
Is a plant male or female? Technically speaking, most ordinary shrubs or trees you see on the lawn are neither male nor female: the pollen grains are male (they produce sperm), and a small anatomical structure in the base of the carpel is female. It is true, however, that some plant species have separate staminate and carpellate individuals; an example on our campus is Bayberry. There are many Bayberry bushes planted along the front of the main dining hall facing the fountain. Each individual Bayberry bush has either flowers with stamens, or flowers with carpels, but no individual Bayberry bush has flowers with both stamens and carpels. People sometimes loosely call the staminate individuals “males” and the carpellate individuals “females,” but calling them staminate and carpellate is more accurate. For this species to reproduce, both staminate individuals and carpellate individuals must be near one another so that pollination (and then fertilization) can take place.
Bird behavior. Spring bird migration in our area extends from February to the beginning of June, with each species arriving according to a well-defined schedule (usually the same week every year). Some species just pass through to breed farther north. Others settle down here for the summer and breed in this area.
In many species the males arrive first and establish territories. A territory is a reproductive area containing a nest, and is usually defended by a male or a pair. Most feeding is done within the territory, like a family farm. Only male birds sing, and song functions both in the courtship of females and in competition with other males. At the end of the breeding season the territories dissolve and are no longer defended. Many birds exhibit nest-site fidelity, however, and return to the same general or specific area to breed each year. (Audubon discovered this in the early 1800s by tying colored threads around the legs of birds near his home. They left for the winter, and then the very same birds came back the next year.)
The annual reproductive cycle of birds native to our area usually involves a strong physiological component controlled by the day length: as the days get longer, sex hormones are secreted in greater volume and the birds’ gonads (testes and ovaries) grow. In the fall when the days get shorter the production of sex hormones declines, the gonads shrink, and the birds become almost asexual during the winter. The following spring the cycle begins again.
Monogamous arrangements are common among the birds species in our area. Other species of birds exhibit various forms of polygamous (either polygynous or polyandrous) reproduction. Lek behavior is an example. A lek is a communal display ground where males congregate to display and mate. It is not a territory. Many species of grouse, some shorebirds, and birds of paradise (among other species) engage in lek behavior. One male may mate with several females, but the females then go off and do the nest building and feeding of young on their own.
Many bird species also exhibit sexual dimorphism (Cardinals are a local example), and in some species this is more pronounced in the breeding season (most local birds moult twice a year). “Reverse” dimorphism occurs in a very few species such as phalaropes, in which the females are brightly colored and the males are dull. Other species, such as Robins, are sexually monomorphic: the males and females are similar in appearance. Lek breeding species (none occur in our area) often display extreme sexual dimorphism: the males have greatly elaborated plumage while the females are plain and brown. Females choose which males to mate with based on their appearance in the lek, and through this pattern of choice generation after generation the males with the most elaborate plumage have tended to reproduce and pass on their genes to the next generation.
Different species of birds build different types of nests: open cups on the ground, in the shrub layer, or in the canopy; cavities in trees; woven hanging bags; burrows in stream banks (as our local Kingfishers do); and so on. Each species builds its nest out of characteristic materials. Robins, for example, always use mud and grass; Chimney Swifts use twigs stuck together with saliva; Orioles hang a woven bag made of grass from the end of a branch. The Brown-headed Cowbird has no nest of its own, but instead lays its eggs in the nests of other birds (it is a “brood parasite”). Remember that a nest is a reproductive site made to hold the eggs, and it is typically used only in the breeding season. Many birds also roost (sleep communally) during the non-breeding season, but roosting birds do not use or build nests for that purpose.
Seasonal Patterns of Adaptation: Nature in Fall and Winter
In autumn in our temperate region the days grow shorter and the nights grow longer.
Deciduous trees change color and drop their leaves in autumn. The color change is caused by the breakdown of chlorophyll, the green pigment in the leaves. Chlorophyll is bundled into cellular organelles called chloroplasts, and these decay as the leaf is dying. When the green pigment breaks down the yellow and red and orange pigments that had always been there, but that had been overwhelmed by the huge amount of green, are revealed. The leaf drops off when a ring of cells around the base of the petiole swells up and clips through the petiole, a process that occurs in response to a combination of temperature and changing day length.
The fallen leaves become a major component of soil as they are gradually broken down. Forests are very good at recycling their parts, and the fallen leaves are a naturally-produced mulch for the next generation of plants.
Fungi are very commonly seen in the autumn. They are there all year round, growing as a loose network of tissue in the ground called “mycelium.” The mushroom that you see above ground in the fall, or the bracket on a tree, is a reproductive organ that is produced by the underground fungus, which may itself be very large. Fungi reproduce by means of single-celled spores that float away from the mushroom cap and germinate to form another network of mycelium in the soil.
Patterns of fruiting and seed dispersal. Just as brightly-colored flowers are an adaptation to attract pollinators, so colorful fruits are an adaptation to attract seed dispersal agents (and plumed seeds, like those of the milkweed, are an adaptation to catch the wind). A fruit is a plant organ that contains one or more seeds. Plant species that reproduce in mid-summer typically produce fruits that are high in sugar and low in fat; these are especially tasty to mammals like us (and also to birds). Examples are blackberries, blueberries, raspberries, and strawberries that are dispersed by year-round resident mammals such as raccoons and opossums. Species that reproduce in the fall commonly produce high-fat/low-sugar fruits that are especially attractive to migratory birds. Examples in our area are Dogwood and Southern Magnolia. The birds eat the fruits and the seeds pass through their digestive system unharmed, to be deposited in a location far away from the parent plant.
Other fall-fruiting plant species (Bayberry, Poison Ivy) produce low quality fruits that are not especially attractive, but that last a long time. These species are “holding out” in the expectation that in the winter when almost all other food sources are gone, their fruits will be eaten as a last resort and dispersed at that time. Many species of fruiting trees in this area display “ripening flags”—color changes that indicate to passing birds that the fruits on the tree are ripe and ready to eat.
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