Mitosis
| I | INTRODUCTION |
Mitosis, process in which a cell’s nucleus replicates
and divides in preparation for division of the cell. Mitosis results in two
cells that are genetically identical, a necessary condition for the normal
functioning of virtually all cells. Mitosis is vital for growth; for repair and
replacement of damaged or worn out cells; and for asexual reproduction, or
reproduction without eggs and sperm.
All multicellular animals, plants, fungi, and
protists, which begin life as single cells, carry out mitosis to develop into
complex organisms containing billions of cells. Mitosis continues in full-grown
organisms as a means of maintaining the organism—replacing dying skin cells, for
example, or repairing damaged muscle cells. In the cells of the adult human
body, mitosis occurs about 25 million times per second. Multicellular organisms
such as sea stars, sea anemones, fungi, and certain plants rely on mitosis for
asexual reproduction at particular stages in their life cycles, and mitosis is
the sole mode of reproduction for many single-celled organisms.
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| interphase |
| II | INTERPHASE |
The life cycle of eukaryotic cells, or cells
containing a nucleus, is a continuous process typically divided into three
phases for ease of understanding: interphase, mitosis, and cytokinesis.
Interphase includes three stages, referred to as G1, S and
G2. In G1, a newly formed cell synthesizes materials
needed for cell growth. In the S stage, deoxyribonucleic acid (DNA), the genetic
material of the cell, is replicated. At this stage, DNA consists of long, thin
strands called chromatin. As each strand is replicated, it is linked to its
duplicate by a structure known as a centromere. When the S stage is complete,
the cell enters a brief stage known as G2, when specialized enzymes
correct any errors in the newly synthesized DNA, and proteins involved with the
next phase, mitosis, are synthesized.
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| Prophase |
| III | MITOSIS |
Mitosis occurs in five steps: prophase,
prometaphase, metaphase, anaphase, and telophase . In prophase the replicated,
linked DNA strands slowly wrap around proteins that in turn coil and condense
into two short, thick, rodlike structures called chromatids, attached by the
centromere. Two structures called centrioles, both located on one side of the
nucleus, separate and move toward opposite poles of the cell. As the centrioles
move apart, they begin to radiate thin, hollow, proteins called microtubules.
The microtubules arrange themselves in the shape of a football, or spindle, that
spans the cell, with the widest part at the center of the cell and the narrower
ends at opposite poles.
Prometaphase is marked by the disintegration
of the nuclear membrane. As the spindle forms, the nuclear membrane breaks down
into tiny sacs or vesicles that are dispersed in the cytoplasm. The spindle
fibers attach to the chromatids near the centromeres, and tug and push the
chromatids so that they line up in the equatorial plane of the cell halfway
between the poles. Like two individuals standing back to back at the equator,
one chromatid faces one pole of the cell, and its linked partner faces the
opposite pole.
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| Metaphase |
In metaphase, exactly half of the chromatids
face one pole, and the other half face the other pole. This equilibrium position
is called the metaphase plate.
Anaphase begins when the centromeres split,
separating the identical chromatids into single chromosomes, which then move
along the spindle fibers to opposite poles of the cell. At the end of anaphase,
two identical groups of single chromosomes congregate at opposite poles of the
cell.
In telophase, the final stage of mitosis, a
new nuclear membrane forms around each new group of chromosomes. The spindle
fibers break down and the newly formed chromosomes begin to unwind. If viewed
under a light microscope, the chromosomes appear to fade away. They exist,
however, in the form of chromatin, the extended, thin strands of DNA too fine to
be seen except with electron microscopes. Mitosis accomplishes replication and
division of the nucleus, but the cell has yet to divide.
| IV | CYTOKINESIS |
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| cytokinesis |
The final phase of the cell cycle is known as
cytokinesis. The timing of cytokinesis varies depending on the cell type. It can
begin in anaphase and finish in telophase; or it can follow telophase. In
cytokinesis, the cell’s cytoplasm separates in half, with each half containing
one nucleus. Animals and plants accomplish cytokinesis in slightly different
ways. In animals, the cell membrane pinches in, creating a cleavage furrow,
until the mother cell is pinched in half. In plants, cellulose and other
materials that make up the cell wall are transported to the midline of the cell
and a new cell wall is constructed. The process of DNA replication, the precise
alignment of the chromosomes in mitosis, and the successful separation of
identical chromatids in anaphase results in two new cells that are genetically
identical. The new cells enter interphase, and the cell cycle begins again.
| V | CONTROL OF CELL DIVISION |
In multicellular organisms, cell division must
be carefully regulated to ensure that growth of the organism is coordinated,
replacement of dead cells takes place in an orderly fashion, and repair of
injured cells is initiated when needed. Cell division must also be halted when
growth and repair are completed. Cell division is controlled by a variety of
factors. One of the most important controls is carried out by molecules called
growth factors.
Growth factors first come into play late in
the G1stage of interphase. Cells cannot pass from G1 to
the S stage unless growth factors bind to the plasma membrane. The binding of
growth factors triggers a cascade of biochemical activity that propels the cell
into the S stage. If the cell does not enter the S stage, it exits from the cell
cycle into the G0 stage, a period of normal metabolic activity where
other control mechanisms prevent it from dividing. Most of the cells in the
adult human body remain in the G0 stage throughout life. Certain
cells, such as bone, muscle, or liver cells, can return to the cell cycle and
divide if they are injured. Injuries release growth factors that override the
controls over the non-dividing state.
Once a cell is committed to dividing, still
other growth factors ensure that steps in mitosis are carried out accurately. At
the end of the G2 stage, mitotic (or maturation) promoting factor
(MPF) triggers prophase, and enzymes condense DNA into chromosomes, break down
the nuclear membrane, and form the spindle. A complex interplay of other growth
factors carries the cell through the remaining steps of mitosis and
cytokinesis.
Scientists have identified over 50 different
growth factors. Some are very specific, and react only with certain cells. Nerve
growth factor, for example, stimulates the growth of nerve cells during
embryonic development, but has no effect on other cells. Others, such as
epidermal growth factor, control division in a variety of cells. Understanding
the production of growth factors and their precise mode of activity pose
significant research challenges. As scientists learn more about the mechanisms
for normal cell division, they gain insight into the causes of the unregulated
cell growth that leads to cancer.
Asexual Reproduction
Asexual Reproduction, the
formation of a new individual from cells of the parent, without meiosis, gamete
formation, or fertilization. There are several types of asexual reproduction.
Fission is the simplest form and involves the division of a single
organism into two complete organisms, each identical to the other and to the
parent. Fission is common among unicellular organisms such as bacteria, many
protists, some algae such as Spirogyra and Euglena, as well as a few higher
organisms such as flatworms and certain species of polychaete worms. A similar
form of asexual reproduction is regeneration, in which an entire organism may be
generated from a part of its parent. The term regeneration normally refers to
regrowth of missing or damaged body parts in higher organisms, but whole body
regeneration occurs in hydroids (see Hydra), starfish, and many plants.
Spores are another form of asexual reproduction and are common among bacteria,
protists, and fungi. Spores are DNA-containing capsules capable of sprouting
into new organisms; unlike most seeds, spores are produced without sexual union
of gametes, that is, reproductive cells.
Budding is another method of asexual reproduction
in which a group of self-supportive cells sprouts from and then detaches from
the parent organism. Unlike eggs or spores, buds are multicellular and usually
contain more than one cell layer. Hydroids and sea squirts reproduce by budding.
Vegetative reproduction is common among plants and consists of certain
parts that grow out from a main parent plant and eventually root and sprout to
form new, independent plants. Examples are the runners of strawberries, the
tubers of potatoes, and the bulbs of onions. Parthenogenesis is an important
means of asexual reproduction in which new individuals are formed from
unfertilized eggs. It occurs in some insects, amphibians, reptiles, and birds
and in some species of plants.
Asexual reproduction may provide a secondary means of
multiplying in organisms that ordinarily reproduce sexually. Under certain
conditions this may be the only way to reproduce. For example, if there are no
other individuals with which to exchange gametes or, in plants, if pollinators
are absent. Asexual reproduction demands less time and energy and may be the
most efficient way for certain species to reproduce under harsh environmental
conditions. Some species switch between asexual and sexual modes of reproduction
in an annual cycle so that each takes place at the most favorable time. Examples
of this are aphids that reproduce asexually in the summer but sexually in the
fall, and water fleas, which have a similar cycle.
Organisms that can reproduce asexually are particularly
well suited to colonize new habitats because a single individual can establish
an entire new population. This is the case in many invasive and weedy species,
including many pests introduced from one region to another. An example is the
common dandelion, which can reproduce from unfertilized seeds.
The offspring of organisms that reproduce asexually are
genetically identical to their parents and to each other. Without sexual
reproduction, the species cannot benefit from the variability introduced by
mixing genes (see Natural Selection). Therefore, evolutionary adaptation
to changing environmental conditions may proceed slowly.
Meiosis
Meiosis, process of cell
division in which the cell’s genetic information, contained in chromosomes, is
mixed and divided into sex cells with half the normal number of chromosomes. The
sex cells can later combine to form offspring with the full number of
chromosomes. The random sorting of chromosomes during meiosis assures that each
new sex cell, and therefore each new offspring, has a unique genetic
inheritance.
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| meiosis |
Meiosis differs from normal cell division, or mitosis, in
that it involves two consecutive cell divisions instead of one and the genetic
material contained in chromosomes is not copied during the second meiotic
division. Whereas mitosis produces identical daughter cells, meiosis randomly
mixes the chromosomes, resulting in unique combinations of chromosomes in each
daughter cell.
To illustrate the steps of meiosis, consider a corn plant
cell with 10 pairs of chromosomes. The normal number of chromosomes, or diploid
number, for corn is 20. In order for the diploid corn cell to reproduce, it must
undergo meiosis to produce cells with half the normal number of chromosomes,
called the haploid number. Each haploid corn cell contains only 10
chromosomes.
Prior to meiosis, the corn cell undergoes interphase, in
which it synthesizes materials needed for cell growth and prepares for cell
division. During this stage the cell’s genetic information, in the form of
deoxyribonucleic acid (DNA), is replicated. Each of the two consecutive cell
divisions consists of four stages: prophase, metaphase, anaphase, and
telophase.
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| meiosis |
In prophase I each long DNA strand wraps around proteins
that in turn coil and condense to form a chromosome. Since the DNA was copied
during interphase, each chromosome condenses to form two identical chromatids,
joined at a centromere. A corn cell has 20 chromosomes at this stage, each with
two identical chromatids, making a total of 40 chromatids.
Chromosomes exist in pairs; one is inherited from the
mother (maternal) and one from the father (paternal). When the chromosomes
duplicate, two maternal and two paternal chromatids are produced. These two
pairs of chromatids gather together in groups of four called tetrads. Each corn
cell contains 10 tetrads. While grouped together in tetrads, sections of the
chromatids from the maternal pair may randomly exchange, or cross over, with
sections of the paternal chromatid pair. Called genetic recombination,
this process is the first of two ways that meiosis mixes genetic information
during sexual reproduction.
Also in prophase I, two structures called centrioles,
both located on one side of the nucleus, separate and move toward opposite sides
of the cell. As the centrioles move apart, they radiate thin hollow structures
called spindle fibers. The membrane around the nucleus of the cell breaks down,
marking the beginning of the next stage.
During metaphase I, the spindle fibers attach to the
chromatids near the centrioles. The spindle fibers move the tetrads so that they
line up in a plane halfway between two centrioles.
Anaphase I begins when the spindle fibers pull the
tetrads apart, pulling the maternal and paternal chromosomes toward opposite
sides of the cell. The first meiotic division concludes with telophase I, when
the two new groups of chromosomes reach opposite sides of the cell. A nuclear
membrane may form around the two new groups of chromosomes and a division of
cell cytoplasm forms two new daughter cells.
Each daughter corn cell receives 10 chromosomes made up
of a random mixture of maternal and paternal chromosomes. This second mixing of
genetic information is called independent assortment. Genetic recombination and
independent assortment make it possible for parents to have many offspring who
are all different from each other.
In the second meiotic division the cell moves directly
into prophase II, skipping the interphase replication of DNA. Each corn cell
begins the second division with 10 chromosomes. Once again the centrioles
radiate spindle fibers as they move to opposite sides of the cell. During
metaphase II, the chromosomes line up along the plane in the center of the cell,
and in anaphase II the pairs of chromatids are pulled apart, each moving toward
opposite ends of the cell.
Telophase II completes meiosis. The spindle fibers
disappear and a new nuclear membrane forms around each new group of chromosomes
to form four haploid cells. The original diploid corn cell with 20 chromosomes
has undergone meiosis to form four haploid daughter cells, each containing 10
chromatids. It is now possible for two haploid sex cells to join during
fertilization to form one egg cell with the normal diploid number of chromatids.
After fusion and DNA replication, two haploid corn cells will yield one diploid
egg cell with 10 pairs of chromosomes.
In humans meiosis occurs only in the reproductive organs,
the testes in males and the ovaries in females. In males, each of the meiotic
divisions result in four equally sized haploid cells that mature into functional
sperm cells. In females, the meiotic divisions are uneven, resulting in three
tiny cells called polar bodies and one large egg that can be fertilized.
Flower
Flowers typically are composed of four parts,
or whorls, arranged in concentric rings attached to the tip of the stem. From
innermost to outermost, these whorls are the (1) pistil, (2) stamens, (3)
petals, and (4) sepals.
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Parts of a Flower
All flowers share several basic
features. Sepals, protective coverings that are closed over the bud
before it blooms, are the outermost flower parts. One step inward lie the
petals, which serve to attract pollinators using both coloration and
scent-producing glands. Inside the petals are the flower's sexual organs, the
stamens and pistil. Each stamen, the pollen producing part of the flower,
includes an anther and a filament. At the center of the flower is
the pistil, composed of a stigma, a style, and an ovary.
Within the ovary is a small cavity that contains the ovule, an egg-shaped
structure that, when fertilized, eventually becomes a seed.
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A. Pistil
The innermost whorl, located in the
center of the flower, is the female reproductive structure, or pistil. Often
vase-shaped, the pistil consists of three parts: the stigma, the style, and the
ovary. The stigma, a slightly flared and sticky structure at the top of the
pistil, functions by trapping pollen grains, the structures that give rise to
the sperm cells necessary for fertilization. The style is a narrow stalk that
supports the stigma. The style rises from the ovary, a slightly swollen
structure seated at the base of the flower. Depending on the species, the ovary
contains one or more ovules, each of which holds one egg cell. After
fertilization, the ovules develop into seeds, while the ovary enlarges into the
fruit. If a flower has only one ovule, the fruit will contain one seed, as in a
peach. The fruit of a flower with many ovules, such as a tomato, will have many
seeds. An ovary that contains one or more ovules also is called a carpel, and a
pistil may be composed of one to several carpels.
| B | Stamens |
The next whorl consists of the male
reproductive structures, several to many stamens arranged around the pistil. A
stamen consists of a slender stalk called the filament, which supports the
anther, a tiny compartment where pollen forms. When a flower is still an
immature, unopened bud, the filaments are short and serve to transport nutrients
to the developing pollen. As the flower opens, the filaments lengthen and hold
the anthers higher in the flower, where the pollen grains are more likely to be
picked up by visiting animals, wind, or in the case of some aquatic plants, by
water. The animals, wind, or water might then carry the pollen to the stigma of
an appropriate flower. The placement of pollen on the stigma is called
pollination. Pollination initiates the process of fertilization.
| C | Petals |
Petals, the next whorl, surround the
stamens and collectively are termed the corolla. Many petals have bright colors,
which attract animals that carry out pollination, collectively termed
pollinators. Three groups of pigments—alone or in combination—produce a
veritable rainbow of petal colors: anthocyanins yield shades of violet, blue,
and red; betalains create reds; and carotenoids produce yellows and orange.
Petal color can be modified in several ways. Texture, for example, can play a
role in the overall effect—a smooth petal is shiny, while a rough one appears
velvety. If cells inside the petal are filled with starch, they create a white
layer that makes pigments appear brighter. Petals with flat air spaces between
cells shimmer iridescently.
In some flowers, the pigments form
distinct patterns, invisible to humans but visible to bees, who can see
ultraviolet light. Like the landing strips of an airport, these patterns, called
nectar guides, direct bees to the nectar within the flower. Nectar is made in
specialized glands located at or near the petal’s base. Some flowers secrete
copious amounts of nectar and attract big pollinators with large appetites, such
as bats. Other flowers, particularly those that depend on wind or water to
transport their pollen, may secrete little or no nectar. The petals of many
species also are the source of the fragrances that attract pollinators. In these
species, the petals house tiny glands that produce essential, or volatile, oils
that vaporize easily, often releasing a distinctive aroma. One flower can make
dozens of different essential oils, which mingle to yield the flower’s unique
fragrance.
| D | Sepals |
The sepals, the outermost whorl, together
are called the calyx. In the flower bud, the sepals tightly enclose and protect
the petals, stamens, and pistil from rain or insects. The sepals unfurl as the
flower opens and often resemble small green leaves at the flower’s base. In some
flowers, the sepals are colorful and work with the petals to attract
pollinators.
| E | Variations in Structure |
Like virtually all forms in nature,
flowers display many variations in their structure. Most flowers have all four
whorls—pistil, stamens, petals, and sepals. Botanists call these complete
flowers. But some flowers are incomplete, meaning they lack one or more whorls.
Incomplete flowers are most common in plants whose pollen is dispersed by the
wind or water. Since these flowers do not need to attract pollinators, most have
no petals, and some even lack sepals. Certain wind-pollinated flowers do have
small sepals and petals that create eddies in the wind, directing pollen to
swirl around and settle on the flower. In still other flowers, the petals and
sepals are fused into structures called a floral tube.
Flowers that lack either stamens or a
pistil are said to be imperfect. The petal-like rays on the edge of a sunflower,
for example, are actually tiny, imperfect flowers that lack stamens. Imperfect
flowers can still function in sexual reproduction. A flower that lacks a pistil
but has stamens produces pollen, and a flower with a pistil but no stamens
provides ovules and can develop into fruits and seeds. Flowers that have only
stamens are termed staminate, and flowers that have only a pistil are called
pistillate.
Although a single flower can be either
staminate or pistillate, a plant species must have both to reproduce sexually.
In some species with imperfect flowers, the staminate and pistillate flowers
occur on the same plant. Such plants, known as monoecious species, include corn.
The tassel at the top of the corn plant consists of hundreds of tiny staminate
flowers, and the ears, which are located laterally on the stem, contain clusters
of pistillate flowers. The silks of corn are very long styles leading to the
ovaries, which, when ripe, form the kernels of corn. In dioecious species—such
as date, willow, and hemp—staminate and pistillate flowers are found on
different plants. A date tree, for example, will develop male or female flowers
but not both. In dioecious species, at least two plants, one bearing staminate
flowers and one bearing pistillate flowers, are needed for pollination and
fertilization.
Other variations are found in the types
of stems that support flowers. In some species, flowers are attached to only one
main stem, called the peduncle. In others, flowers are attached to smaller
stems, called pedicels, that branch from the peduncle. The peduncle and pedicels
orient a flower so that its pollinator can reach it. In the morning glory, for
example, pedicels hold the flowers in a horizontal position. This enables their
hummingbird pollinators to feed since they do not crawl into the flower as other
pollinators do, but hover near the flower and lick the nectar with their long
tongues. Scientists assign specific terms to the different flower and stem
arrangements to assist in the precise identification of a flower. A plant with
just one flower at the tip of the peduncle—a tulip, for example—is termed
solitary. In a spike, such as sage, flowers are attached to the sides of the
peduncle.
Sometimes flowers are grouped together in
a cluster called an inflorescence. In an indeterminate inflorescence, the lower
flowers bloom first, and blooming proceeds over a period of days from the bottom
to the top of the peduncle or pedicels. As long as light, water, temperature,
and nutrients are favorable, the tip of the peduncle or pedicel continues to add
new buds. There are several types of indeterminate inflorescences. These include
the raceme, formed by a series of pedicels that emerge from the peduncle, as in
snapdragons and lupines; and the panicle, in which the series of pedicels
branches and rebranches, as in lilac.
In determinate inflorescences, called
cymes, the peduncle is capped by a flower bud, which prevents the stem from
elongating and adding more flowers. However, new flower buds appear on side
pedicels that form below the central flower, and the flowers bloom from the top
to the bottom of the pedicels. Flowers that bloom in cymes include chickweed and
phlox.
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Flower Pollination and Fertilization
Flowers contain the structures
necessary for sexual reproduction. The male component, or stamen, consists of a
thin stalk called the filament, capped by the anther. The female component, the
pistil, includes the stigma, a sticky surface that catches pollen; the ovary,
which contains the ovule and embryo sac with its egg; and the style, a tube that
connects the stigma and ovary (A). Pollen is produced in the anther (B), and is
released when mature (C). Each mature pollen grain contains two sperm cells. In
self-pollinating plants, the pollen lands on the stigma of the same flower, but
in cross-pollinating plants—the majority of plants—the pollen is carried by
wind, water, insects, or small animals to another flower. If the pollen attaches
to the stigma of a flower from the same species, the pollen produces a pollen
tube, which grows down the neck of the style, transporting the sperm to the
ovule (D). Within the embryo sac of the ovule, one sperm cell fertilizes the
egg, which develops into a seed. The second sperm cell unites with two cells in
the embryo sac called polar nuclei, and this results in the development of the
endosperm, the starchy food that feeds the developing seed. The ovary enlarges
(E) and becomes a fruit.
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| III | SEXUAL REPRODUCTION |
Sexual reproduction mixes the hereditary
material from two parents, creating a population of genetically diverse
offspring. Such a population can better withstand environmental changes. Unlike
animals, flowers cannot move from place to place, yet sexual reproduction
requires the union of the egg from one parent with the sperm from another
parent. Flowers overcome their lack of mobility through the all-important
process of pollination. Pollination occurs in several ways. In most flowers
pollinated by insects and other animals, the pollen escapes through pores in the
anthers. As pollinators forage for food, the pollen sticks to their body and
then rubs off on the flower's stigma, or on the stigma of the next flower they
visit. In plants that rely on wind for pollination, the anthers burst open,
releasing a cloud of yellow, powdery pollen that drifts to other flowers. In a
few aquatic plants, pollen is released into the water, where it floats to other
flowers.
Pollen consists of thousands of microscopic
pollen grains. A tough pollen wall surrounds each grain. In most flowers, the
pollen grains released from the anthers contain two cells. If a pollen grain
lands on the stigma of the same species, the pollen grain germinates—one cell
within the grain emerges through the pollen wall and contacts the surface of the
stigma, where it begins to elongate. The lengthening cell grows through the
stigma and style, forming a pollen tube that transports the other cell within
the pollen down the style to the ovary. As the tube grows, the cell within it
divides to produce two sperm cells, the male sex cells. In some species, the
sperm are produced before the pollen is released from the anther.
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| Seed
Monocot and Dicot Seeds
Monocotyledons (monocots) and
dicotyledons (dicots) make up the two large groups of flowering plants,
differentiated by their seed structures. Monocot seeds contain one cotyledon, or
embryonic leaf. When these seeds germinate, the cotyledon remains below ground,
absorbing nutrients from the endosperm, the starchy food supply in the seed. The
coytledon transports these nutrients to the developing seedling. Dicot seeds
contain two coytledons, which absorb and store the nutrients from the endosperm
before the seed germinates. The cotyledons, thick with stored nutrients, emerge
above ground during germination, and then transport the stored nutrients to the
developing seedling. For a brief time, the cotyledons also serve as the first
photosynthesizing leaves, but they wither and die when the true leaves
emerge.
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Independently of the pollen germination and
pollen tube growth, developmental changes occur within the ovary. The ovule
produces several specialized structures—among them, the egg, or female sex cell.
The pollen tube grows into the ovary, crosses the ovule wall, and releases the
two sperm cells into the ovule. One sperm unites with the egg, triggering
hormonal changes that transform the ovule into a seed. The outer wall of the
ovule develops into the seed coat, while the fertilized egg grows into an
embryonic plant. The growing embryonic plant relies on a starchy, nutrient-rich
food in the seed called endosperm. Endosperm develops from the union of the
second sperm with the two polar nuclei, also known as the central cell nuclei,
structures also produced by the ovary. As the seed grows, hormones are released
that stimulate the walls of the ovary to expand, and it develops into the fruit.
The mature fruit often is hundreds or even thousands of times larger than the
tiny ovary from which it grew, and the seeds also are quite large compared to
the miniscule ovules from which they originated. The fruits, which are unique to
flowering plants, play an extremely important role in dispersing seeds. Animals
eat fruits, such as berries and grains. The seeds pass through the digestive
tract of the animal unharmed and are deposited in a wide variety of locations,
where they germinate to produce the next generation of flowering plants, thus
continuing the species. Other fruits are dispersed far and wide by wind or
water; the fruit of maple trees, for example, has a winglike structure that
catches the wind.
| IV | FLOWERING AND THE LIFE CYCLE |
The life cycle of a flowering plant begins
when the seed germinates. It progresses through the growth of roots, stems, and
leaves; formation of flower buds; pollination and fertilization; and seed and
fruit development. The life cycle ends with senescence, or old age, and death.
Depending on the species, the life cycle of a plant may last one, two, or many
years. Plants called annuals carry out their life cycle within one year.
Biennial plants live for two years: The first year they produce leaves, and in
the second year they produce flowers and fruits and then die. Perennial plants
live for more than one year. Some perennials bloom every year, while others,
like agave, live for years without flowering and then in a few weeks produce
thousands of flowers, fruits, and seeds before dying.
Whatever the life cycle, most plants flower
in response to certain cues. A number of factors influence the timing of
flowering. The age of the plant is critical—most plants must be at least one or
two weeks old before they bloom; presumably they need this time to accumulate
the energy reserves required for flowering. The number of hours of darkness is
another factor that influences flowering. Many species bloom only when the night
is just the right length—a phenomenon called photoperiodism. Poinsettias, for
example, flower in winter when the nights are long, while spinach blooms when
the nights are short—late spring through late summer. Temperature, light
intensity, and moisture also affect the time of flowering. In the desert, for
example, heavy rains that follow a long dry period often trigger flowers to
bloom.
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| Fruit dispersal |
Lesser Burdock depends on its hooks to attach to animal skin (animal dispersal) then the animal moves about and disperses the seed, the rose hip is a fruit eaten by mammals , while their seed remain undigested and passed on with their feces (animal dispersal) , the milkweed and pussy williow depend on wind dispersal with their light-weight parachute like seed carriers, lastly the coconut fruit, and hence seed, is dispersed with the help of water waves on the shore, as far as wave can take them away.
