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Each flower has a specific design which best encourages the transfer
of its pollen. Cleistogamous flowers are self pollinated, after which, they
may or may not open. Many Viola and some Salvia species are known to
have these types of flowers.
Entomophilous flowers attract and use insects, bats, birds or other
animals to transfer pollen from one flower to the next. Flowers commonly
have glands called nectaries on their various parts that attract these animals.
Some flowers have patterns, called nectar guides, that show pollina 136
tors where to look for nectar. Flowers also attract pollinators by scent and
color. Still other flowers use mimicry to attract pollinators. Some species
of orchids, for example, produce flowers resembling female bees in color,
shape, and scent. Flowers are also specialized in shape and have an arrangement
of the stamens that ensures that pollen grains are transferred to
the bodies of the pollinator when it lands in search of its attractant (such as
nectar, pollen, or a mate). In pursuing this attractant from many flowers of
the same species, the pollinator transfers pollen to the stigmas—arranged
with equally pointed precision—of all of the flowers it visits.
Anemophilous flowers use the wind to move pollen from one flower
to the next, examples include the grasses, Birch trees, Ragweed and Maples.
They have no need to attract pollinators and therefore tend not to be
"showy" flowers. Male and female reproductive organs are generally
found in separate flowers, the male flowers having a number of long filaments
terminating in exposed stamens, and the female flowers having
long, feather-like stigmas. Whereas the pollen of entomophilous flowers
tends to be large-grained, sticky, and rich in protein (another "reward" for
pollinators), anemophilous flower pollen is usually small-grained, very
light, and of little nutritional value to insects.
Morphology
Flowering plants are heterosporangiate, producing two types of reproductive
spores. The pollen (male spores) and ovules (female spores) are
produced in different organs, but the typical flower is a bisporangiate strobilus
in that it contains both organs.
A flower is regarded as a modified stem with shortened internodes and
bearing, at its nodes, structures that may be highly modified leaves. In essence,
a flower structure forms on a modified shoot or axis with an apical
meristem that does not grow continuously (growth is determinate). Flowers
may be attached to the plant in a few ways. If the flower has no stem
but forms in the axil of a leaf, it is called sessile. When one flower is produced,
the stem holding the flower is called a peduncle. If the peduncle
ends with groups of flowers, each stem that holds a flower is called a pedicel.
The flowering stem forms a terminal end which is called the torus or
receptacle. The parts of a flower are arranged in whorls on the torus. The
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four main parts or whorls (starting from the base of the flower or lowest
node and working upwards) are as follows:
An example of a perfect flower, this Crateva religiosa flower has both
stamens (outer ring) and a pistil (center).
Calyx: the outer whorl of sepals; typically these are green, but are
petal-like in some species.
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Corolla: the whorl of petals, which are usually thin, soft and colored to
attract insects that help the process of pollination.
Androecium (from Greek andros oikia: man's house): one or two
whorls of stamens, each a filament topped by an anther where pollen is
produced. Pollen contains the male gametes.
Gynoecium (from Greek gynaikos oikia: woman's house): one or more
pistils. The female reproductive organ is the carpel: this contains an ovary
with ovules (which contain female gametes). A pistil may consist of a
number of carpels merged together, in which case there is only one pistil to
each flower, or of a single individual carpel (the flower is then called apocarpous).
The sticky tip of the pistil, the stigma, is the receptor of pollen.
The supportive stalk, the style becomes the pathway for pollen tubes to
grow from pollen grains adhering to the stigma, to the ovules, carrying the
reproductive material.
Although the floral structure described above is considered the "typical"
structural plan, plant species show a wide variety of modifications
from this plan. These modifications have significance in the evolution of
flowering plants and are used extensively by botanists to establish relationships
among plant species. For example, the two subclasses of flowering
plants may be distinguished by the number of floral organs in each whorl:
dicotyledons typically having 4 or 5 organs (or a multiple of 4 or 5) in
each whorl and monocotyledons having three or some multiple of three.
The number of carpels in a compound pistil may be only two, or otherwise
not related to the above generalization for monocots and dicots.
In the majority of species individual flowers have both pistils and
stamens as described above. These flowers are described by botanists as
being perfect, bisexual, or hermaphrodite. However, in some species of
plants the flowers are imperfect or unisexual: having only either male
(stamens) or female (pistil) parts. In the latter case, if an individual plant is
either female or male the species is regarded as dioecious. However, where
unisexual male and female flowers appear on the same plant, the species is
considered monoecious.
Additional discussions on floral modifications from the basic plan are
presented in the articles on each of the basic parts of the flower. In those
species that have more than one flower on an axis — so-called composite
flowers — the collection of flowers is termed an inflorescence; this term
can also refer to the specific arrangements of flowers on a stem. In this regard,
care must be exercised in considering what a "flower" is. In botanical
terminology, a single daisy or sunflower for example, is not a flower but a
flower head — an inflorescence composed of numerous tiny flowers
138
(sometimes called florets). Each of these flowers may be anatomically as
described above. Many flowers have a symmetry, if the perianth is bisected
through the central axis from any point, symmetrical halves are
produced — the flower is called regular or actinomorphic, e. g. rose or trillium.
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When flowers are bisected and produce only one line that produces
symmetrical halves the flower is said to be irregular or zygomorphic, e. g.
snapdragon or most orchids.
Pollination
The primary purpose of a flower is reproduction. Flowers are the reproductive
organs and mediate the joining of the sperm contained within
pollen to the ovules, normally from one plant to another but many plants
also can pollinate there own flowers. The fertilized ovules produce seeds
that are the next generation. Sexual reproduction produces genetically
unique offspring, allowing for adaptation. Flowers have specific designs
which encourages the transfer of pollen from one plant to another of the
same species. Many plants are dependent upon external factors to move
pollen between flowers, including the wind and animals, especially insects.
Even large animals such as birds, bats, and pygmy possums can be employed.
The period of time during which this process can take place (the
flower is fully expanded and functional) is called anthesis.
Fertilization and dispersal
Some flowers with both stamens and a pistil are capable of selffertilization,
which does increase the chance of producing seeds but limits
genetic variation. The extreme case of self-fertilization occurs in flowers
that always self-fertilize, such as many dandelions. Conversely, many species
of plants have ways of preventing self-fertilization. Unisexual male
and female flowers on the same plant may not appear or mature at the
same time, or pollen from the same plant may be incapable of fertilizing its
ovules. The latter flower types, which have chemical barriers to their own
pollen, are referred to as self-sterile or self-incompatible.
LEAF
In botany, a leaf is an above-ground plant organ specialized for photosynthesis.
For this purpose, a leaf is typically flat (laminar) and thin, to expose
the cells containing chloroplast (chlorenchyma tissue, a type of parenchyma)
to light over a broad area, and to allow light to penetrate fully
into the tissues. Leaves are also the sites in most plants where transpiration
and guttation take place. Leaves can store food and water, and are modified
in some plants for other purposes. The comparable structures of ferns
139
are correctly referred to as fronds. Furthermore, leaves are prominent in
the human diet as leaf vegetables.
Leaf anatomy
A structurally complete leaf of an angiosperm consists of a petiole
(leaf stem), a lamina (leaf blade), and stipules (small processes located to
either side of the base of the petiole). The petiole attaches to the stem at a
point called the "leaf axil". Not every species produces leaves with all of
the afformentioned structural components. In some species, paired stipules
are not obvious or are absent altogether. A petiole may be absent, or the
blade may not be laminar (flattened). The tremendous variety shown in
leaf structure (anatomy) from species to species is presented in detail below
under Leaf morphology. After a period of time (i. e. seasonally, during
the autumn), deciduous trees shed their leaves. These leaves then decompose
into the soil.
A leaf is considered a plant organ and typically consists of the following
tissues:
- An epidermis that covers the upper and lower surfaces
- An interior chlorenchyma called the mesophyll
- An arrangement of veins (the vascular tissue).
Epidermis
The epidermis is the outer multi-layered group of cells covering the
leaf. It forms the boundary separating the plant's inner cells from the external
world. The epidermis serves several functions: protection against
water loss, regulation of gas exchange, secretion of metabolic compounds,
and (in some species) absorption of water. Most leaves show dorsoventral
anatomy: the upper (adaxial) and lower (abaxial) surfaces have somewhat
different construction and may serve different functions.
The epidermis is usually transparent (epidermal cells lack chloroplasts)
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and coated on the outer side with a waxy cuticle that prevents water
loss. The cuticle is in some cases thinner on the lower epidermis than on
the upper epidermis, and is thicker on leaves from dry climates as compared
with those from wet climates.
The epidermis tissue includes several differentiated cell types: epidermal
cells, guard cells, subsidiary cells, and epidermal hairs (trichomes).
The epidermal cells are the most numerous, largest, and least specialized.
These are typically more elongated in the leaves of monocots than in those
of dicots.
The epidermis is covered with pores called stomata, part of a stoma
complex consisting of a pore surrounded on each side by chloroplastcontaining
guard cells, and two to four subsidiary cells that lack chloro 140
plasts. The stoma complex regulates the exchange of gases and water vapor
between the outside air and the interior of the leaf. Typically, the stomata
are more numerous over the abaxial (lower) epidermis than the adaxial
(upper) epidermis.
Mesophyll
Most of the interior of the leaf between the upper and lower layers of
epidermis is a parenchyma (ground tissue) or chlorenchyma tissue called
the mesophyll (Greek for "middle leaf"). This assimilation tissue is the
primary location of photosynthesis in the plant. The products of photosynthesis
are called "assimilates".
Leaves are normally green in color, which comes from chlorophyll
found in plastids in the chlorenchyma cells. Plants that lack chlorophyll
cannot photosynthesize.
Leaves in temperate, boreal, and seasonally dry zones may be seasonally
deciduous (falling off or dying for the inclement season). This mechanism
to shed leaves is called abscission. After the leaf is shed, a leaf scar
develops on the twig. In cold autumns they sometimes change color, and
turn yellow, bright orange or red as various accessory pigments (carotenoids
and xanthophylls) are revealed when the tree responds to cold and
reduced sunlight by curtailing chlorophyll production. Red anthocyanin
pigments are now thought to be produced in the leaf as it dies.
Veins
The veins are the vascular tissue of the leaf and are located in the
spongy layer of the mesophyll. They are typical examples of pattern formation
through ramification. The pattern of the veins is called venation.
The veins are made up of:
- xylem, tubes that brings water and minerals from the roots into the
leaf;
- phloem, tubes that usually moves sap, with dissolved sucrose, produced
by photosynthesis in the leaf, out of the leaf.
The xylem typically lies over the phloem. Both are embedded in a
dense parenchyma tissue, called "pith", with usually some structural collenchyma
tissue present.
Leaf morphology
External leaf characteristics (such as shape, margin, hairs, etc.) are
important for identifying plant species, and botanists have developed a rich
terminology for describing leaf characteristics. These structures are a part
of what makes leaves determinant, they grow and achieve a specific pattern
and shape, then stop. Other plant parts like stems or roots are non 141
determinant, and will usually continue to grow as long as they have the resources
to do so.
Classification of leaves can occur through many different designative
schema, and the type of leaf is usually characteristic of a species, although
some species produce more than one type of leaf. The longest type of leaf
is a leaf from palm trees, measuring at nine feet long.
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