Flower specialization and pollination

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

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

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(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.

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

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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)

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.