The part of an angiosperm (flowering
plant) which is concerned with reproducing the species. A flower is essentially
an unlengthened shoot whose leaves are modified to form the floral
organs (petals, etc).
Close-up of a buttercup
The buttercup is a convenient, simple flower to study. The stalk of the
flower is called the pedicel and is swollen at the tip
forming the receptacle. During development of the flower
the rapidly dividing cells of the growing tip produce the floral
leaves in the same way that leaves are produced on ordinary stems.
The floral leaves develop in more or less concentric circles (whorls),
but in some of the more primitive flowers, such as the water-lily, the petals
and other organs are arranged in a spiral on the receptacle. The first formed
organs of the flower are the five sepals.
These are on the lowest part of the receptacle and form the calyx.
The sepals are green, leaf-like organs whose chief purpose is to protect
the developing flower. Above the sepals are the five yellow petals
each with a small pocket at its base which produces nectar
and is called a nectary. The group of petals is called
the corolla and, with the calyx, forms the
perianth. The petals and nectaries attract
insects and other animals to the flower and also help to protect the essential
sexual organs within. These are two types of reproductive organs in the
buttercup flower – the stamens and
the carpels. The stamens are the male, pollen-producing
organs and form the androecium. Each
stamen consists of a stalk (filament) and an anther
which is the pollen sac at the tip. The carpels form the gynoecium.
Each carpel contains a female egg-cell or ovule
which gives rise to a seed when fertilized
by a male cell from a pollen grain. The stigma
is the tip of the carpel through which the pollen grain gains access to
the ovule. The buttercup flower has numerous stamens and carpels but this
is not the case in all flowers.
| Longitudinal section of a buttercup,
an hypogynous flower.
Below the flower there may be one or two tiny leaves (bracteoles)
on the pedicel. These are the very earliest of the floral leaves. The pedicel
tip continues to grow after forming them but its growth almost ceases when
the sepals are formed so that the floral organs are close together on the
receptacle. In a flower such as the snowdrop the bracteole protects the
flower until it opens. It can be seen as a green scale at the back of the
flower. A leaf at the base of the flower stalk is called a bract.
Varieties of flower structure
The buttercup as we have seen has all four types of floral organ and the
parts are arranged in a regular manner. The petals are all of the same size
shape and a section cut down through the flower (a longitudinal section)
will always produce two similar halves. There are, however, many variations
on this structure in the plant kingdom. Many are concerned with pollination
which is the transference of pollen from flower to flower. Pea-flowers and
many orchids are highly irregular with very oddly shaped petals. There is
in such cases only one line along which a section will produce two similar
halves. Frequently the petals (and the sepals) join to form a tube which
gives added protection and may also serve to hold nectar. Examples are the
primrose and bluebell. The carpels are usually above the rest of the flower
because of the manner of growth. This is the hypogynous condition
but it is not universal. In some flowers the receptacle spreads at the top
so that the petals, etc., surround the carpels (perigynous condition)
while in others (e.g., the apple) the receptacle grows up around the carpels
so that they are below the other parts (epigynous condition).
Any type of organ may may be absent from a flower. If there are no petals
the sepals are often brightly colored (e.g., the marsh marigold). Flowers,
especially those of trees (e.g. hazel), may be unisexual having only male
or female parts in each individual flower. The stamens do not vary a great
deal but may be joined to each other (e.g. sweet pea) or to the petals (e.g.,
primrose). The carpels may contain one or more ovules, each of which gives
rise to a seed. The pea pod, for instance, is derived from a single carpel
with several ovules. Carpels may be joined as in the bluebell or free as
in the buttercup. The flowering plants are classified mainly according to
the flower structure.
|Plum flower (left). Although the receptacle has grown
up around the carpel, the flower is still perigynous. The receptacle
of the apple (right) has enclosed the carpels producing an epigynous
Flowers are sometimes borne singly (e.g., anemone, tulip) but more frequently
they occur in a group, called an inflorescence,
whose appearance depends upon the amount and type of branching. There are
however two basic patterns. In one – the racemose
pattern – the main growing point of the stem goes on growing –
or at least does not produce a flower. The flowers are produced laterally
(on the sides) e.g., the bluebell, and the inflorescence is termed a raceme.
If the flowers are not stalked it called a spike. An umbel
is a special raceme in which the main tip stops growing and all the flower
stalks develop at one level producing the familiar head of flowers such
as is found in the hogweed and other hedgerow plants. The umbel must not
be confused with the corymb, however. This is a raceme
in which the pedicels are of different lengths so that the flowers all appear
at one level. The grouping of the small flowers in this way makes them more
attractive to insects. In the second basic pattern – the cymose
pattern – the stalk does end in a flower after giving off obe or two
branches which also end in a flower after branching. The stitchwort is an
example of a cyme.
Flowers of the family Compositae are very specialized.
The dandelion "flower" is really a collection of tiny flowers (florets)
on a flat disk or capitulum. Each floret contains sexual
organs and is a complete flower. The calyx is represented by fine hairs
which later develop and carry away the seed. Each dandelion floret has a
flat blade or ligule but thistles have only tubular florets.
Daises have both types – the outer florets have colored ligules to
attract insects while the inner florets are tubular and produce pollen and
nectar. The Compositae is a very widespread and successful family
Floral formulae and floral diagrams
Botanists do not need to have a lengthy description of a flower to understand
its structure. They use a simple expression – the floral formula.
This tells them the number of parts and a good deal about their arrangement.
The letters K, C, A, and G stand for calyx, corolla, androecium, and gynecium,
respectively. P (for perianth) is used if the sepals and petals are alike.
The formula for a buttercup is K5 C5 A∞ G∞, where ∞ means
"numerous". A line under the carpel figure means that the flower is hypogynous.
Above the figure it indicates epigyny. Where parts are joined, brackers
surround the figure. The formula does not give a complete description. A
floral diagram and a section cut through the flower are required to make
the structure quite clear.
The floral diagram consists of a plan view of the flower with the organs
arranged on circles or spirals, showing the degree of overlapping, any fusion
of parts or irregularity and the position relative to the main stem of the
plant (indicated by a small circle). Bracts and bracteoles are also shown.
The longitudinal section is necessary to show the degree of perigyny if