Flowers

Flowers have many different shapes and sizes, and there are many variations in colour, number of flower parts and the arrangements of these parts. Flowers are the reproductive parts of plants which are responsible for the production of gametes (sex cells). After fusion of the male and female gametes a zygote is produced which develops into an embryo within the seed. This seed gives rise to a new flowering plant of the same kind.

Generally, a flower consists of four whorls, which are a ring of leaves, viz.

• the calyx on the outside;

• the corolla lying inside the calyx;

• the stamens enclosed by the corolla, and

• the pistil in the center of the flower.

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The Calyx.

The calyx is the outermost whorl of a flower. It consists of sepals, that are green. The sepals may be free from each other in the same flowers of some plants or fused to form a cup in flowers of other plants.

Functions

• The calyx encloses and protects the inner whorls in the bud stage.

• Since the sepals contain chlorophyll, they can also synthesize food.

The Corolla.

The corolla is found on the inside of the calyx and is the most conspicuous part in the flower because it is usually white/brightly coloured . This whorl is made up of petals which are much larger than sepals. The petals may be separate from each other or become partly/ completely fused. If the petals are completely fused they form a corolla tube. The calyx and corolla together are called the perianth .

Functions

• The brightly-coloured corolla attracts agents of pollination such as insects and birds.

• The corolla encloses and protects the stamens and pistil.

The Stamens.

The stamens form the third whorl on the inside of the corolla. Each stamen is made up of a slender flexible filament which supports the anther at its end. The anther is the male reproductive organ in a flower.

Functions

• The anther produces pollen grains which contain the male reproductive cells;<
• The filament bears and supports the anther in the most suitable position for pollen transfer to take place.

The Pistils.

The pistil is the fourth and the innermost whorl of the flower. It consists of a basal, swollen portion, the ovary, from which a long and slender style arises. The stigma is found at the tip of the style. The pistil is the female reproductive organ.

Functions

• The ovary is a hollow cavity which contains the ovules.

• Each ovule contains an egg cell.

• The elongated style bears the stigma in the most suitable position for receiving pollen during pollination.

• The sticky stigma at the tip of the style can receive or trap the pollen grains.

The stamens and pistils usually occur in the same flower. These are essential whorls. A flower bearing both the essential whorls is bisexual. Sometimes a flower has only the stamens or only the pistils, in which case it is referred to as a unisexual flower (i.e. the flower bears only one of the essential organs).

The calyx and the corolla are referred to as the non-essential whorls , since they are not responsible for the formation of gametes and seeds. This formation can occur in the absence of the outer whorls.

A flower in which all four whorls are present is said to be a complete flower. If any one of the non-essential whorls is not present, the flower is incomplete.

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Dicotyledonous Flower

Structure of a Dicotyledonous flower e.g. the Potato (Solanum tuberosum)

The Solanum tuberosum plant is fairly widely distributed. It grows very well in disturbed areas. The plants are shrubs/herbs with opposite leaves. Its flowers are large, bisexual and arranged on an inflorescence described as a cyme. Other good examples are the Petunia and Hibiscus.

General characteristics of the Potato flower

1. the floral parts are regular their arrangement;

2. the flowers are bisexual, i.e. the stamens (male parts) and the pistil (female parts) are on the same flower;

3. the flower consists of the four basic whorls (calyx, corolla, stamens and pistil) and each whorl is composed of five parts.

The Floral whorls

1. the calyx is large in relation to other parts. The parts, sepals, are five in number and are joined by a calyx tube. They are green due to the presence of chlorophyll.

2. the corolla consists of five petals which are joined at the base by a short corolla tube. The corolla is brightly purple which attracts insects for pollination.

3. the stamens form the male whorl or androecium. There are five stamens, each arising opposite a corolla segment.

4. the pistil is the female whorl/gynoecium. It has a single stigma, a single style and a single ovary. The ovary is superior (i.e. it is situated above the other floral whorls on the receptacle).

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Monocotyledonous Flower

Among the monocotyledonous flowering families, the Liliaceae, Amaryllidaceae, Iridaceae and the grass families, Gramineae or Poaceae are the most common.

The Aloe genus

• the Aloe genus is widely distributed and different species can exist under extreme dry conditions;
• it is a xerophytic plant, i.e. it is adapted to living in very dry areas;
• most plants of the Aloe genus is short, unbranched fleshy stem with adventitious roots;
• the leaves are triangular in shape, thick and fleshy, with thorns or spines along the edges;
• the flowers are on an inflorescence, which is a compound raceme;
• the youngest flowers appear at the tip of the fleshy peduncle, with the older ones towards its base.The penduncle is an inflorescence stalk.

Structure of a monocotyledonous flower such as the aloe

The aloe flower is attached to the penduncle by means of a short stalk, the pedicel. It is tubular in form.

• The Perianth
  • the perianth is the outer whorl which is red, orange or yellow in colour;
  • the perianth lobes are arranged in two whorls of the three outer (the calyx) and three inner (corolla) lobes;
  • such a perianth is referred to as perigone, and is characteristic of monocotyledons;
  • the perianth segments, the tepals, are fused towards the lower end and is either sepals or petals.
• The Stamens
  • there are six stamens, also arranged in two whorls of three each;
  • each stamen consists of a filament and an anther;
  • each anther has two lobes and the filaments are attached at the middle of the anther.
• The Pistil
  the pistil is situated in the middle of the flower. It consists of:
  • an ovary with three lobes, which represent the three fused carpels;
  • a thin, long style;
  • a simple stigma situated at the end of the style.
• Nectaries
  • nectaries or nectar glands are found at the base of the flower on the receptacle ;
  • they secrete a sweet fluid called nectar which attracts birds;
  • the aloe flower is pollinated by birds with long beaks such as the sugar bird and sunbird.

Comparison between mono- and dicotyledonous flowers

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