All living things can reproduce their own kind. The smallest, single-celled plants, the algae and bacteria, reproduce by dividing themselves. Ferns and fungi form tiny specialized cells called spores, which are highly resistant to adverse growing conditions. They can rest almost indefinitely during poor growing periods, and are ready to sprout as soon as conditions improve.
The most highly evolved plants reproduce themselves by forming the intricate, colourful, delicate, richly scented and beautiful structures called flowers. Flowers are regarded as modifications in form of leaves, converted for the sake of reproduction. Their primary purpose is sexual, to form and house the male and female organs of reproduction and to facilitate their union. From such a union comes the seed of a new generation.
Flowers are formed from the meristem at the ends of stems or shoots or on very short stalks in the axils of leaves on stems.
They may arise singly, or they may arise in groups from a single point of origin when they are spoken of as an in-florescence or flower head. There are many different ways in which these tiny groups of flowers can be arranged and each is given a different name.
In some cases the individual flower has four kinds of parts, all growing from a common base, called the receptacle, at the head of the stalk or peduncle. On dissecting such a flower as that of theor buttercup (which belong to the same flower family, Ranunculaceae), working from the outside in, the first parts to be removed are the sepals, collectively called the calyx. Just inside will be found the coloured petals, together forming what is known as the corolla. Sepals and petals together make up the perianth.
Taking off the petals exposes the male sex organs known as stamens. Each stamen has a stalk or filament at the head of which is attached a sac or anther, containing the pollen grains which are the male cells of the plant. The stamens are collectively known as the androecium (male part).
With the stamens removed, a central structure, the pistil or gynaeceum (female part), is revealed. This structure will be found to be made up of smaller stalked bodies, apparently fused together. These are carpels. Each carpel is made up of a hollowed swelling (ovary) at its base, with a small stalk (style) arising from it and having a sticky top (stigma). In the swelling are tiny round bodies — the female cells, which are known as ovules.
Not all flowers have all four kinds of parts; one or more may be lacking, and in some flowers other parts may be present. But all flowers whose number and arrangement of parts correspond exactly with those of other flowers are said to be in the same flower family or natural order. In every such flower, no matter how complex or peculiar, the parts present are in the same constructional relation to one another and carry out the same functions.
The vital parts of the flower are the pollen and the ovules, since they contain the gametes or sex cells, and the organs — stamens and carpels — that bear them. The majority of plants have flowers containing both stamens and carpels. But on other plants, the flowers contain only one of the sex organs.
The function of flowers is to produce pollen grains and ovules, and, by bringing about their union, to produce seed.
To effect a union, pollen grains must alight on the sticky stigma or carpel. If they are ripe and from the same species of plant, the sticky material on the stigma makes the pollen grains germinate and grow a long tube which reaches down the style to the ovary. There a male nucleus from the pollen unites with the female nucleus of the ovule. Once fertilized, the ovule swells rapidly and builds up the food stores and protective tough outer coat that make it into a seed.
Pollination is the prelude to fertilization and is the process of getting pollen grains on to the sticky stigmas. In many plants equipped with both stamens and carpels, the anthers shed their ripe pollen on the stigmas and the plants are said to be self-pollinated. When the pollen of one plant is transferred to the stigma of another similar plant, what is known as cross-pollination takes place. This gives a stronger and more robust offspring, and is common in Nature and often resorted to in plant breeding.
Nature uses two main agencies to help in the work of pollination, the wind and animals, chiefly insects.
Plants which are adapted for wind-pollination usually have flowers with little or no perianth and of one sex. The male flower has stamens prolific of pollen (often in the form of catkins) and the female flower has comparatively large stigmas and styles but no petals. The pollen grains are released in clouds and wafted far and wide, mostly to be wasted, for chance alone determines that only a few alight on receptive stigmas. Grasses and many trees are wind-pollinated (examples are hornbeam, hazel and willow).
Most flowers that are admired for their beautiful shape, colour or fragrance are devices to lure an insect to the plant for the purpose of pollination. For instance, white flowers, sweet-smelling and opening at dusk, attract night-flying moths. Brightly coloured day flowers attract butterflies; blue, red and white day flowers attract bees. Yellow flowers often attract beetles, and putrid-smelling flowers attract flies. Colour and scent advertise that the plant has food to offer, usually nectar – a dilute sugary solution produced by small organs called nectaries, well inside the flower.
To reach the nectar the insect comes in contact with the pollen released by the anthers and carries some of it away on its body. It is also likely to touch the sticky stigmas of the flowers it visits, and some of the pollen from its body adheres to their surface.
In some flowers the pollen may stick to their own stigmas as the insect with-draws. More often, it is transferred to other flowers on the same plant or on other plants of the same kind. In many plants modifications of flowering or of flower structure prevent self-pollination and ensure crossing. A common trick is for the stigma to mature before the pollen in the same flower, or vice versa; then pollen must come from another flower of suitable maturity for pollination to be accomplished.
Often the construction of a flower is adapted to one particular kind of insect. It requires the burly weight of a bumble-bee to depress the lip of an antirrhinum flower so that it can be entered and pollinated.
Successful pollination is usually followed by fertilization, but not always. Both actions bring about swift changes, but they are not synonymous.