Sexual reproduction

The flower is the organ of sexual reproduction in plants. The most obvious parts are the petals, which form the corolla, and sepals, which form the calyx; together these two constitute the perianth, mounted on the receptacle. The female pistil is made up of the stigma on its style above the ovule-containing ovary. The male stamens consist of pollen-bearing anthers on supporting filaments.

If plants increased by vegetative means only-through budding, runners, and so on—the processes of evolution and adaptation could not take place, because each new plant would have exactly the same genetic makeup as its parent. The element of change relies on sexual reproduction, in which each new individual inherits genetic material from both of its parents. Sexual reproduction has the additional advantage of enabling the individual embryos to be carried away from the parents and to grow some distance away, perhaps in a more favorable environment. This system helps to prevent the plants from becoming overcrowded and assists the spread of the species.

Male and female structures show up well in the flower of a hybrid fuchsia. The female stigma protrudes beyond a cluster of five stamens, some of which bear powdery pollen.

Flowers

The part of a plant specialized for sexual reproduction is the flower. Here the male and female gametes—pollen (male) and ovules (female)—are produced and come together to produce seeds, each containing an embryo plant. In some species, such as buttercup (Ranunculus spp.), all the flowers are hermaphrodite, having both male organs (stamens) and female organs (carpels). Other plants have unisexual flowers, producing either male or female gametes, but not both. Separate male and female flowers may both be borne on the same plant (monoecious), as in hazel ICorylus americana), or male flowers may be produced on some plants and female flowers on others (dioecious), as in willows (Salix spp.). Species with male, female, and hermaphrodite flowers, either on the same plant or on different plants, are known as polygamous.

Flowers may be borne singly or may be grouped together to form an inflorescence. Solitary flowers are seen in anemone (Anemone patens), whereas the flowers of Virginia cowslip (Mertensia virginica) form an inflorescence. In some species, such as oxeye daisy (Chrysanthemum leucanthemum), what appear to be single flowers are in fact inflorescences made up of many tiny individual flowers closely grouped together.

Carpels

The top of a flower stem is often enlarged and forms a platform (the receptacle) upon which the other floral organs are located, arranged in whorls, spirals, or both. In the center are the carpels, collectively forming the pistil. Each carpel consists of a sticky stigma at the top of a stalk (the style), which joins a hollow ovary containing one or more ovules. Buttercups have a single ovule in each carpel; some orchid carpels contain half a million ovules each.

Species with only one carpel in each flower are rare; they include the members of the pea family. Sometimes the carpels are separate as in buttercup, but in most species two or more carpels are fused together, having a common ovary. The edges of the fused carpels may project into the ovary but not reach the center, producing a unilocular ovary; however, more frequently, the carpel edges meet at the center of the ovary, dividing it into as many loculi (chambers) as there are carpels. The styles and stigmas may also be entirely or partly fused together, or they may remain separate.

Each ovule consists of an ovoid mass of cells called the nucellus, which is attached to the ovary wall by a short stalk (the funicle). Protective layers (integuments) cover the entire nucellus except for a small opening called the micropyle. Inside the nucellus is the embryo sac, usually containing eight nuclei. Three are situated near the micropyle, the middle one of these forming the ovum (egg); three are at the opposite end to the micropyle; and two are in the center of the embryo sac. These two central nuclei usually fuse together to form the endosperm nucleus or secondary nucleus.

Stamens

Surrounding the carpels are the stamens. They may be separate, or all or some may be joined. Their number is constant in some species, but variable in others. Each stamen has a swollen head (the anther) and usually a stalk (the filament), which carries nutrients from the plant to the anther. The central part of the anther is called the connective, and attached to this are pollen sacs containing pollen mother cells, each of which divides twice (by meiosis) to form a tetrad of four pollen grains. In some species the grains are dispersed in these groups of four, but usually they separate within the pollen sacs. When ripe, the anther opens, generally by means of two longitudinal slits, and the pollen sacs are ruptured, releasing the pollen grains.

The shape of pollen grains varies, but often they are round or oblong. The outer wall has distinctive pores and may also possess spines, ridges, or other features. These characteristics are remarkably constant in any individual species so that it is usually possible to identify a plant by examining its pollen grains. Each grain has two nuclei, one of which forms the generative cell, and the other is the tube nucleus. The generative cell usually divides to form two male gametes or sperm cells; therefore the mature pollen grain contains three nuclei, one vegetative and two generative (gametes).

Petals and sepals

The stamens are usually surrounded by a corolla, formed of petals. These are net directly involved in the reproductive process, but can be of great assistance to the plant because large, brightly colored petals attract insects that may carry out pollination, transferring pollen grains from stamens to stigmas. In some species the petals are joined to form a corolla tube or trumpet shape.

A calyx, composed of sepals, normally forms the outermost part of the flower, protecting the other organs, especially before the bud opens. The sepals may be joined into a calyx tube. Both petals and sepals are of constant number in some species, and variable in others. Usually the sepals are green, but some flowers, such as those of bluebell and crocus, have sepals that are petaloid—they are just like the petals in size, shape, and color, and the two whorls are known collectively as the perianth. Sometimes, as in gladiolus (Gladiolus tristis), the parts that appear to be petals are in fact large, showy sepals. Some plants, particularly those pollinated by the wind, have neither corolla nor calyx, because they do not need to attract insects. These “naked flowers” grow in plain, tassel-like inflorescences, called catkins, and can be found on plants such as alders, poplars, and willows.

Before the male and female gametes can unite the pollen grains must be carried from the stamens to the carpels (pollination). Pollen may be deposited on the stigma of the same flower or on the stigma of a different flower on the same plant. Both these processes are forms of self-pollination. When the pollen is transported to the stigma of a flower on a different plant, cross-pollination occurs. Self-pollination perpetuates the characteristics of the parent plant, whereas cross-pollination introduces new genetic combinations, bringing the possibility of better, stronger plants.

Self-pollination

Pollen grains vary in texture and shape—depending on the plant species—from smooth and spherical to pitted and slipper-shaped (left). For fertilization to take place, pollen from the male anther has to get to the female stigma. If this takes place in the same flower (A) or between two flowers on the same plant (B), self-pollination occurs. Transfer of pollen to a flower on another plant (C) leads to cross-pollination.

In plants where self-pollination normally occurs, the stamens are usually located so that the anthers are above the stigma, and stamens and stigma ripen simultaneously. The ripe pollen often simply falls onto the receptive stigma below, although sometimes the process is more complex, involving movement of the stamens or stigma.

To prevent self-pollination taking place, many species have stamens and stigmas that do not ripen at the same time. Self-pollination is impossible in dioecious plants, because male and female flowers are borne on separate plants. Sometimes pollen may fall onto a ripe stigma of the same flower but then fail to bring about fertilization, because its genes are incompatible with those of the ovule. Such plants are described as self-sterile.

Another device discouraging self-pollination is called heterostyly, which can be seen in the primrose (Primula vulgaris). The flowers of this species are usually of two types (dimorphic): a “pin-eyed” form, with the stigma at the mouth of the corolla tube on a long style and the stamens halfway up the corolla tube, and a “thrum-eyed” form, with the stigma halfway up the tube on a short style and the stamens located at the top of the corolla tube. The pollen from one form sticks to an insect visitor and is then deposited on a stigma of the other form, as the stamens of one type are at the same level as the stigma of the other. The pollen grains of the two types are different and the plants are self-sterile, a fact which suggests that heterostyly is, by itself, a rather inadequate method of discouraging self-pollination.

Two different designs of flowers of the same species is termed heterostyly. The “pin” form (left) has a long stigma and anthers deep in the flower’s corolla, whereas in the “thrum” form (right) the positions of the organs are reversed. When an insect visits either flower to feed, pollen left on it by the anthers of one type is transferred to the stigma of the other type.

Cross-pollination

Cross-pollination is brought about by the wind, by insects (or birds, bats, or other animals), or, in a few aquatic plants, by water. Wind pollination, the simplest method, is also the most wasteful: vast amounts of pollen are produced in an attempt to ensure that some of it lands on a ripe stigma. The male flowers are often borne on dangling catkins, where they are shaken by the slightest breeze. This form of pollination is particularly common among trees, grasses, rushes, and sedges. The flowers are usually small and inconspicuous, with large anthers exposed to the wind on long, slender filaments, and exposed feathery stigmas to catch airborne pollen. The flowers produce neither nectar nor scent.

The main kinds of fruits can be divided among simple, aggregate, and multiple types. Simple fruits may be fleshy, as in drupes, pomes, and berries, or dry, as in nuts and the “seeds” (actually fruits) of grasses and some trees. An aggregate fruit, like blackberry, has separated seeds in a pulpy flesh (carpell, all derived from one flower. Multiple fruits, like pineapple, derive from several flowers that condense during development to form a single seedbearing structure. Figs are another example of multiple fruits.

Insect pollination is less wasteful, but often much more complex. The flowers attract pollinators by means of large, brightly colored petals, and often by a strong scent. This is not always a sweet perfume; some flowers produce a stench like that of rotting flesh or dung and, thus, attract carrion-feeding or dungfeeding insects. A supply of nectar frequently serves as an invitation to insects, or the visitors may come for the pollen itself. The petals often have bright spots or lines pointing the way to the flower’s nectar-store; such markings are known as nectar-guides. Nectar located deep within the flower can only be reached by insects with a long proboscis, such as bees, butterflies, and moths; flies and beetles, which have short tongues, can take nectar from flowers only where it is easily accessible. Thus different flowers are visited by different types of insects. Pollen becomes attached to the hairy legs and body of a feeding insect and is then brushed onto the stiqma of the next flower visited.

Some more complex pollination systems include mechanical traps that actually hold the pollinating animal long enough for the pollen to be transferred. The jack-in-the-pulpit (Ari-saema triphyllum) attracts flies into an enclosed chamber by the smell of carrion. The flies can escape only when the structures at the mouth of the chamber wither, by which time the flies are thoroughly dusted by pollen during their struggles. A similar device makes sure that the pollen is shaken onto the female structures on another plant. Some orchids, such as the early purple orchid (Orchis mascula), have spring-loaded anthers that snap shut on a visiting bee. The bee’s struggle to escape dislodges the pollen sack onto its back. It then flies to another flower of the same species and the pollen is released on the stigma. This is an excellent example of coevolution.

Water pollination is rare, even among plants that grow in water. Most pollen becomes ineffective when wet, and even in aquatic species that are pollinated by water, it is often the male flowers, and not the individual pollen grains, that are carried by the current to the female flowers, the pollen thus remaining dry.

Fertilization

When a pollen grain of the same species lands on a ripe stigma it swells as it absorbs water, sugar, and other materials from the stigma.

The correct pollen then germinates or grows a tube down the style toward the ovary. The pollen tube, containing two male gametes, grows through the canal and enters an ovule by way of the micropyle. The tip of the pollen tube penetrates the embryo sac and releases the two male gametes. One enters the ovum and fuses with its nucleus, forming a zygote from which an embryo plant develops. The other usually fuses with the central nuclei of the embryo-sac, producing nutritive tissue called endosperm, which nourishes the embryo. Fertilization is thus achieved.

A hoverfly feeds on nectar and has grains of yellow pollen on the hairs of its head and body. By this means, the pollen will be transferred to another flower of the same species and will fertilize it.

Fruit and seeds

Following fertilization, many changes take place in the carpel. Each ovule becomes a seed, its integuments forming the protective seed coat (testa), and the ovary becomes a fruit, its three layers of wall forming the pericarp. Sepals, petals, and stamens usually wither, although their dry remains often persist, sometimes attached to the fruit.

There are three main kinds of fruits, classified as simple (that is, single), aggregate (clusters), and multiple, with various types within these categories as illustrated on the opposite page.

Oranges ripen on a tree in Australia. The fruit, which develops from the ovary of a fertilized flower, contains several seeds, although hor-ticulturalists have also created seedless varieties of oranges and other fruits.