Hello, dear readers. Today's post will be on pollination of flowering plants, their fertilization and the development of an embryo plant.
The reproductive structures of the flowering plant have now formed. As plants cannot move to find a mate, they must transport their pollen, often over large distances, to make contact with a female plant from the same species. Once the pollen has made it to the stigma, the male gamete must make a smaller but equally crucial journey to meet the female gamete within the ovule. Most plants are pollinated either by wind or by insects, but there are other vectors (pollen carriers), including bats, mice, slugs, birds, small reptiles and water.
Wind pollination, or anemophily, is the most straightforward method of pollination. The anthers of a plant ripen in dry weather and release pollen grains into the air. Pollen often spreads to a radius of 5 000 metres around the parent plant, and some pollen grains are known to have travelled thousands of kilometres. The chance of any pollen grain landing on a stigma of the right plant is slim, so large amounts of pollen are produced.
Wind pollination works well for plants that exist in large numbers in a particular area. Grasses and many woodland trees, such as oak, birch and hazel, are successful wind-pollinated plants. Their flowers are not ‘showy’: they do not need to be. Instead, they have large feathery stigmas that stick out from the flower to catch pollen, as seen in the figure below.
Insect-pollinated (or entomophilous) plants are pollinated by insect vectors (carriers), rather than the wind. The shape of their flowers has evolved to ensure that pollen brushes off onto the visitor and is later transferred to a stigma when the insect visits another flower. Such flowers need to be seen and so they advertise themselves with large white or brightly coloured petals. But if the plant invests large amounts of energy in making large petals, it has fewer resources available to make gametes. So, many plants make ‘cheaper’, smaller flowers and crowd them together in clusters, called inflorescences.
Pollen grains are rich in oils and proteins, and producing large amounts drains resources from the plant. Insect-pollinated flowers tend to produce small numbers of larger and stickier pollen grains. Insect pollination is more efficient than wind pollination because less pollen gets wasted and so less is needed. Since pollen itself is nutritious, a plant may have to sacrifice some of it to the insect as a reward for successful pollination. But some plants have found that a more economical bribe of sugary water, nectar, can be just as effective. If insects get plenty of nectar, they are more likely to visit another flower of the same type. Both insect and plant profit. This mutual benefit is the basis of many plant-insect relationships.
With pollen grains produced in astronomical numbers, it is inevitable that much of the pollen that arrives on a particular flower will be of the wrong species. There are precise biochemical recognition processes that ensure that only pollen of the same species is able to complete the fertilization process.
When pollen lands on a stigma of the same species, pollination is complete and fertilization can start. The pollen grain draws water out of the stigma by osmosis and swells. This bulge develops into a pollen tube, which penetrates the style and grows towards the ovule.
Each pollen grain contains two nuclei, the generative nucleus and the tube (or vegetative) nucleus. The tube nucleus directs the growth of the pollen tube. The generative nucleus forms two sperm cells. The nuclei are not separated from each other by cellulose walls: they simply share the cytoplasm, so they do not really occupy separate cells. The sperm and tube nuclei move down the pollen tube as it grows and they enter the ovule through a gap, the micropyle, to reach the embryo sac.
The embryo sac opens, the sperm nuclei enter, and then a double fertilization occurs. In one fertilization, one of the sperm nuclei fertilises the egg cell. The product of this fusion becomes the zygote, the diploid cell that develops into the embryo. In the second fertilization, the other sperm nucleus fuses with the central cell. This fusion produces a triploid cell (it has three copies of each chromosome: two from the diploid central cell and one from the haploid sperm). This triploid cell divides by meiosis to form the endosperm tissue. The endosperm contains the food supply that will nourish the growing embryo. Sexual reproduction in flowering plants always involves a double fertilization.
In an overall summary of spore production, gamete production and fertilization in a flowering plant, after fertilization in most plants, the flower withers and the ovary swells to form the fruit containing the seeds. Inside each seed, the embryo develops.
DEVELOPMENT OF THE EMBRYO
In fertilization, the egg and sperm nuclei fuse to form a diploid nucleus, the zygote, which develops into an embryo plant. This consists of a tiny root or radicle, connected to the young shoot, the plumule, which later develops leaves. The first leaves, the cotyledons, are different from all later leaves. They are usually a different shape and have a simpler network of veins. Sometimes they are swollen with food reserves that nourish the embryo.
When the central cell is fertilised, it divides to form a special triploid tissue called the endosperm, which usually nourishes the embryo. In some plants, such as the maize, endosperm accumulates and is stored inside a seed, ready to feed the young plant when the seed germinates. In other species, such as peas and peanuts, the endosperm food reserves are used by the embryo at a much earlier stage and any spare food is transferred and stored within the cotyledons.
FRUITS AND SEEDS
Once the seed is released from the parent it can develop into a new plant. But most seeds don’t start growing, or germinating, straight away. If seeds are dispersed, the parent is not overcrowded by offspring competing for light, water and nutrients.
Seeds survive dispersal because the testa, a tough coating, prevents them from drying out or being digested by animals that eat them. The testa forms when the outer part of the ovule is strengthened by the polysaccharides cutin and lignin. The testa is sometimes so tough that the seed needs harsh treatment, such as freezing, soaking or a period of wear and tear in the soil, before it can germinate. This ensures that seeds do not germimate until the following year, long after the winter cold, or the summer drought.
Some seeds are light enough to be carried on the wind, like pollen. But some seeds need help to disperse. This help comes from the fruit that surrounds them. In some plants, the ovary wall develops into a fleshy, soft fruit designed to entice animals to eat it. After digestion, the seeds pass out in the faeces, perhaps many miles away, still protected by the testa. Other fruits have explosive mechanisms to throw the seeds about, and others have hooks to catch on animals’ feet or fur to ‘hitch a lift’.
The intimate relationship between plants and their pollinators
Insects and flowering plants have evolved together. Some species are now completely dependent on each other: without the plant the insect doesn’t get food, and without the insect the plant cannot reproduce. Most plant-insect relationships are not as narrow as this: most plants attract a range of potential pollinators. For each species, the range is never very great, because particular colours, shapes, scents, opening times, flowering seasons and so on all tend to be attractive to particular groups of insects.
Red clover, for example, must be cross-pollinated (pollinated by pollen from another plant) before it can produce seeds. It is pollinated by a handful of different insects, but bees are the most important. Red clover is adapted to pollination by bees in several ways. The flowers develop as inflorescences. Each flower in the cluster has a long tube with a tiny hole at the top that gives access to the nectar. A visiting bumblebee inserts its proboscis and long tongue into the tube to feed on the nectar. As the bee does this, it brushes its underside against the anthers and picks up sticky pollen. When the bee visits another red clover flower, its underside brushes against the stigma, some of the pollen is transferred, and so pollination is accomplished.
The tiny access hole and the length of the tube make it hard for other insects to reach the nectar. Alternatively, the shape of the flower may prevent them from landing on it. Honey bees, which have slightly shorter tongues, can reach the nectar when the level is high enough. If not, they may adopt a ‘smash and grab’ approach where the bee bites a hole in the flower near the base and sucks out the nectar.
In my next post, I will discuss plants and sex in relation to their strategies of reproduction and commercial plant breeding.