Pollination is the transfer of pollen (male gamete) from the anther to a stigma. Cross-pollination: pollen is transferred to the stigma of another plant. Increases genetic variation, population more resistant to environmental change.
Self-pollination: pollen transferred to the stigma of the same flower or a flower of the same plant. Guarantees reproduction if pollinating agent is absent or not efficient.
Pollination can be accomplished by the wind or by animals. Insects are the most common animals that will pollinate a carpel.
The most sophisticated relationships between plants and insects are generally those involving bees. Bees collect pollen and nectar not only for themselves but also to feed their young. For this reason bees have developed a number of adaptations that make them particularly good pollen carriers. Bees have special hairs that are arranged to form pollen ‘baskets’ on their hindlegs and the underside of their abdomen. These adaptations allow them to gather and carry large volumes of pollen. Bees are ideal pollinators because they visit many flowers while carrying lots of pollen, before returning to their nest. So the chance that a bee will transfer the pollen between flowers of the same species is very high.
Many insects eat pollen. In the process of eating they become covered in it. Pollination happens when the pollen feeder transfers the pollen to the pollen receivers of the same plant, or another plant of the same species, as the insect looks for more pollen to eat.
Fertilisation is the union of the male and female gametes to form a zygote. Since the male and female gametes are haploid (n) when the two unite the zygote is diploid (2n).
Fertilisation starts when a pollen grain lands on the stigma. The pollen grain then grain germinates forming a pollen tube. The tube nucleus controls the growth of the pollen tube. The pollen tube is an example of chemotropism since it is growing toward chemicals produced from the ovule. The generative nucleus travels down the pollen tube. It undergoes mitosis forming two haploid male gamete nuclei. The pollen tube enters the ovule by way of the micropyle. The two male gamete nuclei are released into the embryo sac. The tube nucleus disintegrates.
Since there are 2 sperm nuclei that have reached the embryo sac both nuclei will fuse with female gametes. One sperm nuclei will fuse with the egg cell to form the zygote (2n) while the other sperm nucleus fuses with the 2 polar nuclei in the embryo sac to form an endosperm nucleus (3n).
The fertilized becomes the seed. The integuments become the wall of the seed called the testa. The micropyle closes. The endosperm nucleus leads to the formation of triploid endosperm, a food tissue. The diploid zygote, by mitosis, develops into a plant embryo. The developing embryo draws nourishment from the endosperm. The embryo ceases development and goes dormant. The ovule becomes a seed, which contains a dormant plant embryo, food reserve, and the protective coat called the testa.
The embryo is made up of the radicle or future root and the plumule or future shoot. The endosperm cells divide many times and absorb the nucellus. This is the nutrition (mainly fats, oils and starch) for the embryo.
There are 2 types of seeds. Some are endospermic while others are non-endospermic. In endospermic seeds the food reserve is the endosperm, which is outside the plant embryo. Examples of this type of seed are maize and wheat. Non-endospermic seeds have food reserve within the cotyledon(s) of the plant embryo. This occurs in broad beans.
Monocots and Dicots
Monocots have one cotyledon in the seed while dicots have two cotyledons. The cotyledons are food reserves for the young plant after it germinates from the soil. It uses these food reserves until it is capable of making its own food. In monocots the food is absorbed from the endosperm while in dicots the food is stored in the cotyledons.
The ovary becomes a fruit. The wall of the ovary becomes the wall of the fruit called the pericarp. The fruit protects the developing seeds and plays an important role in seed dispersal. This process is controlled by auxins produced by the seeds. Once the fruit forms the rest of the flower parts die and fall away.
Fruit and Seed Dispersal
Seed dispersal is the scattering of offspring away from each other and from the parent plant. As a result of dispersal there is an improved chance of success by reducing competition and overcrowding. Dispersal also enables colonisation of new suitable habitats and thus, there is an increased chance of species survival.
Methods of Seed Dispersal:
Wind: The seeds of wind-dispersed plants are lightweight seeds. They have a high air resistance so they can be carried far away from the mother plant.
Fruits which float such as those of the water lily and the coconut palm are carried by water. Coconuts can travel for thousands of kilometres across seas and oceans.
The original coconut palms on South Sea Islands grew from fruits, which were carried there from the mainland by ocean currents.
Some plants have juicy fruit that animals like to eat.
The animal eats the fruit but only the juicy part is digested.
The stones and pips pass through the animal’s digestive system and are excreted to form new plants. This can be far away from the parent plant. Blackberry, cherry and apple seeds are dispersed in this way.
Birds also like to eat fruit and they help to disperse seeds to other areas through their droppings.
Mistletoe has sticky fruits that are attractive to birds. The sticky seeds stick to the bird’s beak. They then rub their beaks clean on the bark of trees. The sticky seeds are left on the bark to grow into new mistletoe plants – mistletoe is a parasitic plant.
Squirrels collect nuts like acorns and bury them for winter food, but they often forget where they have buried them and these grow into new trees.
Some fruits like that of the burdock plant have seeds with hooks. These catch on the fur of animals and are carried away.
Self-Dispersal: Some plants have pods that explode when ripe and shoot out the seeds. Lupins, gorse and broom scatter their seeds in this way. Pea and bean plants also keep their seeds in a pod. When the seeds are ripe and the pod has dried, the pod bursts open and the peas and beans are scattered.
Dormancy is a period of inactivity. There is very little cellular activity and no growth. One or many of the following reasons bring about dormancy:
Auxins that inhibit growth- Growth Inhibitors
The testa is impermeable to water and oxygen- The testa will eventually break down and allow water and oxygen into the seed.
The testa may be too hard for the embryo to germinate.
An Auxin (Growth Regulator) may be absent until suitable environmental conditions develop.
The embryo will germinate from the seed if the proper environmental conditions are present. When this occurs the embryo resumes its growth.
In order for germination to occur the following conditions must be present:
Water must be present. This allows the seed to swell and enzymes to function.
Oxygen must be present in the soil.
The temperature must be suitable for the species of plant. Suitable temperatures are usually between 5-30 degrees Celsius depending on the species.
The dormancy period must be complete.
Some seeds need light and others need darkness.
Events of Germination
When germination begins the first thing that happens is water is absorbed by the seed through the micropyle and through the testa.
Enzymes in the soil now digest the foods stored in the seeds:
Oils become fatty acids and glycerol
Starch becomes glucose
Protein becomes amino acids
These foods now are absorbed by the embryo.
The glucose and amino acids make new structures such as cell walls and enzymes.
The fats and glucose are used in cellular respiration to produce energy.
The stored food of the seed is being used up as the embryo grows larger.
The radicle grows larger and breaks through the testa. It becomes the roots of the new plant.
The plumule grows larger and emerges above the ground.