Plant reproduction (in flowering plants)
Flower structure and function
The flowers of a plant are where sexual reproduction takes place. The female gametes of the flower are the ovules, which are contained in the ovaries. The anthers produce the male gamete, pollen grains. A zygote forms when a pollen grain fuses within an. This process is known as fertilisation. However before this can happen, pollination must occur. This is the process in which pollen grains are transferred from the anthers to the stigma. For pollen grains to be carried to the stigma, an external agent is required. There are two main types of external agents: wind and animals. Depending on the flower, they are adapted to different types of pollination, for example some flowers are formed especially for bee pollination.
When pollen germinates on the stigma, a pollen tube forms. This tube contains the male gametes and carries them down the style to the ovary. The male gametes then fertilises an ovule. The fertilised ovules then develop into seeds. When the ovaries contain fertilised ovules, they develop into fruits. The fruits are for seed dispersal.
Seed structure
The seed is encased in a seed coat, also know as testa. The seed has a scar where it was once attached to the ovary. Within the seed there is an embryo shoot (plumule), an embryo root (radicle) and cotyledons which provide nutrients during germination.
Seed germination
Seeds remain in a dormant state until the external conditions are suitable for the seed to germinate. For the seed to rehydrate, enough water must be available. In most cases, the seed also has to have access to oxygen, for aerobic respiration. Germination requires enzyme activity, which only takes place at suitable temperatures. This is why seeds germinate in a particular season, normally spring.
After the start of germination, the seed coat will have split, due to swelling from water absorption. The cotyledons are a source of nutrients for the growing plant, which is essential in the early stages. Roots begin to grow into the soil and branching out to increase the root surface area. A stem grows, extending the first foliage leaves out from the soil and away from the cotyledons. Stems generally grow with a bend in them, which protects the leaves as they are pushed through the soil.
The cell must become metabolically active and for this to happen, the seed rehydrates by absorbing water. After rehydration, the cotyledons produce a plant hormone known as gibberellin, which stimulates the production of amylase to catalyse the digestion of starch into maltose. The product of starch digestion is maltose, which is transferred to the to the embryo root and embryo shoot. The maltose is then broken down further, to produce glucose which is needed for aerobic respiration, as a source of energy and to make substances needed for growth, such as cellulose. When the seedling's leaves have reached the light and opened, the food stores are no longer needed, as photosynthesis can occur.
