Lesson 5: Reproduction in Plants

The flowers produced by angiosperms contain the plant’s reproductive organs. The female reproductive organs are called the carpel. The male reproductive organs are called the stamen. The seeds form inside the flower. When mature the seeds are enclosed in a case.

Sepals – when in the bud stage the sepals protect the developing flower. Petals – attract insects or other animals for pollination Pollen – contain the male gametes Anthers – produce pollen Filaments – hold anthers up to expose pollen to insects and other animals Stigma – receives pollen grains Style – holds up the stigma to receive pollen

Flowering involves a change in the expression of genes in the cells of the shoot apex. New proteins are produced and the cells and tissues that make up flowers begin to develop from meristems rather than leaves. Plants flowering is influenced the amount of light they receive. This is known as photoperiodism. In flowering, the critical factor is the period of darkness the plant experiences. Plants respond to darkness using a leaf pigment called phytochrome.

This pigment exists in two forms inactive (Pr) and active (Pfr) Active Pfr is rapidly produced from inactive Pr during daylight. This conversion is in response to an increase in red light (660nm) which is absorbed by Pr. In darkness, the Pfr reverts slowly to the more stable Pr.

These plants flower when the days are longest and nights are short. These plants respond to increased amounts of Pfr. Since the days are long more Pfr is produced and the short nights only allow for a small amount of Pfr to be converted back into Pr. These plants flower in the late spring and early summer. Examples of long-day plants include iris, lettuce, wheat and radish.

These plants flower when as nights become longer and days are shorter. In short-day plants Pfr inhibits flowering. As the days get shorter, less Pr is converted to Pfr and during the longer nights more of the Pfr is converted back into Pr allowing to flower. These plants flower in late summer or autumn. The amount of darkness must be over a critical period unique to each species. Examples of short-day plants include soybeans, chrysanthemum, poinsettia and coffee.

Some are unaffected by the photoperiod and will flower when they reach a certain stage of maturity, regardless of day length. Examples of day-neutral plants include tomatoes, rice and dandelions

For the seeds to develop, pollen grains from the anthers must reach the stigma of the carpel. This process is called pollination There are two types of pollination used by angiosperms; Self-pollination - the males and female gametes come from the same plant Cross –pollination – the male and female gametes come from two different plants. The two most common agents of cross-pollination are wind and insects.

Occurs when male and female gametes meet and form a zygote. Occurs in the ovule of the flower. When pollen from the right species arrive they germinate on the style producing a pollen tube. The pollen tube grows down the style to the ovary The pollen tube enters the ovary and a pollen nucleus travels down the tube to fuse with the ovule nucleus.

Fertilized ovules develop into seeds, which protect the developing embryo. Seeds are held within a seed pod, fruit or nut. Seeds have all the necessary components to ensure successful germination: Embryo root (radicle) and shoot (plumule) Cotyledon Testa (seed coat) micropyle Dicot plants seeds contain two cotyledons which store food reserves (starches) for germination. Cotyledons are surrounded by a hard protective seed coat called the testa. The testa protects the tissues from the environment . The testa contains a small pore called the micropyle through which water is absorbed to begin germination.

Seeds need to be moved to locations away from the parents in order to minimize competition. There are many seed dispersal mechanisms: Seed pods dry out and snap causing the seeds to be ejected some distance from the parent Fruits are eaten by animals, which cannot digest the tough seeds. The seeds are then deposited in a rich fertilizer a long distance from the parent to germinate. Nuts are collected by animals which burry them as reserve over winter. The animals may not dig them up and they germinate.

Germination is the development of the seed into a new plant. Seeds remain dormant until conditions are ideal Temperature – suitable temperatures activate enzymes within the seed. Water – most seeds contain about 10% water an therefore require water to germinate. Oxygen – Essential for aerobic respiration

Germination begins as water is absorbed through the micropyle in a process called inbibition Water rehydrates the stored food reserves and activates gibberellin Gibberellin stimulates the release of amylase by the cells in the outer layer of the seed (aleurone) The amylase hydrolyses the starch in the cotyledon converting it into maltose. Maltose is converted into glucose and transported to the embryo in order to be used in respiration to create energy for growth. Absorption of water by the seed splits the testa so that the embryo can emerge and grow. When the leaves of the seedling grow above ground, photosynthesis begins and takes over energy production.

Gymnosperms do not produce flowers Produce seeds inside the cones Seeds have a coat that protects them but are not inside a case

Gymnosperms produce their seeds inside cones. For this reason they are called conifers. In some species male and female cones are produced on separate trees. In most conifers male and female cones are produced by each tree.

Pollination proceeds the same as in angiosperms Fertilization also requires the formation of a pollen tube to allow the pollen to reach the egg.

Plants such as mosses and ferns reproduce through spores Like gametes spores are haploid however, a spore can develop into a new plant without fertilization. The mature form called a sporophyte will release the spores Under the correct conditions the spore will develop into a gametophyte. The two main advantages for dispersal are; that they are light and that they can survive unfavourable conditions by staying dormant until the conditions are ideal.