Nature of science:
Paradigm shift—more than 85% of the world’s 250,000 species of flowering plant depend on pollinators for reproduction. This knowledge has led to protecting entire ecosystems rather than individual species.
Understandings:
Variety in flowers
Flowers vary greatly in size. One of the smallest is the size of a sesame seed and occurs in the aquatic plant Wolffia. At the other end of the scale is the jungle flower Rafflesia arnoldii. This plant grows in Southeast Asia. When fully mature, it measures up to 0.9 m across and can weigh as much as 7 kg. Like many flowers, it depends on insects for pollination. When mature, this flower smells like rotting meat, thus attracting flies that transfer pollen from the male reproductive structures to the female structures.
Angiosperms
Any plant that has a flower is known as an angiosperm. Most flowering plants coevolved with pollinator species, such as insects, bats, and birds. These animals, as with the example of Rafflesia, facilitate the transfer of the male pollen to the female reproductive portions of flowers so that fertilization and seed development within the ovaries can occur.
For a long time, biologists have grouped angiosperms into two classes: the monocots (monocotyledonous plants) and the dicots (dicotyledonous plants). This division is based on morphological characteristics.
Recently, new groupings have been emphasized because of a better understanding of the evolutionary development of the angiosperms. These new groupings involve analysis of DNA.
Paradigm shift—more than 85% of the world’s 250,000 species of flowering plant depend on pollinators for reproduction. This knowledge has led to protecting entire ecosystems rather than individual species.
Understandings:
- Flowering involves a change in gene expression in the shoot apex.
- The switch to flowering is a response to the length of light and dark periods in many plants.
- Success in plant reproduction depends on pollination, fertilization and seed dispersal.
- Most flowering plants use mutualistic relationships with pollinators in sexual reproduction.
- Application: Methods used to induce short-day plants to flower out of season.
- Skill: Drawing internal structure of seeds.
- Skill: Drawing of half-views of animal-pollinated flowers.
- Skill: Design of experiments to test hypotheses about factors affecting germination.
- Students should understand the differences between pollination, fertilization and seed dispersal but are not required to know the details of each process.
- Flowering in so-called short-day plants such as chrysanthemums, is stimulated by long nights rather than short days.
Variety in flowers
Flowers vary greatly in size. One of the smallest is the size of a sesame seed and occurs in the aquatic plant Wolffia. At the other end of the scale is the jungle flower Rafflesia arnoldii. This plant grows in Southeast Asia. When fully mature, it measures up to 0.9 m across and can weigh as much as 7 kg. Like many flowers, it depends on insects for pollination. When mature, this flower smells like rotting meat, thus attracting flies that transfer pollen from the male reproductive structures to the female structures.
Angiosperms
Any plant that has a flower is known as an angiosperm. Most flowering plants coevolved with pollinator species, such as insects, bats, and birds. These animals, as with the example of Rafflesia, facilitate the transfer of the male pollen to the female reproductive portions of flowers so that fertilization and seed development within the ovaries can occur.
For a long time, biologists have grouped angiosperms into two classes: the monocots (monocotyledonous plants) and the dicots (dicotyledonous plants). This division is based on morphological characteristics.
Recently, new groupings have been emphasized because of a better understanding of the evolutionary development of the angiosperms. These new groupings involve analysis of DNA.
Flower structure and function
In both dicots and monocots, animal-pollinated flowers have the same parts in common.
In both dicots and monocots, animal-pollinated flowers have the same parts in common.
Flowers occur in a myriad of colours, shapes, and types. Some different types are:
Pollination and Fertilization
All plants show two different generations in their life cycle. The generations are:
• the gametophyte generation, which is haploid
• the sporophyte generation, which is diploid.
In plants, these two generations alternate with one another. Not surprisingly, this is called alternation of generations. The generations are named according to the reproductive cells they produce. The gametophyte generation produces the plant gametes by mitosis, whereas the sporophyte generation produces spores by meiosis. When we look at a flowering plant such as a cherry tree, we are looking at the sporophyte generation. It grew from a zygote and produces new cells by mitosis. When the cherry tree produces flowers, haploid spores are formed and develop into the haploid bodies referred to as gametophytes. Sperm form within the male gametophytes, and eggs form within the female gametophytes.
Pollination
Pollination is the process by which pollen (containing male sex cells) is placed on
the female stigma. It is the first step in the progression towards fertilization and the production of seeds. Pollen can be carried from anther to stigma by a variety of means. The earliest seed plants relied upon wind as their pollen vector. Later, insects became
a major factor in the process. It appears that the first angiosperms were pollinated by insects. There is very convincing fossil evidence showing that the angiosperms and insects coevolved; they appear to be instrumental in each other’s development.
There are many vectors of pollination besides insects and wind. These include birds, water, and animals other than insects. Most flowering plants use mutualistic relationships with pollinators in sexual reproduction. Flowers of plants that involve insect or other animal pollinators employ various means to attract their vector. For example:
• yellow and orange flowers are noticed by bees
Plants that rely on wind as their pollen vector have inconspicuous, odourless flower parts.
There are two general types of pollination:
• self-pollination
• cross-pollination.
In self-pollination, pollen from the anther of the same plant falls upon its own stigma. Self-pollination is a form of inbreeding and results in less genetic variation within
a species.
When cross-fertilization occurs, pollen is carried from the anther of one plant to the stigma of a different plant of the same species. Cross-pollination increases variation and may result in offspring with better fitness. The problem with cross-pollination is that the female stigma may not receive the male pollen because of the longer distance to travel.
Once pollination occurs, the next step is fertilization.
Fertilization
Fertilization happens when the male and female sex cells unite to form a diploid zygote. The female sex cells that are fertilized by the pollen are present within the ovules of the flower. The ovules are present within the ovary of the carpel. When the pollen grain adheres to the stigma, which is covered by a sticky, sugary substance, it begins to grow a pollen tube. Pollen tube growth and fertilization occur in the following sequence.
The seed
The seed is the means by which an embryo can be dispersed to distant locations. It is a protective structure for the embryo.
- complete flowers, which contain all four basic flower parts, the sepals, petals, stamen, and carpel
- incomplete flowers, which lack at least one of the four basic parts
- staminate flowers, which only have stamens, and no carpels
- carpellate flowers, which only have carpels, and no stamens.
Meiosis occurs in the stamen and carpel to produce the sex cells.
Pollination and Fertilization
All plants show two different generations in their life cycle. The generations are:
• the gametophyte generation, which is haploid
• the sporophyte generation, which is diploid.
In plants, these two generations alternate with one another. Not surprisingly, this is called alternation of generations. The generations are named according to the reproductive cells they produce. The gametophyte generation produces the plant gametes by mitosis, whereas the sporophyte generation produces spores by meiosis. When we look at a flowering plant such as a cherry tree, we are looking at the sporophyte generation. It grew from a zygote and produces new cells by mitosis. When the cherry tree produces flowers, haploid spores are formed and develop into the haploid bodies referred to as gametophytes. Sperm form within the male gametophytes, and eggs form within the female gametophytes.
Pollination
Pollination is the process by which pollen (containing male sex cells) is placed on
the female stigma. It is the first step in the progression towards fertilization and the production of seeds. Pollen can be carried from anther to stigma by a variety of means. The earliest seed plants relied upon wind as their pollen vector. Later, insects became
a major factor in the process. It appears that the first angiosperms were pollinated by insects. There is very convincing fossil evidence showing that the angiosperms and insects coevolved; they appear to be instrumental in each other’s development.
There are many vectors of pollination besides insects and wind. These include birds, water, and animals other than insects. Most flowering plants use mutualistic relationships with pollinators in sexual reproduction. Flowers of plants that involve insect or other animal pollinators employ various means to attract their vector. For example:
• yellow and orange flowers are noticed by bees
Plants that rely on wind as their pollen vector have inconspicuous, odourless flower parts.
There are two general types of pollination:
• self-pollination
• cross-pollination.
In self-pollination, pollen from the anther of the same plant falls upon its own stigma. Self-pollination is a form of inbreeding and results in less genetic variation within
a species.
When cross-fertilization occurs, pollen is carried from the anther of one plant to the stigma of a different plant of the same species. Cross-pollination increases variation and may result in offspring with better fitness. The problem with cross-pollination is that the female stigma may not receive the male pollen because of the longer distance to travel.
Once pollination occurs, the next step is fertilization.
Fertilization
Fertilization happens when the male and female sex cells unite to form a diploid zygote. The female sex cells that are fertilized by the pollen are present within the ovules of the flower. The ovules are present within the ovary of the carpel. When the pollen grain adheres to the stigma, which is covered by a sticky, sugary substance, it begins to grow a pollen tube. Pollen tube growth and fertilization occur in the following sequence.
- Pollen germinates to produce a pollen tube.
- The pollen tube grows down the style of the carpel.
- Within the growing pollen tube is the nucleus that will produce the sperm.
- The pollen tube completes its growth by entering an opening at the bottom of the
ovary.
- The sperm moves from the tube to combine with the egg of the ovule to form a zygote.
The seed
The seed is the means by which an embryo can be dispersed to distant locations. It is a protective structure for the embryo.
Once seeds are formed, a maturation process follows. This process involves dehydration until the water content of the seed is about 10–15% of its weight. At this point, the seed usually goes into a dormancy period. This is a time of very low metabolism and no growth or development. The dormancy period is quite variable for different types of seeds. This represents an adaptation feature to overcome harsh environmental conditions.
If conditions become favourable, the seed will germinate. Germination is the development of the seed into a functional plant. There are several general conditions that must be present for a seed to germinate:
• water is needed, to rehydrate the dried seed tissues
• oxygen is needed, to allow aerobic respiration to produce ATP
• an appropriate temperature for the seed is necessary (temperature is important for
enzyme action).
Besides these general conditions, many plants have specific conditions that must be met in order to germinate. For example, in some seeds the testa must be disrupted or scarified (broken) before water uptake can occur. Other seeds must be exposed to fire or smoke before they germinate. The food product known as liquid smoke can cause seeds of this type to germinate. Liquid smoke is produced when smoke from certain types of burning wood is allowed to condense in water. It can be purchased from several online sources and in many large food stores.
The control of flowering in angiosperms
Light is a very important factor in the life of a plant. It is required for photosynthesis, and it controls many aspects of plant growth and development. Plants are able to detect the presence of light, its direction, wavelength, and even intensity. Photoperiodism is the plant’s response to light involving the relative lengths of day and night: a very important factor in the control of flowering. To ensure continued existence in an area, a plant must flower when pollinators are available and when necessary resources are plentiful.
If conditions become favourable, the seed will germinate. Germination is the development of the seed into a functional plant. There are several general conditions that must be present for a seed to germinate:
• water is needed, to rehydrate the dried seed tissues
• oxygen is needed, to allow aerobic respiration to produce ATP
• an appropriate temperature for the seed is necessary (temperature is important for
enzyme action).
Besides these general conditions, many plants have specific conditions that must be met in order to germinate. For example, in some seeds the testa must be disrupted or scarified (broken) before water uptake can occur. Other seeds must be exposed to fire or smoke before they germinate. The food product known as liquid smoke can cause seeds of this type to germinate. Liquid smoke is produced when smoke from certain types of burning wood is allowed to condense in water. It can be purchased from several online sources and in many large food stores.
The control of flowering in angiosperms
Light is a very important factor in the life of a plant. It is required for photosynthesis, and it controls many aspects of plant growth and development. Plants are able to detect the presence of light, its direction, wavelength, and even intensity. Photoperiodism is the plant’s response to light involving the relative lengths of day and night: a very important factor in the control of flowering. To ensure continued existence in an area, a plant must flower when pollinators are available and when necessary resources are plentiful.
Even though the names refer to day length, it is actually the length of night that controls the flowering process in plants of the long-day and short-day types.
Chrysanthemums are an example of short-day (long-night) plants. To initiate flowering in these plants, some growers will place a black cloth over a plant for 12–15 hours a day until the flower buds begin to show color.
The control by light is brought about by a special blue-green pigment in the plants called phytochrome. There are two forms of phytochrome. One form is inactive and is represented by Pr. The other is active and is represented by Pfr.
When red light (which has a wavelength of 660 nm) is present in available light, the inactive form of phytochrome, Pr, is converted to the active form, Pfr. This conversion occurs rapidly. Pfr has the ability to absorb far-red light (which has a wavelength of 730 nm). This Pfr is rapidly converted back to Pr in daylight. However, in darkness the conversion back to Pr is very slow. It is thought that it is this slow conversion of Pfr back to Pr that allows the plant to time the dark period. This seems to be the controlling factor for flowering in short-day and long-day plants.
In long-day plants the remaining Pfr at the end of a short night stimulates the plant to flower. In other words, it acts as a promoter in these plants. However, in short-day plants the Pfr appears to act as an inhibitor of flowering. For these short-day plants, enough Pfr has to be converted to Pr for flowering to occur.
Chrysanthemums are an example of short-day (long-night) plants. To initiate flowering in these plants, some growers will place a black cloth over a plant for 12–15 hours a day until the flower buds begin to show color.
The control by light is brought about by a special blue-green pigment in the plants called phytochrome. There are two forms of phytochrome. One form is inactive and is represented by Pr. The other is active and is represented by Pfr.
When red light (which has a wavelength of 660 nm) is present in available light, the inactive form of phytochrome, Pr, is converted to the active form, Pfr. This conversion occurs rapidly. Pfr has the ability to absorb far-red light (which has a wavelength of 730 nm). This Pfr is rapidly converted back to Pr in daylight. However, in darkness the conversion back to Pr is very slow. It is thought that it is this slow conversion of Pfr back to Pr that allows the plant to time the dark period. This seems to be the controlling factor for flowering in short-day and long-day plants.
In long-day plants the remaining Pfr at the end of a short night stimulates the plant to flower. In other words, it acts as a promoter in these plants. However, in short-day plants the Pfr appears to act as an inhibitor of flowering. For these short-day plants, enough Pfr has to be converted to Pr for flowering to occur.
Pfr is able to stimulate flowering by activating specific genes of the shoot apex cells in a plant. This activation results in changes in DNA transcription (gene expression), thus allowing the production of flowers.