BOTANY II
This second botany module covers how plants reproduce, photosynthesize, and engage in both respiration and transpiration to grow.
"Green is the primary color of the world, and that from which its loveliness arises."
- Pedro Calderon de la Barca
- Pedro Calderon de la Barca
Introduction
This continuation of your study of Botany will provide you with a basis of understanding of how plants grow and reproduce. We'll start with a discussion of plant parts continued from the previous module. The final two plant structures that we will be studying are intentionally removed from the plant at some point in time, pollen and fruit.
Pollen & Pollination
Pollen is a powdery substance produced by most flowering plants for the purpose of sexual reproduction. and is comprised of pollen grains. These grains have a hard outer shell and contain sperm cells carrying genetic information. If it lands on a compatible pistil, it will germinate. The main use of flowers is to assist in pollination and reproduction. Pollination can occur in two ways: self-pollination, the transfer of pollen from an anther to a stigma in the same flower or to a flower on the same plant; or cross-pollination, the transfer of pollen from one plant to a stigma on a different plant, but of the same species. Self-pollination can be an effective method of pollination, but is has the disadvantage of excluding genetic variability. Cross-pollination, on the other hand, increases genetic variability within species and is promoted by many plants in a variety of ways.
Wind and animals are the major agents of pollination. The flowers of wind-pollinated plants are usually small, greenish, lack fragrance, and have small petals or no petals. Oaks and grasses are examples of wind-pollinated plants. The wind generally does not carry pollen far, so individual plants frequently grow close to one another to help guarantee pollination. Nevertheless, wind delivery of pollen to stigma is not a particularly precise procedure, and some plants have evolved a more accurate pollination mechanism - they employ animals as pollinators. |
Flowers must be able to attract animals if animals are to be efficient pollinators. This has been accomplished through evolution of flower colors, shapes, sizes and fragrances that appeal to different animals. Plants use other strategies to lure pollinators to their flowers as well, including the production of sweet nectar and nutritious pollen. However, once an animal visits a flower and comes in contact with its pollen, it needs to visit another flower of the same species for pollination to occur. Certain types of inflorescences may have evolved to ensure visits to different flowers. The below examples illustrate this evolutionary process.
In some plants, such as those in the Aster family, flowers have undergone a great deal of modification and are grouped together in 'head-like' inflorescences that frequently look like a single flower. Having individual flowers grouped so closely allows for a greater chance of pollination for a greater number of flowers.
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Similarly, Sunflowers are actually made up of hundreds of tiny, separate flowers. The larger yellow outer parts are actually modified leaves. Animals and insects who visit sunflowers frequently, though inadvertently, brush up against many more than just one flower, resulting in successful pollination of multiple flowers.
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Animal pollination has advantages for both the plants and the pollinators. The plants are able to disperse their pollen accurately and widely. The animal pollinators benefit by having a reliable and easily recognizable food source. The development of this relationship between plants and animals has been one of the most important factors in the evolution of both groups. This co-evolution between plants and their animal pollinators has meant that flowers have become adapted to attract certain animals. For example, yellow flowers are primarily attractive to bees, while red flowers attract birds. Some plants produce nectar deep inside their flowers that can be reached only by hummingbirds (with their long bills) or long-tongued insects such as moths and butterflies. We will learn more about this symbiotic relationship in the biodiversity modules later on in this course.
Fruit
Fruits are the vehicles that disseminate (disperse) seeds away from the parent plant. Once a seed is released from its fruit and conditions are favorable for germination, it can grow into a mature plant that produces its own flowers and fruit. Common knowledge about what is and what is not a fruit is sometimes incorrect. Countless items that we classify as vegetables are actually fruits. Many berries are not true berries. Nuts and grains are fruits as well, although not every item we think of as a 'nut' is a nut! It's pretty confusing. So here's the low-down: The definition of a fruit is any structure that develops from a fertilized ovary. All fruits come from ovaries. Examples of items commonly thought to be vegetables that are actually fruits are tomatoes, cucumbers, beans, peas, peppers, corn, eggplant, and squash. Fruits can be fleshy or dry. They can have many seeds, like tomatoes, papaya, and watermelon, or they can have just one, like avocados, almonds, and cherries. They can be quite large, like jackfruit, pineapples, and pumpkins, or very small, like blueberries, raspberries, and grapes.
In some fruits, the seeds are enclosed within the ovary (as in apples, peaches, oranges, squash, and cucumbers).
In other fruits, the seeds are situated on the outside of the plant's fruit tissue (for example, in corn and strawberries).
Did You Know?
The only part(s) of the fruit that contain genes from both the male and female flowers are the seed(s). The rest of the fruit arises from the maternal plant and is genetically identical to it.
The only part(s) of the fruit that contain genes from both the male and female flowers are the seed(s). The rest of the fruit arises from the maternal plant and is genetically identical to it.
Types of Fruit
Fruits are classified as simple, aggregate, or multiple. Simple fruits, the most common type, develop from a single ovary. They include fleshy fruits such as cherries and peaches, pears and apples, and tomatoes. Other types of simple fruits are dry. Their wall is either papery or leathery and hard, as opposed to the fleshy examples just mentioned. Examples are peanuts, poppies, maples, coconuts, and walnuts. |
An aggregate fruit develops from a single flower with many ovaries. Examples are strawberries, raspberries, and blackberries. The flower is a simple flower with one corolla, one calyx, and one stem, but is has many pistils or ovaries. Each ovary is fertilized separately. If some ovules are not pollinated successfully, the fruit will be misshapen.
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In conclusion, simple fruits develop from a single flower, aggregate fruits develop from one flower that produces many tiny fruits, and multiple fruits develop from multiple flowers and then form one large fruit. Here's a little cheat-sheet you may want to jot down in your journal:
Simple: One Flower ---> One Fruit
Aggregate: One Flower ---> Many Fruits
Multiple: Many Flowers ---> One Fruit
Aggregate: One Flower ---> Many Fruits
Multiple: Many Flowers ---> One Fruit
Plant Reproduction
Sexual Reproduction
Seed formation begins when pollen is moved from the male part of the flower (the stamen) to the female part (the pistil). It is important to remember that all the stages of the reproductive life cycle (pollination, fertilization, and seed and fruit production) take place within flowers. Pollination is the transport of pollen from the anther onto the stigma. Pollen can be carried by insects, other animals, or wind. When pollen touches the stigma, a long tube (the pollen tube) grows from the pollen down through the style until it reaches the ovary. This union of a male and female cell is called fertilization. Fertilization results in an embryo which develops inside protective layers of tissue. The embryo and protective layers are collectively called a seed.
Seed formation begins when pollen is moved from the male part of the flower (the stamen) to the female part (the pistil). It is important to remember that all the stages of the reproductive life cycle (pollination, fertilization, and seed and fruit production) take place within flowers. Pollination is the transport of pollen from the anther onto the stigma. Pollen can be carried by insects, other animals, or wind. When pollen touches the stigma, a long tube (the pollen tube) grows from the pollen down through the style until it reaches the ovary. This union of a male and female cell is called fertilization. Fertilization results in an embryo which develops inside protective layers of tissue. The embryo and protective layers are collectively called a seed.
Asexual Reproduction
There are two types of asexual reproduction in plants: vegetative reproduction and apomixis. While the specific processes of each are different, the result is always the same - the creation of clones, or offspring that are genetically identical to the parent plant. Asexual reproduction can be advantageous in plants that are well adapted to their environments; it guarantees that traits that make the pant well suited to its environment will be preserved in future generations. Apomixis is the creation of a genetically identical offspring via seed. Vegetative reproduction is the physical cutting and growing of a part of a plant to produce another identical plant.
There are two types of asexual reproduction in plants: vegetative reproduction and apomixis. While the specific processes of each are different, the result is always the same - the creation of clones, or offspring that are genetically identical to the parent plant. Asexual reproduction can be advantageous in plants that are well adapted to their environments; it guarantees that traits that make the pant well suited to its environment will be preserved in future generations. Apomixis is the creation of a genetically identical offspring via seed. Vegetative reproduction is the physical cutting and growing of a part of a plant to produce another identical plant.
Plants that produce rhizomes, stolens, tubers, and other modified stems are frequently capable of vegetative reproduction. These modified stems may have buds, and each one of these buds is capable of developing into an independent plant. Gardeners commonly take advantage of this trait to propagate such species by dividing modified stem segments with buds into separate pieces that can grow into new plants. By propagating vegetatively, you can be sure that the desirable traits of the parent plant - say, a certain color leaf - will be passed on to the new plants, which will be genetically identical to the parent.
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Growth
Plant growth is defined as the irreversible increase in plant size caused by an increase in cell number or size, which results in the development of new or expanded plant tissues, organs, or other structures. The process of plant growth is controlled by the integration of genetic potential and surrounding environmental conditions. Photosynthesis, respiration, and transpiration are the three major functions that drive plant growth and development; all three are essential to a plant's survival. How well a plant is able to regulate these functions greatly affects its ability to compete and reproduce.
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Photosynthesis
One of the major differences between plants and animals is that plants have the ability to manufacture their own food; they are essentially little living factories that both produce their own food and serve as the source of food and support for almost all other living organisms. The process plants use to create energy from sunlight is called photosynthesis, which literally means 'to put together with light'. To produce food, a plant requires energy from the sun, carbon dioxide from the air, and water and nutrients from the soil. During photosynthesis, it splits carbon dioxide into carbon and oxygen, adds water, and forms carbohydrates (starches and sugars). Oxygen is a by-project of this process. Check out the equation for photosynthesis below:
One of the major differences between plants and animals is that plants have the ability to manufacture their own food; they are essentially little living factories that both produce their own food and serve as the source of food and support for almost all other living organisms. The process plants use to create energy from sunlight is called photosynthesis, which literally means 'to put together with light'. To produce food, a plant requires energy from the sun, carbon dioxide from the air, and water and nutrients from the soil. During photosynthesis, it splits carbon dioxide into carbon and oxygen, adds water, and forms carbohydrates (starches and sugars). Oxygen is a by-project of this process. Check out the equation for photosynthesis below:
Carbon Dioxide + Water + Sunlight = Sugar + Oxygen
6 CO2 + 6 H2O + Energy = C6H12O6 + 6O2
6 CO2 + 6 H2O + Energy = C6H12O6 + 6O2
After producing carbohydrates, a plant either uses them as energy, stores them, or builds them into complex energy compounds such as oils and proteins. All of these food products are called photosynthates. The plant uses them when light is limited, or transports them to its roots or developing fruits.
Chlorophyll, the pigment that makes leaves green, is found in chloroplasts. It is responsible for trapping light energy from the sun. Often chloroplasts are arranged perpendicular to incoming sun rays so they can absorb maximum sunlight. If any of the ingredients for photosynthesis - light, water, and carbon dioxide - are lacking, photosynthesis stops. Furthermore, if any factor is absent for a long period of time, a plant will die. |
Respiration Carbohydrates made during photosynthesis are of value to a plant when they are converted to energy. This energy is used for cell growth and building new tissues. The chemical process by which sugars and starches are converted to energy is called oxidation and is similar to the burning of wood or coal to produce heat. Controlled oxidation in a living cell is called respiration and is shown by this equation: C6H12O6 = 6 CO2 + 6 H2O + Energy |
Does this equation look familiar? It is essentially the opposite of photosynthesis. Photosynthesis is a building process, and respiration is a breaking-down process. The differences between the two processes are outlined below:
PhotosynthesisProduces Food
Stores Energy Uses Water Uses Carbon Dioxide Releases Oxygen Occurs in Sunlight |
RespirationUses Food
Releases Energy Produces Water Produces Carbon Dioxide Uses Oxygen Occurs in the Dark as well as Light |
Unlike photosynthesis, respiration does not depend on light, so it occurs at night as well as during the day. Respiration occurs in all life forms and in all cells, and the rate of respiration changes depending on temperature and the availability of oxygen and carbohydrates. Respiration nearly doubles for every 18 degree Fahrenheit increase in temperature between 40 degrees and 96 degrees Fahrenheit. The rate of respiration also varies within a plant, with the highest respiration occurring in rapidly growing tissues and the lowest respiration occurring in dormant plant parts. Respiration even occurs in plant parts removed from the parent, including branches that have been cut off and fruit that has been harvested.
Transpiration
Transpiration is the evaporative loss of water vapor from plant leaves through the stomata. When a leaf's guard cells shrink, its stomata open, and water is lost through them. As a result, more water is pulled into the plant through its roots. The rate of transpiration is directly related to whether stomata are open or closed. Stomata account for only 1 percent of a leaf's surface but 90 percent of the water transpired. Transpiration is responsible for several things, including transporting minerals from the soil throughout the plant, cooling the plant through evaporation, maintaining turgor pressure, and moving sugars and plant chemicals.
Transpiration is the evaporative loss of water vapor from plant leaves through the stomata. When a leaf's guard cells shrink, its stomata open, and water is lost through them. As a result, more water is pulled into the plant through its roots. The rate of transpiration is directly related to whether stomata are open or closed. Stomata account for only 1 percent of a leaf's surface but 90 percent of the water transpired. Transpiration is responsible for several things, including transporting minerals from the soil throughout the plant, cooling the plant through evaporation, maintaining turgor pressure, and moving sugars and plant chemicals.
Transpiration is a necessary process and uses about 90 percent of the water that enters a plant's roots. The other 10 percent is used in chemical reactions and in plant tissues. The amount and rate of water lost through transpiration depends on factors such as temperature, humidity, and wind or air movement. Transpiration often is greatest in hot, dry (low relative humidity), windy weather, and almost ceases at night due to the closure of stomata. Transpiration, in addition to its role in water movement through plants, also helps to cool plants on warmer days, and transports minerals from soil and organic compounds produced in roots to plant cells.
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In succulent plants, transpiration works on a reverse schedule. Stomata on the storage leaves of these plants open during nighttime hours and are closed during the day. This schedule allows carbon dioxide to build up in plant leaves at night (which is then used during the day during photosynthesis) and reduces water loss via transpiration during the day.
Balancing the Cycle
In order for a plant to grow and develop properly, it must balance photosynthesis, respiration, and transpiration. Left to their own devices, plants do a good job managing this intricate balance. If a plant photosynthesizes at a high rate, but its respiration rate is not high enough to break down the photosynthates produced, photosynthesis will either slow down or stop. On the other hand, if respiration is much more rapid than photosynthesis, the plant won't have adequate photosynthates to produce energy for growth. In this case, growth either will slow down or stop altogether. Transpiration can also be controlled in response to environmental variables. When stomata are open, transpiration occurs, sometimes at a very high rate. A corn plant may transpire 50 gallons of water per season, but a large tree may move 100 gallons per day! Plants have problems if they lose too much water, so stomata close during hot, dry periods when transpiration is highest. However, CO2, which is needed for photosynthesis, also enters the plant through open stomata. Thus, if stomata stay closed a long time to stop water loss, not enough CO2 will enter for photosynthesis. As a result, photosynthesis and respiration will slow down, in turn reducing plant growth. Many herb plants produce lots of high-energy oils, which help them survive in the dry landscapes where they evolved. These oils help them survive extended periods where their stomata are closed.
In order for a plant to grow and develop properly, it must balance photosynthesis, respiration, and transpiration. Left to their own devices, plants do a good job managing this intricate balance. If a plant photosynthesizes at a high rate, but its respiration rate is not high enough to break down the photosynthates produced, photosynthesis will either slow down or stop. On the other hand, if respiration is much more rapid than photosynthesis, the plant won't have adequate photosynthates to produce energy for growth. In this case, growth either will slow down or stop altogether. Transpiration can also be controlled in response to environmental variables. When stomata are open, transpiration occurs, sometimes at a very high rate. A corn plant may transpire 50 gallons of water per season, but a large tree may move 100 gallons per day! Plants have problems if they lose too much water, so stomata close during hot, dry periods when transpiration is highest. However, CO2, which is needed for photosynthesis, also enters the plant through open stomata. Thus, if stomata stay closed a long time to stop water loss, not enough CO2 will enter for photosynthesis. As a result, photosynthesis and respiration will slow down, in turn reducing plant growth. Many herb plants produce lots of high-energy oils, which help them survive in the dry landscapes where they evolved. These oils help them survive extended periods where their stomata are closed.
Water and Nutrient Uptake
In addition to the photosynthates produced via photosynthesis, plants also rely on the uptake of water and nutrients to function. The mineral nutrients gathered support various internal plant processes, including photosynthesis and respiration, and are combined with carbohydrates produced by photosynthesis to form important growth compounds. Most plants absorb all of the water and nutrients they need for these purposes via chemical and physical processes that occur in roots.
Translocation
Translocation is the movement of water, mineral nutrients, food (in the form of carbohydrates), and other dissolved compounds from one part of a plant to another. It is a process that takes place mostly in phloem and xylem tissues, but can also occur in the space between cells. Via translocation, plants can distribute water and essential minerals from roots to other tissues, and move carbohydrates produced in leaves to meristematic areas, storage organs, and other tissues in need.
In addition to the photosynthates produced via photosynthesis, plants also rely on the uptake of water and nutrients to function. The mineral nutrients gathered support various internal plant processes, including photosynthesis and respiration, and are combined with carbohydrates produced by photosynthesis to form important growth compounds. Most plants absorb all of the water and nutrients they need for these purposes via chemical and physical processes that occur in roots.
Translocation
Translocation is the movement of water, mineral nutrients, food (in the form of carbohydrates), and other dissolved compounds from one part of a plant to another. It is a process that takes place mostly in phloem and xylem tissues, but can also occur in the space between cells. Via translocation, plants can distribute water and essential minerals from roots to other tissues, and move carbohydrates produced in leaves to meristematic areas, storage organs, and other tissues in need.
plant development
The development of a plant is an integrated expression of its genetic potential and its environment as it goes through periods of vegetative growth, reproductive growth, and dormancy. A general cycle begins with a seed which has germinated and sprouted. The seedling will then progress through juvenility, maturity, flowering, and fruiting. Once the fruit matures, the cycle is considered complete. At this time, annual plants will perish, and perennial plants start the cycle over again with vegetative growth after a period of dormancy. Internal regulation of plant growth is controlled by a few things. including the availability of sunlight, nutrients, water, carbohydrates, chemical energy, and hormones.
Plant Hormones
The hormones present in plant life are produced within plants (mainly in meristematic tissue) and are translocated to other parts of the plant where they are used in relatively small quantities. These hormones provide control or regulation of most plant processes. There are five major groups of plant hormones: Gibberellins, auxins, cytokinins, ethylene, and abscisic acid. Auxins are the general managers of cell enlargement, and are responsible for creating apical dominance of main buds and shoots, directing shoots toward light sources and horizontal growth upward, and promoting formation of adventitious roots from stems or leaves. This hormone also suppresses leaf and fruit drop and promotes fruit set and development or fruit abortion, depending on the situation.
Giberellins regulate cell division that leads to the elongation of stems and in some plant species also supports and promotes flowering. This hormone also activates enzymes in germinating seeds. Cytokinins stimulate cell division as well, often working alongside giberellins and auxins to regulate the processes that they are responsible for. Ethylene is a hormone that accelerates fruit ripening, and in some plant species also induces flowering. Finally, abscisic acid regulates and promotes dormancy in shoots and seeds, and is responsible for the loss of leaves in deciduous plants.
Plant Hormones
The hormones present in plant life are produced within plants (mainly in meristematic tissue) and are translocated to other parts of the plant where they are used in relatively small quantities. These hormones provide control or regulation of most plant processes. There are five major groups of plant hormones: Gibberellins, auxins, cytokinins, ethylene, and abscisic acid. Auxins are the general managers of cell enlargement, and are responsible for creating apical dominance of main buds and shoots, directing shoots toward light sources and horizontal growth upward, and promoting formation of adventitious roots from stems or leaves. This hormone also suppresses leaf and fruit drop and promotes fruit set and development or fruit abortion, depending on the situation.
Giberellins regulate cell division that leads to the elongation of stems and in some plant species also supports and promotes flowering. This hormone also activates enzymes in germinating seeds. Cytokinins stimulate cell division as well, often working alongside giberellins and auxins to regulate the processes that they are responsible for. Ethylene is a hormone that accelerates fruit ripening, and in some plant species also induces flowering. Finally, abscisic acid regulates and promotes dormancy in shoots and seeds, and is responsible for the loss of leaves in deciduous plants.
Dormancy
Occurring in both seeds and sprouted plants, dormancy represents a period of time when plant life is alive, but not actively growing. In engaging in dormancy, plants can survive unfavorable conditions by living off of stored energy. Seed dormancy will be covered in our Seed Module. Plant dormancy is triggered by the onset of environmental variables not conducive to active growth, such as shorter days, cooler temperatures, or drought. Parts of plants can be dormant, as well, while the rest of the plant is still active. A great example of this occurrence is apical dominance, where the hormone auxin suppresses lateral growth in favor of the dominant growing shoot. If this dominant growing shoot is pruned off of the plant, lateral buds and growth will will wake from dormancy and begin to grow.
Occurring in both seeds and sprouted plants, dormancy represents a period of time when plant life is alive, but not actively growing. In engaging in dormancy, plants can survive unfavorable conditions by living off of stored energy. Seed dormancy will be covered in our Seed Module. Plant dormancy is triggered by the onset of environmental variables not conducive to active growth, such as shorter days, cooler temperatures, or drought. Parts of plants can be dormant, as well, while the rest of the plant is still active. A great example of this occurrence is apical dominance, where the hormone auxin suppresses lateral growth in favor of the dominant growing shoot. If this dominant growing shoot is pruned off of the plant, lateral buds and growth will will wake from dormancy and begin to grow.
Vegetative Development
A plant is considered a juvenile from the time it is a seedling until it is fully mature and able to produce flowers. During this time, which can last from a few months to a few years, plants will grow vigorously. The plant will add to its structure, width, and height, growing stems, shoots, roots, and leaves. Because of their aptitude for putting on growth, juvenile plants more readily initiate adventitious roots and are more readily grafted. When the plant is fully developed, it is said to have reached maturity. At this point, it is ready to put out flowers and other specialized organs. Plants with the genetic ability to do so will produce bulbs, tubers, fleshy roots, and runners. |
Reproductive Development
In flowering plants, the reproductive growth phase begins when vegetative meristems are induced to produce reproductive organs (flowers), and ends with the formation of fruit or the senescence (death) of the plant. When a meristem is ordered to produce a flower, it follows an (in most cases) irreversible process of initiating cells that form new tissues of a flower or flower cluster (an inflorescence). Meristem induction time can last from a few weeks to several months, and can occur on both old and new wood, depending on the species grown. Some species are self-induced to flower, others need to have certain environmental factors present for flowering to occur. Primary environmental factors that can induce flowering include the photoperiod (length of daylight in a 24 hour period), light intensity, temperature, soil moisture content, and the internal nutrition of the plant.
After flowers have formed. a few events must take place for fruit to grow. The flower's stigma must receive viable pollen, which must successfully germinate and fertilize the ovule or ovules. Each ovule, if there are multiple, needs to be fertilized by a separate pollen grain. The size of the fruit increases rapidly after successful pollination, using materials and energy supplied by photosynthesis. It takes about 40 illuminated leaves on a mature apple tree to support the growth of one apple fruit, for example. Soil moisture must be adequate, as well, to prevent premature fruit drop and smaller fruit than normal. In some cases, it is possible for fruits to form without fertilization occurring. These fruits are seedless, and are genetically identical to the parent plant. Crops in this group include bananas, navel oranges, some varieties of grapes, pineapple, persimmon, and some cucumber species. On most fruit crops, only a fraction of the flowers produced will create mature fruit. A significant number of fruit will drop from fruit trees about a month to a month and a half after flowers have faded; this is known as June Drop. At this time what the tree is doing is adjusting its fruit load to a quantity it can support.
In flowering plants, the reproductive growth phase begins when vegetative meristems are induced to produce reproductive organs (flowers), and ends with the formation of fruit or the senescence (death) of the plant. When a meristem is ordered to produce a flower, it follows an (in most cases) irreversible process of initiating cells that form new tissues of a flower or flower cluster (an inflorescence). Meristem induction time can last from a few weeks to several months, and can occur on both old and new wood, depending on the species grown. Some species are self-induced to flower, others need to have certain environmental factors present for flowering to occur. Primary environmental factors that can induce flowering include the photoperiod (length of daylight in a 24 hour period), light intensity, temperature, soil moisture content, and the internal nutrition of the plant.
After flowers have formed. a few events must take place for fruit to grow. The flower's stigma must receive viable pollen, which must successfully germinate and fertilize the ovule or ovules. Each ovule, if there are multiple, needs to be fertilized by a separate pollen grain. The size of the fruit increases rapidly after successful pollination, using materials and energy supplied by photosynthesis. It takes about 40 illuminated leaves on a mature apple tree to support the growth of one apple fruit, for example. Soil moisture must be adequate, as well, to prevent premature fruit drop and smaller fruit than normal. In some cases, it is possible for fruits to form without fertilization occurring. These fruits are seedless, and are genetically identical to the parent plant. Crops in this group include bananas, navel oranges, some varieties of grapes, pineapple, persimmon, and some cucumber species. On most fruit crops, only a fraction of the flowers produced will create mature fruit. A significant number of fruit will drop from fruit trees about a month to a month and a half after flowers have faded; this is known as June Drop. At this time what the tree is doing is adjusting its fruit load to a quantity it can support.
plant functioning
Plants, like all other living organisms, will do what they can to survive in a world that can sometimes be harsh. Environmental conditions and plant nutrition can drastically influence plat growth and development. Factors that affect plants include light, temperature, soil moisture content, and nitrogen nutrition. These conditions commonly interact in complex ways to impact growth and development, and induce myriad reactions in plants.
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Plant Responses to Day Length, Light Intensity, and Quality
There are some horticultural and ornamental plants that will respond to a specific amount of daylight in a 24-hour period (a photoperiod) with the initiation of flowering, the formation of specialized vegetative organs, or the beginning of dormancy. These plants are referred to as photoperiodic. This process begins in the leaves, which sense changes in the photoperiod and then send signals via hormones or biochemical mechanisms (that are not yet fully understood) to meristems. An elongated period of change in the photoperiod is usually needed to initiate these signals, usually 60 successive days or longer. Once this growth pattern is initiated, it cannot be reversed. Plants generally fall into three photoperiod-sensitive categories: short-day, long-day, and day-neutral. Short-day plants begin to change their functioning when light periods are less than 12 hours long on successive days. One of the most noticeable short-day plant responses is the shedding of leaves in deciduous plants in response to the loss in daylight hours in fall. Conversely, long-day plants will initiate responses if photoperiods last for 12-14 hours or more. Day-neutral plants do not have specific responses to day length.
There are some horticultural and ornamental plants that will respond to a specific amount of daylight in a 24-hour period (a photoperiod) with the initiation of flowering, the formation of specialized vegetative organs, or the beginning of dormancy. These plants are referred to as photoperiodic. This process begins in the leaves, which sense changes in the photoperiod and then send signals via hormones or biochemical mechanisms (that are not yet fully understood) to meristems. An elongated period of change in the photoperiod is usually needed to initiate these signals, usually 60 successive days or longer. Once this growth pattern is initiated, it cannot be reversed. Plants generally fall into three photoperiod-sensitive categories: short-day, long-day, and day-neutral. Short-day plants begin to change their functioning when light periods are less than 12 hours long on successive days. One of the most noticeable short-day plant responses is the shedding of leaves in deciduous plants in response to the loss in daylight hours in fall. Conversely, long-day plants will initiate responses if photoperiods last for 12-14 hours or more. Day-neutral plants do not have specific responses to day length.
Quick Question: What causes fall color change in deciduous plants?
As mentioned above, deciduous plants are short-day plants. This means that in fall when the daylight hours shorten, these plants initiate internal changes to compensate. When this occurs, deciduous plants will send signals to stop chlorophyll production. Because chlorophyll is what gives leaves their green color, the leaves begin to turn various shades of red, orange, yellow, pink, purple, and brown (depending on the species). A special zone of cells also forms at the base of each leaf petiole that weakens the connection to the rest of the plant. These leaves eventually fall off, leaving bare silhouettes behind until spring comes around again. |
Photosynthesis and transpiration are also plant functions that change depending on the amount of daylight available. Photosynthetic production and transpiration are lower on shorter days of the year, because photosynthesis directly depends on the amount of sunlight available for processing, and transpiration depends on light to trigger the opening of stomata. In addition to daylight length, plants also are affected by the intensity of light they receive.
Every plant has a specific range of light intensity at which they function best. Changes in light intensity affect plants at all stages of development, affecting germination of seeds, the coloring and sugar content of fruits, leaf thickness, and overall growth rate and shape. Light intensity is controlled by horticulturalists via plant placement and the provision of supplemental light sources (like grow lamps). Plants can also respond to light intensity issues by physically bending shoots so that leaves are at the correct height and angle to maximize photosynthetic activities. Auxin, the hormone that manages cell growth, is found in larger concentrations on the shaded side of stems. The higher auxin levels stimulate growth on that side, causing the entire stem to bend towards the light. Plants that are receiving lower than required light intensity will also tend to get 'leggy', meaning that their internodes lengthen as they stretch out and search for more light. Although this does solve light issues for the plant, it also makes the stems weaker, and if the plant isn't able to find sufficient light, it can die. When moving plants around, changes in light intensity is an important factor to consider. Abrupt changes, even if they are for the better, can stress plants out. In most species, increasing light abruptly can cause foliage to yellow and in some cases get sunburned. Reducing light abruptly can cause leaf drop. It is best to gradually introduce plants to their new light situation to avoid causing additional harm.
Finally, plants are affected by light quality, which is the expression of the color of the light source. Sunlight encompasses all colors and appears white, but most artificial light sources do not have this balance of color and impart other colors, like blue or red. Photosynthesis is most efficiently conducted with red and blue light. In indoor growing rooms, red-blue lights are commonly used to mimic natural sunlight. It has been discovered recently, however, that cool-white fluorescent lights are the most effective for growing foliage plants without natural light.
Every plant has a specific range of light intensity at which they function best. Changes in light intensity affect plants at all stages of development, affecting germination of seeds, the coloring and sugar content of fruits, leaf thickness, and overall growth rate and shape. Light intensity is controlled by horticulturalists via plant placement and the provision of supplemental light sources (like grow lamps). Plants can also respond to light intensity issues by physically bending shoots so that leaves are at the correct height and angle to maximize photosynthetic activities. Auxin, the hormone that manages cell growth, is found in larger concentrations on the shaded side of stems. The higher auxin levels stimulate growth on that side, causing the entire stem to bend towards the light. Plants that are receiving lower than required light intensity will also tend to get 'leggy', meaning that their internodes lengthen as they stretch out and search for more light. Although this does solve light issues for the plant, it also makes the stems weaker, and if the plant isn't able to find sufficient light, it can die. When moving plants around, changes in light intensity is an important factor to consider. Abrupt changes, even if they are for the better, can stress plants out. In most species, increasing light abruptly can cause foliage to yellow and in some cases get sunburned. Reducing light abruptly can cause leaf drop. It is best to gradually introduce plants to their new light situation to avoid causing additional harm.
Finally, plants are affected by light quality, which is the expression of the color of the light source. Sunlight encompasses all colors and appears white, but most artificial light sources do not have this balance of color and impart other colors, like blue or red. Photosynthesis is most efficiently conducted with red and blue light. In indoor growing rooms, red-blue lights are commonly used to mimic natural sunlight. It has been discovered recently, however, that cool-white fluorescent lights are the most effective for growing foliage plants without natural light.
Plant Responses to Temperature
Effects of temperature on plants generally depends on the length of exposure time; for the most part, plant metabolic processes shut down when sustained temperatures exceed 96 degrees Fahrenheit or drop below 40 degrees Fahrenheit. Respiration rates and growth rates increase as temperature increases within this range. Germination of seeds in certain species is temperature-sensitive. Stratification is the process by which seeds germinate after a period of time in cold (45 degrees Fahrenheit or lower) soil. Bud and shoot growth and dormancy is also affected by temperatures, commonly requiring a certain number of hours (usually between 200 and 800 hours) at or below a certain temperature (usually 45 degrees Fahrenheit) to break dormancy. This information is well-studied by growers of fruit trees, as hour and temperature requirements vary depending on the species.
Vernalization is another process that occurs in biennial plants in response to temperature changes. In some species, exposure to cold temperatures for a certain period of time is required before flower growth is initiated (usually below 40 or 50 degrees Fahrenheit for between 6 and 12 weeks). This cold snap sends signals to meristematic tissue that triggers the production of flowers. Some temperature-driven processes are also affected by light availability. The two variables combine to create an environment in which plants will initiate flowering. Examples of this phenomenon are outlined below:
Effects of temperature on plants generally depends on the length of exposure time; for the most part, plant metabolic processes shut down when sustained temperatures exceed 96 degrees Fahrenheit or drop below 40 degrees Fahrenheit. Respiration rates and growth rates increase as temperature increases within this range. Germination of seeds in certain species is temperature-sensitive. Stratification is the process by which seeds germinate after a period of time in cold (45 degrees Fahrenheit or lower) soil. Bud and shoot growth and dormancy is also affected by temperatures, commonly requiring a certain number of hours (usually between 200 and 800 hours) at or below a certain temperature (usually 45 degrees Fahrenheit) to break dormancy. This information is well-studied by growers of fruit trees, as hour and temperature requirements vary depending on the species.
Vernalization is another process that occurs in biennial plants in response to temperature changes. In some species, exposure to cold temperatures for a certain period of time is required before flower growth is initiated (usually below 40 or 50 degrees Fahrenheit for between 6 and 12 weeks). This cold snap sends signals to meristematic tissue that triggers the production of flowers. Some temperature-driven processes are also affected by light availability. The two variables combine to create an environment in which plants will initiate flowering. Examples of this phenomenon are outlined below:
Poinsettia plants initiate flowers in 65 days when grown in short day-conditions at 70 degrees Fahrenheit, but require 85 days to flower if the temperature is kept at 60 degrees Fahrenheit.
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Species of strawberries initiate flowers under short-day conditions and runners under long-day conditions unless temperatures remain below 67 degrees Fahrenheit (when the plants will flower under any day-length conditions).
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Christmas Cactus has a short-day response when night temperatures stay between 60 and 65 degrees Fahrenheit, and will flower at any day length if temperatures are kept below 55 degrees Fahrenheit. If nighttime temperatures stay above 70 degrees, no flowers will be produced.
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Plant Responses to Soil Moisture Conditions
When soil moisture is inadequate (or the roots or vascular system is impaired and can't transport water normally), it cam affect all plant growth and developmental processes. The first functions affected are photosynthesis, transpiration, and nutrient uptake. Drought-stressed plant tissues tend to become less succulent, and continued drought will reduce growth rate of shoots and roots and affect flowers, fruits, and storage organ development. In general, plants will assume a green-grey color, shoots will wilt and die, and growth will be stunted. Seeds are also affected; without soil moisture, they will not germinate. Although some plants will respond to drought conditions by trying to control water loss (closing stomata), if water deficit continues for too long, the plants will not be able to compensate and will die.
When soil moisture is inadequate (or the roots or vascular system is impaired and can't transport water normally), it cam affect all plant growth and developmental processes. The first functions affected are photosynthesis, transpiration, and nutrient uptake. Drought-stressed plant tissues tend to become less succulent, and continued drought will reduce growth rate of shoots and roots and affect flowers, fruits, and storage organ development. In general, plants will assume a green-grey color, shoots will wilt and die, and growth will be stunted. Seeds are also affected; without soil moisture, they will not germinate. Although some plants will respond to drought conditions by trying to control water loss (closing stomata), if water deficit continues for too long, the plants will not be able to compensate and will die.
Plant Responses to Carbon Dioxide, Oxygen, and Nitrogen Concentrations
Both carbon dioxide and oxygen are essential to plant functioning and growth. The air surrounding plants is usually sufficient to provide these gases to shoots. However, plants also need to derive oxygen through the air in soil via their roots. If the soil is compacted or waterlogged, oxygen may be insufficient for root respiration. If these conditions are not remedied, plants will die. Seeds will also struggle to germinate if they cannot pull oxygen from the soil to support respiration. Soil used to germinate seeds should be well-aerated. This is usually accomplished by sowing seeds under a light covering of soil and not packing it down too much on top.
Nitrogen is one of the most important elements for plant growth, but abundant levels of nitrogen can also cause a lot of issues. Although it contributes to new vegetative growth, this growth usually occurs at the expense of flowering. High nitrogen levels can cause problems in disease susceptibility and reduced fruit quality. Once nitrogen levels have fallen, increased carbohydrates from the abundant photosynthetic material is then used to initiate flower growth. If nitrogen levels subside too drastically, however, little photosynthesis will occur, leading to not enough carbohydrate being produced to support growth, flowering and fruit development. For plants to flower and fruit in a healthy manner, a good balance of nitrogen and carbohydrates must be achieved.
Both carbon dioxide and oxygen are essential to plant functioning and growth. The air surrounding plants is usually sufficient to provide these gases to shoots. However, plants also need to derive oxygen through the air in soil via their roots. If the soil is compacted or waterlogged, oxygen may be insufficient for root respiration. If these conditions are not remedied, plants will die. Seeds will also struggle to germinate if they cannot pull oxygen from the soil to support respiration. Soil used to germinate seeds should be well-aerated. This is usually accomplished by sowing seeds under a light covering of soil and not packing it down too much on top.
Nitrogen is one of the most important elements for plant growth, but abundant levels of nitrogen can also cause a lot of issues. Although it contributes to new vegetative growth, this growth usually occurs at the expense of flowering. High nitrogen levels can cause problems in disease susceptibility and reduced fruit quality. Once nitrogen levels have fallen, increased carbohydrates from the abundant photosynthetic material is then used to initiate flower growth. If nitrogen levels subside too drastically, however, little photosynthesis will occur, leading to not enough carbohydrate being produced to support growth, flowering and fruit development. For plants to flower and fruit in a healthy manner, a good balance of nitrogen and carbohydrates must be achieved.
Plant Responses to Stress
Plant stress is defined as any combination of non-optimal growing conditions for a given plant. These conditions can include extremes in temperature, insufficient light or water, inadequate nutrients, or poor soil aeration. Plants under stress will have a shorter juvenile growing phase, low vigor, and weak but tough vegetative growth. Stressed plants can enter dormancy prematurely in order to conserve energy for a more growth-conducive time. Under severe stress, plants can defoliate (lose their leaves) or turn unnatural colors. Sometimes, however, plant stress is used intentionally to help plants survive transplanting. This process, called 'hardening' involves withholding water and nitrogen fertilizer from greenhouse-grown plants alongside exposing them to increasingly long periods of natural sunlight. This ensures that the plants, once they are purchased, brought home, and planted, will survive the change in environment.
Plant stress is defined as any combination of non-optimal growing conditions for a given plant. These conditions can include extremes in temperature, insufficient light or water, inadequate nutrients, or poor soil aeration. Plants under stress will have a shorter juvenile growing phase, low vigor, and weak but tough vegetative growth. Stressed plants can enter dormancy prematurely in order to conserve energy for a more growth-conducive time. Under severe stress, plants can defoliate (lose their leaves) or turn unnatural colors. Sometimes, however, plant stress is used intentionally to help plants survive transplanting. This process, called 'hardening' involves withholding water and nitrogen fertilizer from greenhouse-grown plants alongside exposing them to increasingly long periods of natural sunlight. This ensures that the plants, once they are purchased, brought home, and planted, will survive the change in environment.
Conclusion
Now that you've completed your second botany module, you've hopefully got a better understanding of how your garden survives on a day-to-day basis. This information may help you to fully grasp how to help plants that are struggling, why it is important to follow instructions on watering and sun requirements of different species, and how important it is to encourage biodiversity in your yard to ensure pollination is occurring. Before moving on to you next module - which is all about dirt - check out the list of key terms for this module by clicking the button below, go over the resources and references, and do the activities on the homework page.
Botany II HOmework
Click the button below to access the activities and homework questions for this Module. If you'd like to print the homework page, visit the printables page.
Resources & References
Plant Form
Adrian D. Bell and Alan Bryan The first part of the book describes and clearly illustrates the major plant structures that can be seen with the naked eye or a hand lens. The second part focuses on how plants grow: bud development, the growth of reproductive organs, leaf arrangement, branching patterns, and the accumulation and loss of structures. Aimed at students of botany and horticulture, enthusiastic gardeners, and amateur naturalists, it functions as an illustrated dictionary, a basic course in plant morphology, and an intriguing and enlightening book to dip into. |
Botany Illustrated
Joyce Glimn-Lacy and Peter B. Kaufman This easy-to-use book helps you acquire a wealth of fascinating information about plants. There are 130 pages with text, each facing 130 pages of beautiful illustrations. Each page is a separate subject. Included is a coloring guide for the realistic illustrations. The illustration pages are composed of scientifically accurate line drawings with the true sizes of the plants indicated. Using colored pencils and the authors' instructions, you can color the various plant structures to stand out in vivid clarity. Your knowledge of plants increases rapidly as you color the illustrations. There is a balanced selection of subjects that deal with all kinds of plants. |
Botany for Gardeners Brian Capon A bestseller since its debut in 1990, this indispensable and handy reference has now been expanded and updated to include an appendix on plant taxonomy and a comprehensive index. Two dozen new photos and illustrations make this new edition even richer with information. Its convenient paperback format makes it easy to carry and access, whether you are in or out of the garden. An essential overview of the science behind plants for beginning and advanced gardeners alike. |