External
Environmental Factors Tropisms Growth Regulators IAA’s Role in Phototropism
Commercial Applications General Plant Adaptation for Protection The Desert The Grassland
The Tropical Rainforest The Temperate Rainforest
As we have studied in our Nervous System study animals
respond to stimuli using electrical and chemical signals. We have also learned
that chemical messengers in our Endocrine System are used to
accomplish life’s needs.
Plants also have the means to respond to
their environment in order to successfully live. They use chemicals similar to
the endocrine system to accomplish this. This web page will examine these plant
responses.
The Regulation of Growth
Plants’ growth is controlled by many
factors. Some of the factors are environmental and climatic while others come
from within the plants themselves.
External Environmental Factors
Some external factors that regulate the
growth of plants are light
intensity, day length, gravity, and temperature.
Light- Light, obviously affects
the plants ability to grow because light is needed for photosynthesis. Through
photosynthesis the plant makes its energy carrying molecules. It is also needed
for the production of chlorophyll.
Day Length- Day length causes
the plants to flower. Many other roles of day length are being studied such as
fruit and seed germination, dormancy, and leaf loss.
Gravity- Gravity causes roots
to grow down toward the soil and roots to grow up away from the source of
gravity.
Temperature- Temperature affects
the rate of enzyme reaction. Higher temperature, to a point, is best for plant
growth while low temperatures are needed for some plants to flower.
Tropisms
A tropism is a
biological phenomenon, indicating growth or turning movement of a biological
organism, usually a plant, in response to an environmental stimulus. In
tropisms, this response is dependent on the direction of the species. The word
tropism comes from the Greek trope ("to turn" or "to
change"). Tropisms are usually named for the stimulus involved and may be
either positive (towards the stimulus) or negative (away from the stimulus).
Phototropism
Phototropism is the growth
response of a plant in response to light direction. Different parts of a plant
exhibit different reactions to light. Stems exhibit positive phototropism while
most roots exhibit negative phototropism.
View movies showing phototropism: Phototropism
movie 1 Phototropism movie 2 Tomato phototropism
Corn phototropism Cool corn phototropism Sunflower phototropism
Geotropism
is the growth response of a plant in response to gravity. Roots exhibit
positive geotropism while stems and leaves exhibit negative geotropism.
View movies showing geotropism: Shoot negative geotropism Root positive geotropism
Shoot positive geotropism Sunflower positive geotropism
Thigmotropism
is the growth response of a plant to physical contact (touch). Plants that
cling to physical structures such as walls exhibit positive thigmotropism.
Hydrotropism
is the growth response of a plant to water. Roots exhibit positive
hydrotropism.
Chemotropism
is the growth response of a plant to a particular chemical. Roots grow toward
useful minerals in the soil but away from acids.
Growth
regulators are chemicals that control the growth of a plant. Some of these are growth
promoters while others are growth inhibitors.
Growth
Promoters
Auxins- Auxins are
chemicals that are produced at the meristems, young leaves, and seeds of
plants. The most important auxin is indoleacetic acid (IAA). Auxins have
a role in the following:
Development of the embryo- From the very first mitotic division of the zygote,
auxins guide the patterning of the embryo into the parts that will become the
organs of the plant:
Leaf formation- The formation of new leaves in the apical meristem is initiated by the
accumulation of auxin. Already-developing leaves deplete the surrounding cells
of auxin so that the new leaves do not form too close to them. In this way, the
characteristic pattern of leaves in the plant is established.
Phototropism- Plant shoots display positive phototropism: when illuminated from one
direction, the shoot proceeds to grow in that direction.
Geotropism- Geotropism is a plant growth response to gravity.
Plant shoots display negative geotropism: when placed on its
side, a plant shoot will grow up. Roots display positive
geotropism: they grow down.
Apical dominance- Growth of the apical bud usually inhibits the
development of the lateral buds on the stem beneath. This phenomenon is called
apical dominance. If the apical bud of a plant is removed, the inhibition is
lifted, and lateral buds begin growth. Gardeners use this principle by pruning
the apical buds of ornamental shrubs, etc. The release of apical dominance
enables lateral branches to develop and the plant becomes bushier. The process
usually must be repeated because one or two laterals will eventually outstrip
the others and apical dominance will result again. Apical dominance seems to
result from the downward transport of auxin produced in the apical meristem. In fact, if the apical meristem is
removed and IAA applied to the stump, inhibition of the lateral buds is
maintained.
Fruit
development- Pollination
of the flowers of angiosperms initiates the formation of seeds.
As the seeds mature, they release IAA to the surrounding flower parts,
which develop into the fruit
that covers the seeds. Some commercial growers deliberately initiate fruit
development by applying IAA to the flowers. Not only does this ensure
that all the flowers will "set" fruit, but it also maximizes the
likelihood that all the fruits will be ready for harvest at the same time. This
process develops seedless fruit.
Root initiation- IAA in epidermal cells of the root initiates the
formation of lateral or secondary
roots. Auxin also stimulates the formation of adventitious roots in many
species. Adventitious roots grow from stems or leaves rather than from the
regular root system of the plant. Horticulturists may propagate desirable
plants by cutting pieces of stem and placing them base down in moist soil.
Eventually adventitious roots grow out at the base of the cutting. The process
can often be hastened by treating the cuttings with a solution or powder
containing a synthetic auxin.
Cell elongation- Auxins cause the cells of plants to elongate
because they soften the cell walls. They also stimulate enzymes that cause cell
elongation.
Growth
Inhibitors
The auxin ethane
is a gas. It is made in the nodes of stems, in ripe fruits, and in decaying
leaves. This auxin causes leaves and fruits to fall from trees. It causes a
special layer of cells — the abscission layer — to form at the base of
the petiole or fruit stalk. Soon the petiole or fruit stalk breaks free at this
point and the leaf or fruit falls to the ground. It also causes fruits to
colour, flavour, and ripen. It also causes plants to age.
Abscisic
acid (
Unlike animals, plants cannot flee from potentially
harmful conditions like drought the approach of winter. They must adapt or die.
The plant hormone abscisic acid (
Bud dormancy
Seed maturation and dormancy
Seeds are not only important agents of
reproduction and dispersal, but they are also essential to the survival of
annual and biennial plants. These angiosperms
die after flowering and seed formation is complete.
Closing of stomata
Some
90% of the water taken up by a plant is lost in transpiration.
Most of this leaves the plant through the pores — called stomata
— in the leaf. A pair of guard
cells flanks each stoma. When the guard cells are turgid, the stoma is
open. When turgor
is lost, the stoma closes.
IAA’s Role in Phototropism
IAA is produced in the meristems of the stem. The auxin diffuses down the shady side of a stem and not the sunny side. As a result, the cells on the shady side elongate more than the sunny side and the stem bends toward the sun.
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Commercial Applications
Plant hormones have
many commercial uses, relating to agriculture and flower growing.
Rooting Powder: Naphthylacetic acid (NAA) is used to stimulate root
formation of plant cuttings.
Cytokinin: An auxin, which is used in tissue culturing.
Pieces of a plant can be grown into a new plant. First a callus forms.
This is a group of cells. Auxins are then used in varying concentrations to
produce the parts of the plant.
Ethelene: Ethelene promotes the ripening of bananas
for the market.
Auxins are used as
selective weed killers. They reduce competition and so promote crop
growth.
Production of seedless
fruits e.g. oranges.
General Plant Adaptation for Protection
Plants must be adapted to the environment and predators of their habitat. Generally these adaptations include:
The epidermis and bark of the plants protect it from the
entry of pathogens as well as from water loss.
The cuticle of the leaf protects the leaf against water
loss as well as infection by bacteria, fungi and viruses.
The stomata closing protects against excessive water loss.
Stinging dermal hairs of plants such as the nettle
protects against herbivores. These hairs contain chemicals that harm the
organism that touches the plant.
Spines and thorns protect against herbivores.
Toxic substances are part of some plants to protect against
insect pests and herbivores.
Heat shock proteins are chemicals
found in the plant that, when the temperature rises, surround other proteins of
the plant so that they keep their shape.
Stress proteins allow the plant
to protect itself from invading microorganisms. Some will kill the invaders,
others will form a strong cell wall to stop the pathogen, and others stimulate
nearby cells to protect themselves.
Specific
Plant Adaptation for Various Habitats
The Desert
The desert is very dry and often hot. Annual
rainfall averages less than 10 inches per year, and that rain often comes all
at the same time. The rest of the year is very dry. There is a lot
of direct sunlight shining on the plants. The soil is often sandy or
rocky and unable to hold much water. Winds are often strong, and dry out
plants. Plants are exposed to extreme temperatures and drought
conditions. Plants must cope with extensive water loss.
Desert Plant Adaptations
- Some plants, called succulents, store water in their stems or
leaves;
- Some plants have no leaves or small seasonal leaves that only
grow after it rains. The lack of leaves helps reduce water loss
during photosynthesis. Leafless plants conduct photosynthesis in
their green stems.
- Long root systems spread out wide or go deep into the ground to
absorb water;
- Some plants have a short life cycle, germinating in response to
rain, growing, flowering, and dying within one year. These plants
can evade drought.
- Leaves with hair help shade the plant, reducing water loss.
Other plants have leaves that turn throughout the day to expose a minimum
surface area to the heat.
- Spines to discourage animals from eating plants for water;
- Waxy coating on stems and leaves help reduce water loss.
- Flowers that open at night lure pollinators who are more likely
to be active during the cooler night.
- Slower
growing requires less energy. The plants don’t have to make as much
food and therefore do not lose as much water.
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This
cactus displays several desert adaptations: it has spines rather than leaves
and it stores water in its stem. |
This
cactus displays light-colour hair that helps shade the plant. |
This
plant has a waxy coating on its leaves. |
The Temperate Grasslands
The temperate grasslands, also called prairie, feature hot summers and cold winters. Rainfall is uncertain and drought is common. The temperate grasslands usually receive about 10 to 30 inches of precipitation per year. The soil is extremely rich in organic material due to the fact that the aboveground portions of grasses die off annually, enriching the soil. The area is well suited to agriculture, and few original prairies survive today.
Temperate Grassland (Prairie) Plant Adaptations
- During a fire, while above-ground portions of grasses may perish,
the root portions survive to sprout again
- Some prairie trees have thick bark to resist fire
- Prairie shrubs readily resprout after fire
- Roots of prairie grasses extend deep into the ground to absorb as
much moisture as they can
- Extensive root systems prevent grazing animals from pulling roots
out of the ground
- Prairie grasses have narrow leaves which lose less water than
broad leaves
- Grasses grow from near their base, not from tip, thus are not
permanently damaged from grazing animals or fire
- Many grasses take advantage of exposed, windy conditions and are
wind pollinated
- Soft stems enable prairie grasses to bend in the wind
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Soft
stems enable prairie grasses to bend in the wind. Narrow leaves
minimize water loss. |
Many
grasses are wind pollinated and are well suited to the exposed, windy
conditions of the grasslands. |
The Tropical Rainforest
The tropical rainforest is hot and it rains a lot, about 80 to 180 inches per year. This abundance of water can cause problems such as promoting the growth of bacteria and fungi, which could be harmful to plants. Heavy rainfall also increases the risk of flooding, soil erosion, and rapid leaching of nutrients from the soil (leaching occurs when the minerals and organic nutrients of the soil are "washed" out of the soil by rainfall as the water soaks into the ground). Plants grow rapidly and quickly use up any organic material left from decomposing plants and animals. This results is a soil that is poor. The tropical rainforest is very thick, and not much sunlight is able to penetrate to the forest floor. However, the plants at the top of the rainforest in the canopy must be able to survive 12 hours of intense sunlight every day of the year. There is a great amount of diversity in plant species in the tropical rainforest.
Tropical Rainforest Plant Adaptations
- Drip tips and waxy surfaces allow water to run off, to discourage
growth of bacteria and fungi
- Buttresses and prop and stilt roots help hold up plants in the
shallow soil
- Some plants climb on others to reach the sunlight
- Some plants grow on other plants to reach the sunlight
- Flowers on the forest floor are designed to lure animal
pollinators since there is relatively no wind on the forest floor to aid
in pollination
- Smooth bark and smooth or waxy flowers speed the run off of water
- Plants have shallow roots to help capture nutrients from the top
level of soil.
- Many bromeliads are epiphytes (plants that live on other plants);
instead of collecting water with roots they collect rainwater into a
central reservoir from which they absorb the water through hairs on their
leaves
- Epiphytic orchids have aerial roots that cling to the host plant,
absorb minerals, and absorb water from the atmosphere
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Drip-tips
on leaves help shed excess water. |
Prop
roots help support plants in the shallow soil. |
Some
plants collect rainwater into a central reservoir. |
The Temperate Rain Forest
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The
Temperate Rainforest
The temperate rain forest features minimal seasonal fluctuation of temperature: the winters are mild and the summers cool. The temperate rain forest receives a lot of precipitation, about 80 to 152 inches per year. Condensation from coastal fogs also add to the dampness. The soil is poor in nutrients. Large evergreen trees, some reaching 300 feet in height, are the dominant plant species.
Temperate Rain Forest Plant
- Epiphytes such as mosses and ferns grow atop other plants to
reach light.
- Cool temperatures lead to slow decomposition but seedlings grow
on "nurse logs" to take advantage of the nutrients from the
decomposing fallen logs.
- Trees can grow very tall due to amount of precipitation.
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Epiphytes
live on other plants to reach the sunlight. |
Trees
can grow very tall in this very moist environment. |
