Plant Responses

C10L2

Plant Growth

Plants respond to their environment by the way they grow or do not grow.

Plant Growth

Growth in plants can result in an increase in length, or an increase in thickness, or both.

Plant Growth

The special areas where growth is occurring in plants are called meristems. These areas are easily spotted under a microscope because the recently divided or dividing plant cells are smaller and more dense and have either larger nuclei or visible chromosomes. Meristems are these regions where many cells are undergoing cell division.

Plant Growth

Meristems are located near the tips of stems and roots and between a plant’s xylem and phloem.

Two Types of Meristems

apical meristems: those meristems at the tips of stems and roots.

lateral meristems: a region of cell division located parallel to the sides of a plant (responsible for growth in thickness; vascular cambium and cork cambium)

Two Types of Meristems

apical meristems: those meristems at the tips of stems and roots.

Two Types of Meristems

lateral meristems: a region of celldivision locatedparallel to the sidesof a plant(responsible for growth in thickness; vascular cambium and cork cambium)

Stimuli are any changes in an organism’s environment that cause a response.

A plant responds to light by growing toward it.

Stimuli and Plant Responses

Plant Growth: Chemical Stimuli

Hormones are one way aplant’s growth is controlled.Hormones are chemicalsubstances which are madeby plants and which affecthow plant tissue growth by stimulating plant cells to divide, to enlarge, or to stop growing.

Plant Hormones

auxin: a plant growth hormone produced in the growing tips of plants.

They cause the cells on the dark side of the plant’s stem to grow longer.

Plant Hormones

The plant hormone ethylene helps stimulate the ripening of fruit.

Ethylene is a gas that can be produced by fruits, seeds, flowers, and leaves.

Plant Hormones

Rapidly growing areas of a plant, such as roots and stems, produce gibberellins, which increase the rate of cell division and cell elongation.

Plant Hormones

Root tips produce cytokinins, a hormone that increases the rate of cell division and, in some plants, slows the aging process of flowers and fruits.

Humans make plants more productive by using plant hormones.

Some crops now are easier to grow because humans understand how plants respond to hormones.

Humans and Plant Responses

Tropism: Environmental Stimuli

• the growth responses of plants to their environment

• a plant's directional growth response to a physical stimulus

• the growth of plants in response to external stimuli such as light, gravity, or contact

Tropism

• positive tropism: when the plant grows toward the stimulus

• negative tropism: when the plant grows away from the stimulus

Types of Tropism

1. phototropism – light

2. gravitropism– gravity

3. thigmotropism - touch

4. chemotropism - chemicals

5. hydrotropism - water

Phototropism

• plant's response to light

• positive phototropism - turns toward light (stem and leaves)

• negative phototropism - away from light (roots)

Gravitropism

• plant's response to gravity

• positive gravitropism - grows toward the pull of gravity (roots)

• negative gravitropism - grows away from the pull of gravity ( stem and leaves)

Gravitropism

Thigmotropism

• plant's response to touch

• positive thigmotropism - grows toward touch

• negative thigmotropism - grows away from touch

Thigmotropism

Special structures that respond to touch, called tendrils, can wrap around or cling to objects.

Thigmotropism

Chemotropism

• plant's response to chemicals

• positive chemotropism - grows toward chemicals

• negative chemotropism - grows away from chemicals

Hydrotropism

• plant's response to water

• positive hydrotropism - grows toward water

• negative hydrotropism - grows away from water

Plants and Light

Characteristics of light that are significant for plant growth

1. Intensity

etiolated - the condition of a plant when grown in the absence of light elongated stems with small, pale leaves

Characteristics of light that are significant for plant growth

2. Duration

duration - the length of daylight affects the photoperiodism of plants the chief factor affecting flowering

Photoperiodism

a plant’s response to the number of hours of darkness in its environment

the response of a plant to changes in the length of daylight

(the reponses of a plant to changes in light intensity and length of days)

It often determines whether ornot a plant produces flowers.

Photoperiodism

Some plants can accurately measure the length of light and darkness to within minutes so they will flower at precisely the right time of year.

Types of Plants regarding Photoperiodism

• Short-day plants

• Long-day plants

• Neutral-day plants (also called Day-neutral plants)

Short-dayplants

plants that flower when exposed to less than 12 hours of sunlight

Short-day plants require 12 or more hours of darkness for flowering to begin.

(bloom when the daysare short and the nights are long)

Short-day plants

examples: chrysanthemums,corn, strawberries, apples,soybeans, violets, ragweed

flower naturally out-of-doors in the early spring or in late summer and fall

Long-day plants

Plants that flower when exposed to less than 10-12 hours of darkness are called long-day plants.

require more than 12 hours of light

bloom with long periods of light and short periods of darkness

generally flower during late spring and summer

examples: clover, gladiolus, sunflowers, beets, lettuce, grains

Neutral-dayplants

Day-neutral plants flower when they reach maturity and the environmental conditions are right.

flower independently of a photoperiod

bloom whenever conditions like moisture and temperature are acceptable regardless of the amount of light or darkness

Neutral-day plants

usually flower continuouslyif other conditions (temp.,moisture, etc.) are favorable

examples: tomato, dandelion, hybrid roses, beans, zinnias, cotton

The number of hours of darkness controls flowering in many plants.