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 cell
division located
parallel to the sides
of a plant
(responsible for growth in thickness;
vascular cambium and cork cambium)
Stimuli and Plant Responses


Stimuli are any changes in an organism’s
environment that cause a response.
A plant responds to light by growing toward it.
Plant Growth:
Chemical Stimuli
Hormones are one way a
plant’s growth is controlled.
Hormones are chemical
substances which are made
by plants and which affect
how 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 and Plant Responses


Humans make plants more productive
by using plant hormones.
Some crops now are easier to grow because
humans understand how plants respond to
hormones.
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 or
not 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-day
plants
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 days
are 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-day
plants
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 continuously
if 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.