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Control Systems in
Plants
Chapter 22
Plant Hormones

Chemical messengers in plants which control:
1. germination
2. growth
3. flowering
4. fruit production
Five Major Plant Hormones
1. Auxins—promotes plant growth; produced in
the apical meristems at the tips of shoots
2. Cytokinins—stimulate cell
division; produced in
actively growing tissues
such as embryos, roots,
and fruits; slows the aging
of flowers and fruit;
promote cell division in
axillary buds
3. Giberellins—produced at
the tips of both stems and
roots; stimulate growth of
stems; also promote seed
germination in some
species
4. Abscisic Acid—inhibits cell division in buds
and in the vascular cambium during periods
of dormancy; promotes dormancy in seeds;
acts as a stress hormone causing stomata to
close
5. Ethylene—stimulates fruit ripening
Plants Respond to Changes in
Environment
TropismsA. Thigmotropism—response to touch
B. Phototropism—response to light
C. Gravitropism—response to gravity
Coping with Stressful Environments
1. Drought—
a. Close stomata
b. Cacti store water in
fleshy stems; their
leaves are modified to
spines
c. Succulents have a
thick cuticle; their green
stems are the main
photosynthetic organs
2. Flooding—plants that are in flooded
conditions release ethylene; some cells in
the roots are killed creating air tubes which
act as snorkels
Mangrove trees have roots that are partially
above ground.
3. Salt Stress—When salt builds up
in the soil, roots can lose their
water through osmosis
Halophytes—have salt glands that
pump salt out of the plant across
the leaf epidermis
Pickleweed pumps salt to the stems
at the tips of the plant, then
sheds
its stems
Plant Defenses
1. Epidermis—first line of defense against disease
2. Chemicals—second line of defense
a. Some are microbial
b. Some signal lignin production to harden
cell walls around the infected area
3. Inherited ability to recognize and attack
pathogens
4. Thorns, chemical poisons—defend against
being eaten