1. What Is Life? A Guide To Biology © 2011 W. H. Freeman and Company
Jay Phelan
What Is Life? A Guide To Biology
First Edition
CHAPTER 19
Plants Respond to Their Environments
© 2011 W. H. Freeman and Company

4. PLANTS RESPOND TO THEIR ENVIRONMENT

5. PLANTS RESPOND TO THEIR ENVIRONMENT
Three cuttings are taken from the same plant and grown in different environments.
Deep water
Shallow water
Land
The plant grows long, ribbon-like leaves.
The plant grows large, round leaves.
The plant grows arrow-shaped leaves. 
The leaves of the arrowleaf plant take on dramatically different forms depending on the environment in which the plant grows.

6. PLANT HORMONES HORMONE FUNCTION LOCATION
Increase the speed of seed germination; promote stem elongation; induce early blooming of flowers; increase fruit size
Shoot and root
apical meristems;
seeds
Gibberellins
Stimulate stem elongation; control seedling orientation; stimulate root branching; promote fruit development
Apical meristems; immature plant tissue
Auxins
Increases the speed at which fruit ripens; stimulates leaf dropping and the death of flowers
All parts of the plant including the fruits
Ethylene
Inhibits growth and reproduction; inhibits seed germination; stimulates closure of stomata
Leaves; fruits; root tips; seeds
Abscisic acid
Cause rapid cell division, in conjunction with auxin; induce seed germination; initiate new branches from lateral buds
Roots and fruits,
primarily
Cytokinins

7. THE EFFECTS OF GIBBERELLINS

8. THE EFFECTS OF GIBBERELLINS
SPEEDING SEED GERMINATION
Gibberellins initiate the production of enzymes that help break down nutrients stored within the seed’s endosperm, allowing quicker and more efficient use of the seed’s energy reserves.
STEM ELONGATION
Gibberellins affect stem elongation by increasing the distance between nodes, thus spacing the branch points farther apart.
INDUCING EARLY BLOOMING OF FLOWERS
Gibberellins can cause flower production in the absence of a triggering event from the external environment.
ENLARGEMENT OF FRUITS
Seedless grapes sprayed with large amounts of gibberellins grow larger and, due to the stem-elongation effects, have more space between the grapes on the bunch.

9. 
Gibberellins are powerful growth stimulators and, when applied in unnaturally large concentrations, can produce giant plants!

10. THE EFFECTS OF AUXINS

11. THE EFFECTS OF AUXINS STIMULATE SHOOT ELONGATION
Auxins enhance the effect of gibberellins in shoot elongation.
CONTROL SEEDLING ORIENTATION
Auxins direct the growth of shoots and roots, making sure the correct ends are up and down.
STIMULATE ROOT BRANCHING
Auxins induce the formation of roots.
PROMOTE FRUIT DEVELOPMENT
Auxins produced within an embryo promote the maturation of the ovary wall and development of the fruit.

12. AUXINS’ INFLUENCE ON PLANT ORIENTATION1
Sunlight
Auxin molecules 2 3 1
Auxins are produced near the growing tips of shoot, roots, and branches. 2
The auxin molecules are directed downward by gravity and move away from light. 3
In regions of higher auxin concentration, cells elongate more rapidly than in regions of lower auxin concentration, causing the shoot to bend toward the light.

14. ETHYLENE
Bananas picked before they are ripe are exposed to ethylene gas just prior to their delivery to market, initiating the ripening of all the fruit simultaneously.

15. Some flower merchants briefly soak cut-flower stems in a chemical solution of silver salts, which inhibits the deteriorating effects of ethylene on a flower’s petals.

16. ABSCISIC ACID THE PRIMARY EFFECTS OF ABSCISIC ACID
Inhibits growth and reproductive activities when environmental conditions are stressful
Signals the stomata on a plant’s leaves to close, increasing water conservation

17. CYTOKININS THE PRIMARY EFFECTS OF CYTOKININS
Cause rapid cell division in conjunction with auxins
Induce seed germination
Initiate new branches from lateral buds
Retard leaf death

18. PHOTOTROPISM

19. PHOTOTROPISM
Auxin molecules
Auxins produced in the plant move away from the light source to the shaded side of a stem, stimulating a greater rate of growth than on the side with less auxin. The uneven growth causes the plant to bend toward the light.

20. GRAVITROPISM

21. GRAVITROPISM
Auxin molecules
Starches within the cells of the stem sink downward in response to gravity, triggering the movement of auxin toward them. Auxin then stimulates faster growth in the regions where it occurs in higher concentration, causing the stem to bend upward.

22. THIGMOTROPISM
Climbing plants produce tendrils, which are specialized thread-like leaves or stems that wrap around whatever they touch.

23. THE BIOLOGICAL CLOCK IN PLANTS
Sunrise
Noon
Sunset
Midnight 
Plants have internal methods of keeping time—influenced by the external environment—that enable them to initiate various actions at the appropriate time.

24. 
When it comes to producing flowers—an energetically expensive task—a plant’s life can depend on choosing the right moment.

25. PHOTOPERIODISM LONG-DAY PLANTS
All flowering plants fall into one of three categories when it comes to regulating their flower production.
Amount of daylight J F M A M J J A S O N D
Month
LONG-DAY PLANTS
Flower production is triggered by shorter periods of darkness (generally in spring).

26. PHOTOPERIODISM SHORT-DAY PLANTS
All flowering plants fall into one of three categories when it comes to regulating their flower production.
Amount of daylight J F M A M J J A S O N D
Month
SHORT-DAY PLANTS
Flower production is triggered by longer periods of darkness (generally in late summer or fall).

27. PHOTOPERIODISM
All flowering plants fall into one of three categories when it comes to regulating their flower production.
Amount of daylight J F M A M J J A S O N D
Month
DAY-NEUTRAL PLANTS
Flower production is triggered by a sufficient state of maturity and not by periods of darkness. 
Some plants are triggered to produce flowers when the length of the nights is long (and the amount of daylight is relatively small). Others are triggered when nights are shorter, and the daylight lasts longer.

29. MECHANICAL DEFENSES
IN PLANTS
THORNS, SPINES, AND HAIRS
Structures such as sharp spines or fine hairs can significantly reduce herbivory.

30. MECHANICAL DEFENSES
IN PLANTS
WAXES AND SAPS
Leaf secretions such as slippery waxy compounds or sticky saps significantly reduce herbivory.

31. MECHANICAL DEFENSES
IN PLANTS
DEFENSIVE MOVEMENTS
Rapid movements, such as flattening leaves in response to touch, can decrease available surface area and significantly reduce herbivory.

32. Monarch butterfly caterpillars feed on milkweed and are able to tolerate the cyanide-containing molecules produced by the plant that are toxic to most other insects. The caterpillars are then able to store the toxic chemical and, in turn, become poisonous to the animals that try to eat them.

33. 
The chemical compounds produced by plants to reduce herbivory can also have medicinal effects in humans.

34. Auxins produced in the plant move away from the light source to the shaded side of a stem, stimulating a greater rate of growth than on the side with less auxin. The uneven growth causes the plant to bend toward the light.

36. SUCCULENT LEAVES AND STEMS
METHODS OF SURVIVING
DRY HABITATS
SUCCULENT LEAVES AND STEMS
Cacti and other succulent plants have thick, fleshy, water-storing tissue within their leaves and stems that helps minimize water loss due to evaporation.

37. METHODS OF SURVIVING
DRY HABITATS
DEEP TAPROOTS
Plants such as mesquite send down unusually hardy and deep taproots that can utilize water far beneath the surface.

38. METHODS OF SURVIVING
DRY HABITATS
LONG-DORMANT SEEDS
Many plants have seeds that can remain dormant for long periods of time, then quickly germinate and grow in response to brief periods of moisture.

39. Mangroves are able to transport much of the salt absorbed through their roots and excrete it through their leaves. The salt sits on the leaf surfaces until it dries and blows away.

40. 
Plants living in cold and windy habitats tend to grow close to the ground and have smaller-than- average leaves and shallow root systems.

42. PLANT HORMONES HORMONE FUNCTION LOCATION Gibberellins Auxins Ethylene
Abscisic acid
Cytokinins