1. Plant Responses and Adaptations
In what may be its last moments, an ant peers down into a pitcher plant's specialized leaf
The leaf is lined with slippery hairs and is filled with digestive enzymes that will extract nutrients from any unsuspecting prey

2. Plant Responses and Adaptations

3. Hormones and Plant Growth
Unlike most animals, plants do not have a rigidly set organization to their bodies
Cows have four legs, ants have six, and spiders have eight; but tomato plants do not have a predetermined number of leaves or branches
However, plants show distinct patterns of growth
As a result, you can easily tell the difference between a tomato plant and a corn plant, between an oak tree and a pine tree

4. Patterns of Plant Growth
Although plant growth is not determined precisely, it still follows general patterns that differ among species
What controls these patterns of development?
Biologists have discovered that plant cells send signals to one another that indicate when to divide and when not to divide, and when to develop into a new kind of cell

5. Patterns of Plant Growth
There is another difference between growth in plants and animals
Once most animals reach adulthood, they stop growing
In contrast, even plants that are thousands of years old continue to grow new needles, add new wood, and produce cones or new flowers, almost as if parts of their bodies remained “forever young”
As you have learned, the secrets of plant growth are found in meristems, regions of tissue that can produce cells that later develop into specialized tissues
Meristems are found at places where plants grow rapidly—the tips of growing stems and roots, and along the outer edges of woody tissues that produce new growth every year

6. Patterns of Plant Growth
If meristems are the source of plant growth, how is that growth controlled and regulated?
Plants grow in response to environmental factors such as light, moisture, temperature, and gravity
But how do roots “know” to grow down, and how do stems “know” to grow up toward light?
How do the tissues of a plant determine the right time of year to produce flowers?
How do plants ensure that their growth is evenly balanced—that the trunk of a tree grows large enough to support the weight of its leaves and branches?
The answers to these questions involve the actions of chemicals that direct, control, and regulate plant growth

7. Plant Hormones
In plants, the division, growth, maturation, and development of cells are controlled by a group of chemicals called hormones
A hormone is a substance that is produced in one part of an organism and affects another part of the same individual
Plant hormones are chemical substances that control a plant's patterns of growth and development, and the plant's responses to environmental conditions

8. PLANT HORMONES
Chemicals that control the internal factors of plant growth
Organic compounds that are effective in small concentrations
Synthesized in one part of the plant and transported to target tissue elsewhere in the plant triggering a physiological response
Many hormones work together

9. Plant Hormones
The general mechanism of hormone action in plants is shown in the diagram
As you can see, the hormone moves through the plant from the place where it is produced to the place where it triggers its response
The portion of an organism affected by a particular hormone is known as its target cell or target tissue
To respond to a hormone, the target cell must contain a hormone receptor—usually a protein—to which the hormone binds
If the appropriate receptor is present, the hormone can exert an influence on the target cell by changing its metabolism, affecting its growth rate, or activating the transcription of certain genes
Cells that do not contain receptors are generally unaffected by hormones

10. Hormone Action in Plants
Plant hormones are chemical substances that control patterns of development as well as plant responses to the environment
Hormones are produced in apical meristems, in young leaves, in roots, and in growing flowers and fruits
From their place of origin, hormones move to other parts of the plant, where target cells respond in a way that is specific to the hormone

11. Hormone Action in Plants

12. Hormone Action in Plants
Different kinds of cells may have different receptors for the same hormone
As a result, a single hormone may affect two different tissues in different ways
For example:
a particular hormone may stimulate growth in stem tissues but inhibit growth in root tissues

13. Auxins
The experiment that led to the discovery of the first plant hormone was carried out by Charles Darwin
In 1880, Darwin and his son Francis published a book called The Power of Movement in Plants
In this book, they described an experiment in which oat seedlings demonstrated a response known as phototropism
Phototropism is the tendency of a plant to grow toward a source of light

14. Auxins
The activity Effect of Light on a Growing Plant shows an experiment similar to the one carried out by the Darwins
Notice that the tip of one of the oat seedlings was covered with an opaque cap
This plant did not bend toward the light, even though the rest of the plant was uncovered
However, if an opaque shield was placed a few centimeters below the tip, the plant would bend toward the light as if the shield were not there
Clearly, something was taking place at the tip of the seedling

15. AUXINS Hormones that regulate the growth of plant cells
Stimulates/inhibits cell elongation depending on concentration
Tropisms:
Phototropism: response to light
Geotropism (gavitropism): response to gravity
Thigmotropism: response to touch
Synthetic:
Weed killers: 2,4-D
Fruit harvest: naphthaleneacetic acid (NAA)
Harvest fruit at sametime
Stimulates root development

16. Auxins and Phototropism
The Darwins suspected that the tip of each seedling produced substances that regulated cell growth
Forty years later, these substances were identified and named auxins
Auxins are produced in the apical meristem and are transported downward into the rest of the plant
They stimulate cell elongation
When light hits one side of the stem, a higher concentration of auxins develops in the shaded part of the stem
This change in concentration stimulates cells on the dark side to elongate
As a result, the stem bends away from the shaded side and toward the light
Recent experiments have shown that auxins migrate toward the shaded side of the stem, possibly due to changes in membrane permeability in response to light

17. Auxins and Gravitropism
Auxins are also responsible for gravitropism, which is the response of a plant to the force of gravity
By mechanisms that are still not understood, auxins build up on the lower sides of roots and stems
In stems, auxins stimulate cell elongation, helping turn the trunk upright, as shown in photo
In roots, however, the effects of auxins are exactly the opposite
There, auxins inhibit cell growth and elongation, causing the roots to grow downward

20. Gravitropism in a Stem 
Auxins are responsible for the plant response called gravitropism
Auxins caused the tip of this tree stem to grow upright

21. Gravitropism in a Stem 

23. Gravitropism in a Stem 
Auxins are also involved in the way roots grow around objects in the soil
If a growing root is forced sideways by an obstacle such as a rock, auxins accumulate on the lower side of the root
Once again, high concentrations of auxins inhibit the elongation of root cells
The uninhibited cells on the top elongate more than the auxin-inhibited cells on the bottom of the root
As a result, the root grows downward

24. Auxins and Branching Auxins also regulate cell division in meristems
As a stem grows in length, it produces lateral buds
A lateral bud is a meristematic area on the side of a stem that gives rise to side branches
Most lateral buds do not start growing right away
The reason for this delay is that growth at the lateral buds is inhibited by auxins
Because auxins move out from the apical meristem, the closer a bud is to the stem's tip, the more it is inhibited
This phenomenon is called apical dominance

25. Apical Dominance 
Apical dominance, shown here, is controlled by the relative amounts of auxins and cytokinins
During normal growth (A), lateral buds are kept dormant because of the production of auxins in the apical meristem
If the apical meristem is removed (B), the concentration of auxins drops

26. Apical Dominance 

27. Apical Dominance 
Although not all gardeners have heard of auxins, most of them know how to overcome apical dominance
If you snip off the tip of a plant, the side branches begin to grow more quickly, resulting in a rounder, fuller plant
Why does this happen?
When the tip is removed, the apical meristem—the source of the growth-inhibiting auxins—goes with it
Without the influence of auxins, meristems in the side branches grow more rapidly, changing the overall shape of the plant

28. Auxinlike Weed Killers
Chemists have produced many compounds that mimic the effects of auxins
Because high concentrations of auxins inhibit growth, many of these compounds are used as herbicides, which are compounds that are toxic to plants
Herbicides include a chemical known as 2,4-D (2,4-dichlorophenoxyacetic acid), which is used to kill weeds
A mixture containing 2,4-D was used as Agent Orange, a chemical defoliant sprayed during the Vietnam War

29. CYTOKININS Promote cell division
Influence the development of root, stems, and differentiation of xylem and phloem

30. Cytokinins
Cytokinins are plant hormones that are produced in growing roots and in developing fruits and seeds
In plants, cytokinins stimulate cell division and the growth of lateral buds, and cause dormant seeds to sprout
Cytokinins also delay the aging of leaves and play important roles in the early stages of plant growth

31. Cytokinins
Cytokinins often produce effects opposite to those of auxins
For example, auxins stimulate cell elongation, whereas cytokinins inhibit elongation and cause cells to grow thicker
Auxins inhibit the growth of lateral buds, whereas cytokinins stimulate lateral bud growth
Recent experiments show that the rate of cell growth in most plants is determined by the ratio of the concentration of auxins to cytokinins
In growing plants, therefore, the relative concentrations of auxins, cytokinins, and other hormones determine how the plant grows

32. GIBBERELLINS Promote cell enlargement 65 different types
Increasing length between nodes in stem
Elongation of stem
Taller plant
65 different types
Stimulates seed germination
Promotes formation of seedless fruits

33. Gibberellins
For years, farmers in Japan knew of a disease that weakened rice plants by causing them to grow unusually tall
They called the disease the “foolish seedling” disease
In 1926, Japanese biologist Eiichi Kurosawa discovered that this extraordinary growth was caused by a fungus: Gibberella fujikuroi
His experiments showed that the fungus produced a growth-promoting substance that was named gibberellin

34. Gibberellins
Before long, other researchers had learned that plants themselves produce more than 60 similar compounds, all of which are now known as gibberellins
Gibberellins produce dramatic increases in size, particularly in stems and fruit
Gibberellins are also produced by seed tissue and are responsible for the rapid early growth of many plants

35. Ethylene
When natural gas was used in city street lamps in the nineteenth century, people noticed that trees along the street suffered leaf loss and stunted growth
This effect was eventually traced to ethylene, one of the minor components of natural gas

36. Ethylene
Today, scientists know that plants produce their own ethylene, and that it affects plants in a number of ways
In response to auxins, fruit tissues release small amounts of the hormone ethylene
Ethylene then stimulates fruits to ripen

37. Ethylene
Commercial producers of fruit sometimes use this hormone to control the ripening process
Many crops, including lemons and tomatoes, are picked before they ripen so that they can be handled without damage to the fruit
Just before they are delivered to market, the fruits are treated with synthetic ethylene to produce a ripe color quickly
This trick does not always produce a ripe flavor, which is one reason why naturally ripened fruits often taste much better

38. Plant Responses
Like all living things, plants respond to changes in their environments
Some biologists call these responses “plant behavior,” which is a useful way of thinking about them
Plants generally do not respond as quickly as animals do, but that does not make their responses any less effective
Some plant responses are so fast that even animals cannot keep up with them!

39. Tropisms
Plants change their patterns and directions of growth in response to a multitude of cues
The responses of plants to external stimuli are called tropisms, from a Greek word that means “turning”
Plant tropisms include gravitropism, phototropism, and thigmotropism
Each of these responses demonstrates the ability of plants to respond effectively to external stimuli, such as gravity, light, and touch

40. Gravitropism and Phototropism
You have already read about gravitropism, the response of a plant to gravity, and phototropism, the response of a plant to light
Both of these responses are controlled by the hormone auxin
Gravitropism causes the shoot of a germinating seed to grow out of the soil—against the force of gravity
It also causes the roots of a plant to grow with the force of gravity and into the soil

41. Gravitropism and Phototropism
Phototropism causes a plant to grow toward a light source
This response can be so quick that young seedlings reorient themselves in a matter of hours

42. Thigmotropism in a Grapevine
Plant tropisms include gravitropism, phototropism, and thigmotropism
One effect of thigmotropism—growth in response to touch—is that plants curl and twist around objects, as shown by the stems of this grapevine

43. Thigmotropism in a Grapevine

44. Rapid Responses Some plant responses do not involve growth
In fact, they are so rapid that it would be a mistake to call them tropisms
If you touch a leaf of Mimosa pudica, appropriately called the “sensitive plant,” within only two or three seconds, its two leaflets fold together completely
The secret to this movement is changes in osmotic pressure
Recall that osmotic pressure is caused by the diffusion of water into cells
The leaves are held apart due to osmotic pressure where the two leaflets join
When the leaf is touched, cells near the center of the leaflet pump out ions and lose water due to osmosis
Pressure from cells on the underside of the leaf, which do not lose water, force the leaflets together

45. Rapid Responses
The carnivorous Venus' flytrap also demonstrates rapid responses
When a fly triggers sensory cells on the inside of the flytrap's leaf, electrical signals are sent from cell to cell
A combination of changes in osmotic pressure and cell wall expansion causes the leaf to snap shut, trapping the insect inside

46. Photoperiodism To every thing there is a season
Nowhere is this more evident than in the regular cycles of plant growth
Year after year, some plants flower in the spring, others in summer, and still others in the fall
Plants such as chrysanthemums and poinsettias flower when days are short and are therefore called short-day plants
Plants such as spinach and irises flower when days are long and are therefore known as long-day plants

47. Photoperiodism
How do all these plants manage to time their flowering so precisely?
In the early 1920s, scientists discovered that tobacco plants flower according to the number of hours of light and darkness they receive
Additional research showed that many other plants also respond to periods of light and darkness, a response called photoperiodism
This type of response is summarized in the figure
Photoperiodism in plants is responsible for the timing of seasonal activities such as flowering and growth.

48. PHOTOPERIODISM Plant response to changes in day length
Long-day plants: flower when exposed to longer days (Spring/Summer)
Short-day plants: flower when exposed to shorter days (Fall)
Day neutral plants: flowering not affected by length of day (tomato, dandelion)

49. Effect of Photoperiod on Flowering
Photoperiodism controls the timing of flowering and seasonal growth
The response of flowering, shown here, is controlled by the amount of darkness plants receive
Short-day plants, such as chrysanthemums, flower only when exposed to an extended period of darkness every night—and thus a short period of light during the day
Long-day plants, such as irises, flower when exposed to a short period of darkness or to a long period of darkness interrupted by a brief period of light

50. Effect of Photoperiod on Flowering

51. Effect of Photoperiod on Flowering
It was later discovered that a plant pigment called phytochrome is responsible for photoperiodism
Phytochrome absorbs red light and activates a number of signaling pathways within plant cells
By mechanisms that are still not understood completely, plants respond to regular changes in these pathways
These changes determine the patterns of a variety of plant responses

52. Winter Dormancy
Phytochrome also regulates the changes in activity that prepare many plants for dormancy as winter approaches
Dormancy is the period during which an organism's growth and activity decrease or stop

53. Winter Dormancy
The changes that prepare a plant for dormancy are important adaptations that protect plants over the cold winter months
As cold weather approaches, deciduous plants turn off photosynthetic pathways, transport materials from leaves to roots, and seal leaves off from the rest of the plant
In early autumn, the shorter days and lower temperatures gradually reduce the efficiency of photosynthesis
With these changing conditions, the plant gains very little by keeping its leaves alive
In fact, the thin, delicate leaves produced by most flowering plants would have little chance of surviving a tough winter, and their continued presence would be costly in terms of water loss

54. Leaf Abscission 
In temperate regions, most flowering plants lose their leaves during the colder months
During the warm growing season, auxins are produced in leaves
At summer's end, the phytochrome in leaves absorbs less light as days shorten and nights become longer
Auxin production drops, but the production of ethylene increases
The change in the relative amounts of these two hormones starts a series of events that gradually shut down the leaf

55. Leaf Abscission 
The chemical pathways for chlorophyll synthesis stop first
When light destroys the remaining green pigment, other pigments that have been present all along—including yellow and orange carotenoids—become visible for the first time
Production of new plant pigments—the reddish anthocyanins—begins in the autumn
The brilliant colors of autumn leaves are a direct result of these processes

56. Leaf Abscission 
Behind the scenes, enzymes extract nutrients from the broken-down chlorophyll
These nutrients are then transported to other parts of the plant, where they are stored until spring
Every available carbohydrate is transported out of the leaf, and much of the leaf's water is extracted
Finally, an abscission layer of cells at the petiole seals the leaf off from the plant's vascular system
The location of the abscission layer is shown in the diagram
Before long, the leaf falls to the ground, a sign that the tree is fully prepared for winter

57. Leaf Abscission  
Deciduous plants undergo changes in preparation for winter dormancy
Photosynthetic pathways in leaves shut down
An abscission layer of cells forms at the petiole to seal the leaf off from the rest of the plant
Eventually, the leaf falls off.

58. Leaf Abscission  

59. Overwintering of Meristems
Hormones also produce important changes in apical meristems
Instead of continuing to produce leaves, meristems produce thick, waxy scales that form a protective layer around new leaf buds
Enclosed in its coat of scales, a terminal bud can survive the coldest winter days
At the onset of winter, xylem and phloem tissues pump themselves full of ions and organic compounds
These molecules act like antifreeze in a car, preventing the tree's sap from freezing, thus making it possible to survive the bitter cold

60.  Plant Adaptations
Flowering plants grow in a variety of biomes—in deserts, savannas, and tundras—to name a few
They also grow in various aquatic ecosystems, such as ponds and streams
Angiosperms can survive in many different locations
How is this possible?
Through natural selection they have evolved tolerances and structural and physiological adaptations to meet the conditions of each biome
In this section, we explore how plants have become adapted to various environments through evolutionary change

61. Aquatic Plants
Aquatic plants are able to tolerate mud that is saturated with water and nearly devoid of oxygen
To take in sufficient oxygen, many aquatic plants have tissues with large air-filled spaces through which oxygen can diffuse
In waterlilies there are large open spaces in the long petioles that reach from the leaves down to the roots at the bottom
Oxygen diffuses from these open spaces into the roots

62. Waterlilies  
Aquatic plants have air-filled spaces in their tissues that allow for the uptake and diffusion of oxygen
These waterlilies transport oxygen from the air to their roots through large spaces in their petioles

63. Waterlilies  

64. Aquatic Plants Many other plants show similar adaptations
Several species of mangrove trees grow in shallow water along tropical seacoasts
Mangroves tolerate this environment by means of specialized air roots with air spaces in them, just like waterlily stems
These spaces conduct air down to the buried roots, allowing the root tissues to respire normally
Stately bald cypress trees thrive in freshwater swamps in the southern United States
These trees grow structures called knees, which protrude above the water
The knees bring oxygen-rich air down to the roots

65. Aquatic Plants
The reproductive adaptations of aquatic plants include seeds that float in water and delay germination for long periods
Many aquatic plants grow quickly after germination, extending the growing shoot above the water's surface

66. Salt-Tolerant Plants
When plant roots take in dissolved minerals, a difference in the concentration of water molecules is created between the root cells and the surrounding soil
This concentration difference causes water to enter the root cells by osmosis
For plants that grow in salt water, such as mangroves, this means taking in much more salt than the plant can use
The roots of salt-tolerant plants are adapted to salt concentrations that would quickly destroy the root hairs on most plants
The leaves of these plants have specialized cells that pump salt out of the plant tissues and onto the leaf surfaces, where it is washed off by rain

67. Desert Plants
Plants that live in the desert biome are called xerophytes
Xerophytes must tolerate a variety of extreme conditions, including strong winds, daytime heat, sandy soil, and infrequent rain
Rainwater sinks rapidly through desert soils instead of staying near the surface
The hot, dry air quickly removes moisture from any wet surface, making life difficult for plants
Plant adaptations to a desert climate include extensive roots, reduced leaves, and thick stems that can store water

68. Desert Plants
One familiar group of desert plants is the cactus (family Cactaceae)
Cactuses have root systems that either spread out for long distances just beneath the soil surface or that reach deep down into the soil
In addition, the roots have many hairs that quickly absorb water after a rainstorm, before the water sinks too deeply into the soil

69. Cactuses  
Desert plants have evolved different adaptations to survive desert conditions
For example, the shallow root systems of cactuses allow them to pick up surface water
The deep taproots of the mesquite tree and the sagebrush collect underground water
Spines, which are found on many desert plants, are actually reduced leaves that carry out little or no photosynthesis and, as a result, lose little water
Most of a plant's photosynthesis is carried out in its fleshy stem

70. Cactuses 

71. Cactuses 
To reduce water loss due to transpiration, cactus leaves have been reduced to thin, sharp spines
Cactuses also have thick green stems that carry out photosynthesis and are adapted to store water
The stems of cactuses swell during rainy periods and shrivel during dry spells, when the plants are forced to use up their water reserves

72. Desert Plants
Seeds of many desert plants can remain dormant for years, germinating only when sufficient moisture guarantees them a chance for survival
Other desert plants have bulbs, tubers, or other specialized stems that can remain dormant for years
When rain does come, the plants mature, flower, and set seed in a matter of weeks or even days, before the water disappears

73. Nutritional Specialists
Some plants grow in environments that have low concentrations of nutrients in the soil
Plants that have specialized features for obtaining nutrients include carnivorous plants and parasites

74. Carnivorous Plants 
Some plants live in bogs, wet and acidic environments where there is very little or no nitrogen present
Because conditions are too wet and too acidic, bacteria that cause decay cannot survive
Without these bacteria, neither plant nor animal material is broken down into the nutrients plants can use

75. Carnivorous Plants 
A number of plants that live in these habitats obtain nutrients using specialized leaves that trap and digest insects
Pitcher plants drown their prey in pitcher-shaped leaves that hold rainwater and digestive enzymes
Sundews trap insects on leaf hairs tipped with sticky secretions
The best known of the carnivorous plants is the Venus' flytrap
This plant has leaf blades that are hinged at the middle
If an insect touches the trigger hairs on the leaf, the leaf folds up suddenly, trapping the animal inside
Over a period of several days, the leaf secretes enzymes that digest the insect and release nitrogen for the plant to use

76. Venus' Flytrap  
Plants that have specialized features for obtaining nutrients include carnivorous plants and parasites
Carnivorous plants, such as the Venus' flytrap, digest insects—and occasionally frogs—as a source of nutrients
Parasites grow into the tissues of their host plant and extract water and nutrients, causing harm to the host

77. Venus' Flytrap  

78. Parasites
Some plants extract water and nutrients directly from a host plant
Like all parasites, these plants harm their host organisms and sometimes even pose a serious threat to other species
The dodder plant Cuscuta is a parasitic plant that has no chlorophyll and thus does not produce its own food
The plant grows directly into the vascular tissue of its host
There, it extracts nutrients and water
Mistletoe grows as a parasite on many plants, including conifers in the western United States

79. Epiphytes
Epiphytes are plants that are not rooted in soil but instead grow directly on the bodies of other plants
Most epiphytes are found in the tropical rain forest biome, but they grow in other moist biomes as well
Epiphytes are not parasite
They gather their own moisture, generally from rainfall, and produce their own food
One of the most common epiphytes is Spanish moss
This plant is actually not a moss at all but a member of the bromeliad family
Over half the species of orchids are epiphytes

80. Chemical Defenses
Seed plants and insects have had such a long relationship that each has had plenty of time to adapt to the other
The beginnings of the relationship are obvious—plants represent an important source of food for insects
Plants, therefore, fall prey to a host of plant-eating insects
Because plants cannot run away, you might think that they are defenseless against insects that are armed with biting and sucking structures
But plants have their own defenses

81. Chemical Defenses
Many plants defend themselves against insect attack by manufacturing compounds that have powerful effects on animals
Some of these chemicals are poisons that can be lethal when eaten
Other chemicals act as insect hormones, disrupting normal growth and development and preventing insects from reproducing
These chemicals include those used in aspirin, codeine, and scores of other drugs that humans use as medicines

82. Chemical Defenses
As you may know, nicotine is a chemical that is found in tobacco plants
When a person smokes tobacco in the form of cigarettes, the nicotine in the tobacco affects the human nervous system
Biologists hypothesize that nicotine is a natural insecticide that disrupts the nervous system of many insects, protecting tobacco plants from potential predators