Biology 2201 Ch 11 –Plants and Homeostasis11.1 – THE VASCULAR PLANT BODY (PP. 410-416)

Vascular Plant Systems

• Vascular plants have two organ systems: • above-ground shoot system

• underground root system

• shoot system - the stems and leaves of a plant; the stems provide structural support and, in some cases, perform photosynthesis; the leaves are specialized for photosynthesis

• root system - the roots of a plant; anchors the plant and absorbs the water and mineral nutrients the plant needs

Plant Tissues

• Plants have four main types of tissues:

• Meristematic tissue

• Dermal tissue

• Ground tissue

• Vascular tissue

Meristematic tissue

• meristematic tissue - undifferentiated embryonic plant

tissue from which all other plant tissues develop

• Throughout their lives, plants can continue to produce

new cells by mitosis in their meristematic tissues, which all

vascular plants have. These embryonic tissues make up

meristems—areas of rapidly dividing cells. As these cells

mature, they can develop into different types of

specialized plant cells.

Dermal tissue

• dermal tissue - the outer layers of cells that form a protective covering for the plant; includes epidermis and periderm

• epidermis - the dermal tissue that makes up a plant’s outer covering

• Older woody plants also form dermal tissue called periderm, which is produced as part of secondary growth. It replaces the epidermis to form cork in woody stems and roots.

Dermal tissue cont…

• guard cell - a specialized epidermal cell; functioning in pairs, they regulate the opening of stomata

• stoma - a small opening, usually in the leaf, that allows gas exchange to occur

• Guard cells control the size of the stomata. When stomata are open, gas exchange can occur. During the day, carbon dioxide diffuses in through the stomata and oxygen diffuses out. Water in the form of gaseous water vapour also diffuses out of the plant and into the atmosphere through stomata.

• root hair - the fine, hair-like structures that cover the surface of the root of a plant; they increase the surface area available for gas exchange and the absorption of water and nutrients

• Some epidermal cells have specialized structures or extensions on their surface that have important functions.

• Ex. trichomes

• tiny growths on the surface of the epidermis.

• They are often found on stems and leaves, and can make the plant appear fuzzy or woolly.

• Trichomes keep leaf surfaces cool and reduce evaporation.

• Some trichomes may secrete sticky or toxic substances that repel herbivores.

• Some trichomes, such as those on passion vines, are deadly and actually puncture the skin of herbivores that attempt to walk across them

Ground Tissue

• ground tissue - a plant tissue that has multiple functions

and that makes up most of the inside of a plant

• Ground tissue forms most of the plant’s internal and

external material. Ground tissues have a wide range of

functions, including photosynthesis, storage, and support.

In some stems, roots, and seeds, the cells of ground tissue

store starch and oils. Ground tissue also provides support

for the plant when it grows between other types of tissue.

Vascular Tissue

• Vascular tissue is an internal system of tubes that run lengthwise

throughout the stem of a plant, connecting the roots and the

leaves. The function of vascular tissue is to transport water and

dissolved substances throughout the plant.

• There are two types of vascular tissue: xylem and phloem.

• xylem - vascular tissue that transports water and minerals from the

roots to the leaves

• phloem - vascular tissue that transports organic nutrients, often from

the leaves to the roots, but also from roots and mature leaves to new

leaves

Xylem

• Xylem is the water-conducting tissue of plants. In gymnosperms, xylem consists of cells called tracheids. In angiosperms, xylem consists of two types of cells: tracheids and vessel elements.

• Tracheids and vessel elements begin as living cells growing end to end in an immature stem. When tracheids and vessel elements mature, their living contents die, leaving the non-living cell walls in place. Fluids are passed from one tracheid or vessel element to the next through pores known as pits.

Xylem diagram

Phloem

• Phloem is the food-conducting tissue in vascular plants.

• Two types of phloem cells: • sieve tube elements

• Companion cells.

• Both of these are alive at maturity, unlike the cells making up xylem.

• Sieve tube elements have no nuclei. Sieve tubes have plates at both ends that are perforated with holes, making them resemble a sieve. Each sieve tube element has an associated companion cell that does have a nucleus.

• Companion cells carry out life functions to maintain both types of cells.

Phloem diagram

Biology 2201 Ch 11 –Plants and Homeostasis11.2 – PLANT ORGANS AND THEIR FUNCTIONS (PP. 417-422)

Plant Organs

• In general, plants have three organs

• Roots

• Stems

• Leaves

Roots

• Functions of roots

• Roots take in water and dissolved minerals that are then

transported to where they are needed in the rest of the plant.

• Roots anchor the plant in soil or to some other plant or object,

supporting the plant against forces such as wind and water.

• Roots anchor the plant in soil or to some other plant or object,

supporting the plant against forces such as wind and water.

Root Structure

• Root cap - a protective covering on the tip of the root

• Outer root• cortex - a layer of cells between the epidermis and the vascular tissues of the

root

• The cortex is composed of ground tissue made of cells that transport and store water, minerals, and food in the plant. All materials that enter the root, including water and minerals, must pass through the cortex to get to the vascular tissue, which will then transport them to the rest of the plant.

• endodermis - a layer of cells between the cortex and the vascular tissue of the root

• The endodermis is surrounded by a waterproof band called a Casparianstrip. The Casparian strip creates a barrier that forces water and dissolved minerals to cross the plasma membrane and pass through the cytoplasm of endodermal cells rather than moving around the cells.

Root Structure cont…

• Inner Root Vascular Tissue

• Inside the endodermis are the vascular tissues, xylem and

phloem. They make up the centre of the root. The arrangement

of xylem and phloem is different in monocot and dicot flowering

plants.

• In monocots, xylem cells typically form a ring around a central core of

cells. The core of cells is called pith. Phloem cells surround the xylem cells in

this ring.

• In dicots, the xylem cells often form an X or star shape, and the phloem cells are between the arms of the star.

Monocot vs Dicot Vascular Tissue

Root Systems

• There are two types of root systems • Fibrous

• Taproot

• fibrous root - a root system made up of many small branching roots• roots are all about the same size and grow from a central point.

• Fibrous roots usually do not grow as deeply into soil as taproots do.

• Grass roots, for example, may grow only a few centimetres into soil.

• taproot - a root system made up of a thick root with few smaller lateral branching roots

Stems

• herbaceous – soft, flexible stems. Plants that are annuals (only live one growing season) are herbaceous

• Woody – hard, rigid stems. Many perennials (plants that live for more than one season) have woody stems

• The main function of a stem is to provide support for the plant’s leaves and flowers, a plant’s reproductive structures.

• As woody plants grow taller, the diameter of the stem also increases. New vascular tissue is produced each year, resulting in a pattern of annual growth rings that is visible in the xylem tissue. The age of a tree can be estimated by counting the annual growth rings in the xylem at the base of its trunk

• There are many adaptations of stems that help plants to survive. In some plants, stems are used to store excess food. In others, stems help plants withstand drought, cold, or heat.

Leaves

• Function - to convert the light energy from the Sun into the chemical energy of food through photosynthesis

• Structure• The external structure of a typical leaf has a flat portion called the blade, which has

a relatively large surface area. In some species, such as grasses, the blade attaches directly to the stem. In other species, the blade attaches to the stem via a structure called a petiole.

• The epidermal cells of leaves secrete a waxy substance that forms the cuticle. This protective layer helps reduce water loss from leaves by reducing evaporation.

• Cuticle - a waxy layer on the epidermis that is secreted by epidermal cells

• Vascular tissue runs through the petiole, connecting the leaf’s vascular tissue—the veins—to the stem’s vascular tissue. The veins conduct water and dissolved minerals into the leaf and they conduct dissolved carbohydrates, made during photosynthesis, from the leaf to the rest of the plant.

Leaves cont…

• Mesophyll - the layer between the upper and lower epidermis of a leaf that contains numerous chloroplasts

• The cuticle and the epidermis of a leaf are transparent, allowing light to penetrate to the next layer. The only cells of the epidermis that have chloroplasts are the guard cells.

• Immediately below the upper epidermis is a row of tightly packed cells that contain many chloroplasts. Their location below the epidermis and their shape give them maximum exposure to light. Most photosynthesis takes place in this layer of a leaf, called the palisade mesophyll.

• Below the palisade mesophyll layer is the spongy mesophyll. In this layer, the cells are irregularly shaped and loosely packed. The open air spaces between the cells in the spongy mesophyll allow oxygen, carbo dioxide, and water vapour to move easily around the cells. Cells in the spongy mesophyll also contain chloroplasts, but there are fewer of these per cell than in the palisade mesophyll layer.

Biology 2201 Ch 11 –Plants and Homeostasis11.3 – TRANASPORT IN PLANTS (PP. 423-429)

Transport in the Xylem

• The root cells contain a higher concentration of dissolved nutrients than the surrounding soil, so water moves into the roots by osmosis. Water moves through the root cells or through the intercellular spaces within the root, and enters the xylem.

• The water is then transported in the xylem tissue up through the root into the stem. Within the stem, water moves by diffusion into other tissues of the plant. Minerals usually move across cell membranes through active transport.

• transpiration - the process in which water evaporates from the inside of a leaf to the outside through the stomata

• Long-distance transport in the xylem is accomplished by two processes: root pressure and transpirational pull. Both positive pressure (pushing) and negative pressure (pulling) contribute to water moving up through the xylem. Root pressure is responsible for the positive pressure, while transpirational pull is responsible for the negative pressure.

Root Pressure

• root pressure - the mechanism by which positive pressure in the roots moves water upward in a plant

• Root pressure is a mechanism that pushes water and minerals upward in a plant. Water entering the roots creates a positive pressure that tends to push water upward. Minerals are moved from the soil into the xylem against their concentration gradient by active transport. The high concentration of dissolved materials in the xylem causes more water to move in by osmosis. This increases the positive pressure that pushes the water column up. This process is aided by the adhesion (sticking) of water molecules to the cell walls of the xylem tissue.

Transpirational Pull

• cohesion-tension model - a model of water transport that explains how water is moved from the roots to the leaves of a plant

• Most of the water that reaches the leaves is lost through transpiration. The loss of this water creates the pull that moves water up the plant. The transpiration of water from the leaves of a plant creates a negative pressure that acts to pull water up to replace the lost water.

• Three main factors that play a role in cohesion-tension model:• Transpiration

• Cohesion

• Adhesion

Cohesion-Tension Model

• Transpiration: Transpiration, the evaporation of water molecules from the shoot system, is the major force responsible for the movement of water and dissolved minerals upward in a plant stem. Dry air, heat, and wind cause transpiration, which results in negative pressure as water molecules continuously leave the plant through the stomata. This negative pressure exerts tension on the water confined in the xylem’s conducting tubes all the way down to the roots.

• Cohesion: The columns of water in the xylem have a property called cohesion. The force of attraction between the water molecules in each narrow xylem tube provides a force that keeps the water column unbroken while it is being pulled up under tension

• Adhesion: Adhesion causes the water molecules to adhere, or stick, to the xylem walls. Along with cohesion, adhesion keeps the water column from breaking as it is pulled upward.

Nutrient Transport in Phloem

• translocation - the transport of sucrose and other organic molecules through the phloem of a plant

• Translocation moves sugars from where they are made in the plant to where they are needed for growth, metabolism, and storage.

• Translocation in phloem moves sucrose from a source to a sink. The source of the sucrose is any region of the plant where the sugars are entering into sieve tubes. The sink is any region in the plant where sugars are used or stored.

• While water and minerals are mostly pulled up by transpiration, plant nutrients are mostly pushed through phloem.

• pressure-flow model - a model that explains how organic molecules move from source to sink through phloem in a flowering plant

Pressure Flow in Phloem

• When nutrients are pumped into or removed from the phloem, the change in concentration causes a movement of water in that same direction. As a result, internal pressure builds up at the source end of the sieve tube and pushes the sucrose-rich solution toward any sink, where the sucrose is then removed.

• The direction of flow is always from source to sink, and the sinks change depending on the plant’s stage of growth and development. • For example, in older plants, newly forming leaves are a sink, and the sucrose travels

from photosynthesizing leaves to growing leaves. When fruit is forming, sucrose moves there by translocation, and growth in other areas of the plant slows down.

• Because the flow moves from source to sink, sometimes flow can happen in both directions at once. Different sieve tubes can be conducting phloem sap in opposite directions, from different sources to different sinks.

Biology 2201 Ch 11 –Plants and Homeostasis11.4 – FACTORS THAT AFFECT PLANT GROWTH (PP. 430-438)

Plant Hormones

• hormone - a chemical compound produced in one part of the

plant that controls growth activity in another part of that plant

• See Table 11.2 p. 430

• Five main plant hormones:

• Auxins

• Cytokinins

• Gibberellins

• Ethylene

• Abscisic acid (ABA)

Auxins

• stimulate cell division and elongation in stems and roots

• regulate cell expansion in plant responses to light and gravity

• apical dominance - a condition of a plant stem in which

growth is mainly upward, with little growth laterally from side

branches

• Many popular weed killers contain a synthetic auxin , called

2,4-D. The synthetic compound works b accelerating growth so

much that a plant quickly uses up its food reserves and

effectively starves to death.

Cytokinins

• Cytokinins promote cell division by stimulating the

production of the proteins needed for mitosis and

cytokinesis.

• Cytokinins also delay the aging of leaves and fruit.

• The presence of other hormones, especially auxins,

influences the effects of cytokinins.

Gibberellins

• gibberellins are produced in the apical meristem (same as auxins)

• transported in the vascular tissue.

• They stimulate plant growth by changing the plant’s cell walls, stop dormancy in seeds, and can reverse genetic dwarfism in plants.

• They are used in commercial crops all over the world to increase fruit size and to increase cluster size in grapes. However, increasing yield can also create problems. Grapes, for example, require extra care because the vines do not naturally support such heavy fruit.

Ethylene

• Only known gaseous hormone

• Mainly affects the ripening of fruit

• Ethylene weakens the cell walls of unripe fruit and breaks down

complex carbohydrates (such as starches) into simple sugars

(such as sucrose and fructose).

• Because ripe fruits and vegetables are bruised easily during

shipping, growers often pick and ship unripe fruits and

vegetables. Once they reach their destinations, a treatment with

ethylene is used to speed up the ripening process.

Abscisic Acid (ABA)

• Synthesized in mature green leaves, fruits, and root caps

• generally inhibits growth.

• ABA inhibits the growth of buds in plant stems and blocks the

intake of carbon dioxide by controlling the opening and closing

of leaf stomata.

• Abscisic acid also blocks the action of growth-promoting

hormones.

Plants’ Responses to Environmental Stimuli

• tropism - a plant’s growth response to external stimulation coming from one direction in the environment• Examples: The leaves of house plants grow toward light-bringing

windows, vines climb trellises, and roots grow downward and around obstructions.

• nastic response - a plant’s movement in response to a stimulus that is not associated with the direction of the stimulus• Example: the opening of flower petals during the day and the

closing of petals at night to conserve heat.

Types of Tropic Responses

• phototropism - a plant’s growth response to light

• caused by an unequal distribution of auxin. There is less auxin on the

side of the plant toward the light source than there is on the side away

from the light source. Because auxin can cause cell elongation, the

cells on the side away from the light elongate, making that side of the

stem longer. As a result, the stem curves in the direction of the light.

• gravitropism a plant’s growth response to gravity

• Roots grow down with gravity, stems grow in reverse against gravity

• Thigmotropism - a plant’s growth response to touch or contact

• Example: Vines that twist around fences, trees

Other Factors that Affect Plant Growth

• Nutrients (see table 11.3, p. 434)• Macronutrients – needed in large amounts

• Ex. Nitrogen, potassium, phosphorous, etc.

• Micronutrients – needed in small amounts• Ex. Chlorine, iron, copper, etc.

• Soil pH• Most plants grow well within only a very narrow range of pH values.

• Most plants thrive in slightly acidic soils ranging from pH 6 to 7. Common examples include pine, spruce, dogwood, blueberry, hydrangea, magnolia, holly, potatoes, peanuts, and cranberries.

• Fewer plant species tolerate basic soils of pH 7 to 8. Examples include geraniums, petunias, lawn grass, beans, beets, lettuce, pears, and plums.