Biology Honors: Chapter 23

Water Transport

Active transport and root pressure cause water to move from soil into plant roots, and the pressure created by water entering the tissues of a root can push water upward in a plant stem, but this pressure does not exert nearly enough force to lift water up into trees

Roots

Anchor plants in the ground, holding soil in place and preventing erosion; often work with soil bacteria and fungi in mutualistic relationships that help the roots absorb water and dissolved nutrients, and transport these materials to the rest of the plant, store food, and hold plants upright against forces such as wind and rain; goes with gravity, which is gravitoprism

Ground Tissue of Leaves

Area between leaf veins is filled with mesophyll

Sieve Tube Elements

Arranged from end to end, forming sieve tubes, the end walls of sieve tube elements have many small holes through which nutrients move from cell to cell in a watery stream; as they mature, they lose their nuclei and most other organelles, while the remaining organelles hug the inside of the cell wall and are kept alive by companion cells

Secondary Growth

As a plant grows larger, the older stems and roots have more mass to support and more fluid to move through their vascular tissues, so they must increase in thickness; very common among dicots and nonflowering seed plants, but is rare in monocots which limits their girth

Formation of Bark

As a tree expands in width, the phloem layer must grow as well, and this expansion may cause the oldest tissues to split and fragment as the expanding stem stretches them; the cork cambium surrounds the cortex and produces a thick, protective layer of waterproof cork that prevents the loss of water from the stem, and as the stem increases in size, outer layers of dead bark often crack and flake off the tree

Heartwood

As woody stems grow thicker, the older xylem near the center of the stem no longer conducts water and instead becomes what is known as heartwood; usually darkens with age because it accumulates colored deposits

Adhesion

Attraction between unlike molecules; water molecules can also form hydrogen bonds with other substances

Cohesion

Attraction of molecules of the same substance to each other; water cohesion is especially strong because of the tendency of water molecules to form hydrogen bonds with each other

Petiole

Blade is attached to the stem by a thin stalk called the petiole

Plant Growth

Carefully controlled and regulated; depending upon the species, follows general patterns that produce the characteristic size and shape of the adult plant

Companion Cells

Cells that surround sieve tube elements and keep their nuclei and other organelles through their lifetime; support the phloem cells and aid in the movement of substances in and out of the phloem

Pressure Flow Hypothesis

Changes in nutrient concentration drive the movement of fluid through phloem tissue in directions that meet the nutritional needs of the plant; gives plants enormous flexibility in responding to changing seasons, as during the growing season, sugars from the leaves are directed into ripening fruits or into roots for storage, as the growing season ends, the plant drops its fruits and stores nutrients in the roots, and as spring approaches, the chemical signals stimulate phloem cells in the roots to pump sugars back into phloem sap, then the pressure flow system raises these sugars into stems and leaves to support rapid growth

Vascular Bundles

Clusters of xylem and phloem tissue; in most dicots and gymnosperms, vascular bundles are arranged in a cylinder

Lignin

Complex molecule that resists water and gives wood much of its strength

Vascular Tissue of Leaves

Connected directly to the vascular tissues of stems, making them part of the plant's fluid transport system; xylem and phloem tissues are bundles in leaf veins that run from the stem throughout the leaf

Information from Tree Rings

Each ring has light wood at one edge and dark wood at the other, making a sharp boundary between rings, and usually a ring corresponds to a year of growth, so by counting the rings in a cross section of a tree, you can estimate its age; the size of the rings may even provide information about weather conditions, as thick rings indicate that weather conditions were favorable for tree growth whereas thin rings indicate less favorable conditions

Taproot System

Found mainly in dicots; in some plants, the primary root grows long and thick and gives rise to smaller branch roots, and the large primary root is called a taproot

Fibrous Root System

Found mainly in monocots; in other plants, the system begins with one primary root but is soon replaced by many equally sized branch roots that grow separately from the base of the system, and these fibrous roots branch to such a n extent that no single root grows larger than the rest; extensive fibrous root systems produced by many plants help prevent topsoil from being washed away by heavy rain

Nodes

Growing stems contain distinct nodes, where leaves are attached

Primary Growth

Growth of new cells produced by the apical meristems of roots and stems that adds length to the plant; takes place in all seed plants

Parenchyma

Have a thin cell wall and a large central vacuole surrounded by a thin layer of cytoplasm; in leaves, these cells contain many chloroplasts and are the site of most of a plant's photosynthesis

Sclerenchyma

Have extremely thick, rigid cell walls that make ground tissue tough and strong

Collenchyma

Have strong, flexible cell walls that help support plant organs

Dicot Stems

Have vascular bundles that are generally arranged in an organized, ringlike pattern; parenchyma cells inside the ring of vascular tissue are known as pith, while those outside form the cortex of the stem; those relatively simple tissue patterns become more complex as the plant grows larger and the stem increases in diameter

Meristems and Flower Development

Highly specialized cells found in cones and flowers are produced in meristems; flower or cone development begins when the pattern of gene expression changes in a stem's apical meristem, and these changes transform the apical meristem of a flowering plant into a floral meristem, which produce the tissues of flowers that include the plant's reproductive organs as well as the petals

Guard Cells

Highly specialized cells in the epidermis of each leaf that surround the stomata and control their opening and closing; regulate the movement of gases, especially water vapor and carbon dioxide, into and out of leaf tissues

Bark

In a mature stem, all of the tissues found outside the vascular cambium make up the bark, and these tissues include phloem, the cork cambium, and cork

Growth From the Vascular Cambium

In a young dicot stem, bundles of xylem and phloem are arranged in a ring, and once secondary growth begins, the vascular cambium appears as a thin, cylindrical layer of cells between clusters of vascular tissue; this new meristem forms between the xylem and phloem of each vascular bundle, and divisions in the vascular cambium give rise to new layers of xylem and phloem, so the stem becomes wider; each year, the cambium continues to produce new layers of vascular tissue, causing the stem to become thicker and thicker

Tree Rings

In most of the temperate zone, tree growth is seasonal, and when growth begins in the spring, the vascular cambium begins to grow rapidly, producing large, light colored xylem cells with thin cell walls, which results in a light colored layer of early wood, and as the growing season continues, the cells grow less and have thicker cell walls, forming a layer of darker late wood; this alternation of dark and light wood produces what we commonly call tree rings

Palisade Mesophyll

Layer of cells beneath the upper epidermis, containing closely packed cells that absorb light that enters the leaf

Endodermis

Layer of ground tissue; completely encloses the vascular cylinder and plays and essential role in the movement of water and minerals into the center of the root

Dermal Tissue of Leaves

Leaves are covered on their top and bottom surfaces by epidermis; leaf epidermis is made of a layer of tough, irregularly shaped cells with thick outer walls that resist tearing, and is also covered by a waxy covering; cuticle is a waterproof barrier that protects tissues and limits the loss of water through evaporation

Gas Exchange

Leaves take in carbon dioxide and give off oxygen during photosynthesis; when plant cells use the food they make, the cells respire, taking in oxygen and giving off carbon dioxide; plant leaves allow gas exchange between air spaces in teh spongy mesophyll and the exterior by opening their stomata

Tracheids

Long and narrow, with tough cell walls that help to support the plant; as they mature, tracheids die, leaving only their cell walls that contain lignin; openings in the walls connect neighboring cell sand allow water to flow from cell to cell, and pits allow water to diffuse from tracheids into surrounding ground tissue

Spongy Mesophyll

Loose tissue beneath the palisade mesophyll, which has many air spaces between its cells that connect with the exterior through stomata

Transpiration

Loss of water through leaves that helps to cool leaves on hot days, but may also threaten the leaf's survival if water is scarce; walls of mesophyll cells are kept moist so that gases can enter and leave the cells easily, and the trade off to this feature is that water evaporates from these surfaces and is lost to the atmosphere, but this lost water may be replaced by water drawn into the leaf through xylem vessels in the vascular tissue

Leaves

Main photosynthetic organs, as the broad, flat surfaces of many leaves increase the amount of sunlight plants absorb; expose a great deal of tissue to the dryness of the air, and, therefore, have adaptations that protect against water loss; adjustable pores in leaves help conserve water while letting oxygen and carbon dioxide enter and exit the leaf

Transpiration and Water Transport

Major force in water transport is provided by the evaporation of water from leaves during transpiration;as water evaporates through open stomata, the cell walls within the leaf begin to dry out, and cell walls contain cellulose, so dry cell walls draw water from cells deeper inside the leaf, which extends into vascular tissue so that water is pulled up through xylem; the hotter and drier the air, and the windier the day, the greater the amount of water loss, and as a result of this water loss, the plant draws up even more water from the roots

Anatomy of a Root

Mature root has an epidermis and contains vascular tissue and a large area of ground tissue

Vascular Cambium

Meristem that produces vascular tissues and increases the thickness of stems over time

Apical Meristems

Meristems at the tip of a stem or root; unspecialized cells produced in apical meristems divide rapidly as stems and roots increase in length

Photosynthesis

Mesophyll tissue in most leaves is highly specialized for photosynthesis

Wood

Most of what we call wood is actually layers of secondary xylem produced by the vascular cambium, and these cells build up year after year, layer after layer

Transpiration and Wilting

Osmotic pressure keeps a plant's leaves and stems rigid, but high transpiration rates can lead to wilting, which results from the loss of water, and therefore pressure, in a plant's cells; without this internal pressure to support them, the plant's cell walls bend inward, and the plant's leaves and stems wilt; when a leaf wilts, its stomata close, and as a result, transpiration slows down significantly, which helps a plant to conserve water

Root Epidermis

Performs dual functions of protection and absorption, and its surface is covered in root hairs

Homeostasis

Plants maintain homeostasis by keeping their stomata open just enough to allow photosynthesis to take place but not so much that they lose an excessive amount of water; in general, stomata are open during the daytime, when photosynthesis is active, and closed at night, when open stomata would only lead to water loss; however, stomata may be closed even in bright sunlight under hot, dry conditions in which water conservation is a matter of life and death, and guard cells respond to conditions in the environment, such as wind and temperature, helping to maintain homeostasis within a leaf

Ground Tissue

Produces and stores sugars, and contributes to physical support of the plant

Cork Cambium

Produces the outer covering of stems

Dermal Tissue

Protective outer covering of a plant; in young plants consists of epidermis, and in older plants may be many cell layers deep and may be covered with bark; in roots, includes root hair cells that help absorb water, and in shoots, prevents the loss of water

Stems

Provide a support system for the plant body, a transport system that carries nutrients, and a defensive system that protects the plant against predators and disease; produce leaves and reproductive organs; support system must be strong enough to hold up leaves and branches, transport system contains tissues that lift water from the roots up to the leaves and carry the products of photosynthesis from the leaves back down to the roots; goes against photosynthesis

Cortex

Region of ground tissue; water and minerals move through the cortex from the epidermis toward the center of the root, and also stores the products of photosynthesis

Meristems

Regions of unspecialized cells in which mitosis produces new cells that are ready for differentiation; found in places where plants grow rapidly, such as the tips of stems and roots

Root Cap

Root tip is covered by a tough root cap that protects the fragile meristem as the root tip forces its way thought the soil; as the root grows, the root cap secretes a slippery substance that eases the progress of the root through the soil; cells at the very tip of the root cap are constantly being scraped away, the new root cap cells are continually added by the meristem

Monocot Stems

Shows all three tissue systems clearly; has a distinct epidermis that encloses ground tissue and a series of vascular bundles, which are scattered throughout the ground tissue; ground tissue is fairly uniform, consisting mainly of parenchyma cells

Epidermis

Single layer of cells; outer surfaces of epidermal cells are often covered with the cuticle, and some epidermal cells have trichomes

Buds

Small buds are found where leaves attach to the nodes and contain apical meristems that can produce new stems and leaves

Stomata

Small openings in the epidermis that allow carbon dioxide, water, and oxygen to diffuse into and out of the leaf

Mesophyll

Specialized ground tissue where photosynthesis occurs; sugars produced in mesophyll move to leaf veins, where they enter phloem sieve tubes for transport to the rest of the plant

Anatomy of a Stem

Stems contain dermal, vascular, and ground tissue, and are surrounded by a layer of epidermal cells that have thick cell walls and a waxy protective covering

Stomata and Guard Cells

Stomata open and close in response to changes in water pressure within the guard cells, as when water is abundant, it flows into the leaf, raising water pressure in the guard cells, which then open the stomata, and the thin outer walls of the cells are force into a curved shape, which pulls the thick inner walls of the guard cells away from one another, opening the stoma, and carbon dioxide can then enter through the stoma, and water is lost by transpiration; when water is scarce, the opposite occurs, as water pressure within the guard cells decreases, the inner walls pull together, and the stoma closes, which reduces further water loss by limiting transpiration

Leaf Structure and Function

Structure of a leaf is optimized to absorb light and carry out photosynthesis; like roots and stems, leaves have an outer covering of dermal tissue and inner regions of ground and vascular tissues

Vascular Tissue

Supports the plant body and transports water and nutrients throughout the plant; in tracheophytes

Sapwood

Surrounds heartwood, is active in fluid transport and is therefore usually lighter in color

Capillary Action

Tendency of water to rise in a thin tube; water is attracted to the walls of the tube, and water molecules are attracted to one another, and the thinner the tube, the higher the water will rise inside it

Cuticle

Thick waxy layer that protects against water loss

Root Hairs

Thin cellular projections that penetrate the spaces between soil particles and produce a large surface area that allows water and minerals to enter

Pits

Thinner regions of the wall

Seed Plant Structure

Three principal organs of seed plants are roots, stems, and leaved; organs are linked together by systems that run the length of the plant, and these systems produce, store, and transport nutrients, and provide physical support and protection

Trichomes

Tiny projections that help protect the leaf and may give the leaf a fuzzy appearance

Phloem

Tissue that carries dissolved food

Blade

To collect sunlight, most leaves have a thin, flattened part called a blade; flat shape of a leaf blade maximizes the amount of light it can absorb

Secondary Growth in Conifers and Dicots

Unlike monocots, most dicots have meristems within their stem and roots that can produce true secondary growth, which enables many dicots to grow to great heights because the increase in width supports the extra weight; in conifers and dicots, secondary growth takes place in the vascular cambium and cork cambium, and similar types of cambium tissue enable roots to grow; addition of new tissue in these cambium layers increases the thickness of stems and roots

Nutrient Transport

Unlike the cells that form xylem, the sieve tube cells in phloem remain alive and active transport moves sugars into the sieve tube from surrounding tissues, water then follows by osmosis, creating pressure in the tube at the source of the sugars, and if another region of the plant has a need for sugars, they are actively pumped out of the tube and into the surrounding tissues, and osmosis then causes water to leave the tube, reducing pressure in the tube at such places, which results in a pressure driven flow of nutrient rich fluid from the sources of sugars to the places in the plants where sugars are used or stored

Xylem

Water conducting tissue

Vessel Element

Wider than tracheids and are arranged end to end on top of one another; after they mature and die, cell walls at both ends are left with slitlike openings through which water can move freely, while in some vessel elements, the end walls disappear altogether, producing a continuous tube

Vascular Cyliner

Xylem and phloem together make up region; dicot roots have a central column of xylem cells

Transpiration and Capillary Action

Xylem tissue is composed of tracheid and vessel elements that form may hollow, connected tubes that are lined with cellulose cell walls, to which water adheres very strongly, so when transpiration removes some water from the exposed walls strong adhesion forces pull in water from the wet interior of the leaf, which is so powerful that it extends even down to the tips of roots, and through them, to the water in the soil; combination of transpiration and capillary action provides over 90 percent of the force that moves water through the xylem tissues of a plant