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