e-Learning Session

36.1. Plant Organs

A. Flowering Plant Structure (p. 638)

1. Structures of flowering plants are well-adapted to varied environments.
2. Flowering plants have three vegetative organs: root, stem and leaf. (Figs. 36.1 and 36.2)

B. Roots

1. We speak of "a root" but it is more appropriate to call it a root system.
2. Root system is the main root plus its lateral (side) branches. (Fig. 36.2a)
3. It is generally equal in size to the shoot system, the part above ground.
4. Root systems have the following functions.
1. Roots anchor a plant in soil and give support.
2. Roots absorb water and minerals from soil; root hairs are central to this process.
1. Root hair cells are in a zone near root tip.
2. Root hairs are numerous to increase absorptive surface of a root.
3. Transplanting plants without preserving root hairs damages a plant.
4. Water and nutrients absorbed are distributed to rest of the plant.
5. Roots produce hormones that must be distributed to the plant
3. Perennials "die back" to regrow next season; roots store food. (e.g., carrots, sweet potatoes).

C. Stems (p. 639)

1. Stem forms main axis of a plant, along with lateral branches. (Fig. 36.2b)
2. Stem produces leaves and arrays them to be exposed to as much sun as possible..
3. A node occurs where a leaf attaches and internode is region between nodes; nodes and internodes identify a stem even if it is underground.
4. Stem has vascular tissue to transport water and minerals from roots and sugar from leaves.
5. Non-living cells form a continuous pipeline in vascular tissue.
6. A cylindrical stem expands in girth and length; trees use woody tissue to strengthen stems.
7. Stems function in storage: cactus stems store water; tubers are horizontal stems that store nutrients.

D. Leaves

1. A foliage leaf is the usual organ of photosynthesis in vascular plants. (Fig. 36.2c)
2. Leaves receive water from roots by way of the stem.
3. Broad, thin leaves have maximum surface area to absorb CO₂ and collect solar energy.
4. A blade is the wide portion of a leaf with most photosynthetic tissue.
5. Petiole is a stalk that attaches a leaf blade to stem.
6. Leaf axil is upper acute angle between petiole and stem where an axillary (lateral) bud originates.
7. Some leaves protect buds, attach to objects, store food, or capture insects.

36.2. Monocot Versus Dicot Plants

B. Epidermal Tissue

1. Epidermis is an outer protective covering tissue of plant roots, leaves, and stems of nonwoody plants.
2. It contains closely packed epidermal cells.
3. Waxy cuticle covers walls of epidermal cells, minimizing water loss and protecting against bacteria.
4. In roots, certain epidermal cells are modified into root hairs that increase surface area of the root for absorption of water and minerals and help to anchor plants. (Fig. 36.4a)
5. Different protective hairs are produced by epidermal cells of stems and leaves.
6. Epidermal cells are modified as glands to secrete protective substances.
7. On lower epidermis of leaves, special guard cells form microscopic pores (stomates) and regulate gas exchange between leaf interior and exterior. (Fig. 36.4b)
8. In older woody plants, epidermis of the stem is replaced by cork tissue. (Fig. 36.4c)
1. Cork is outer covering of the bark of trees; composed of dead cork cells that may be sloughed off.
2. Cork cambium is lateral meristem that produces new cork cells.
3. As cork cells mature, they encrust with lipid suberin that renders them waterproof and inert.
4. Cork protects a plant and makes it resistant to attack by fungi, bacteria, and animals.

C. Ground Tissue

1. Ground tissue fills the inside of plants with parenchyma, collenchyma and sclerenchyma cells.
2. Parenchyma are least specialized of all plant cell types. (Fig. 36.5a)
1. Cells of this type contain plastids (e.g., chloroplasts or colorless storage plastids).
2. They are found in all organs of a plant.
3. They divide to form more specialized cells (e.g. roots develop from stem cuttings in water).
3. Collenchyma resemble parenchyma but has thicker primary cell walls. (Fig. 36.5b)
1. Collenchyma cells are uneven in the corners.
2. They usually occur as bundles of cells just beneath epidermis.
3. They give flexible support to immature regions of plants (e.g. celery stalk is mostly collenchyma).
4. Sclerenchyma cells have thick secondary cell walls. (Fig. 36.5c)
1. They are impregnated with lignin that renders walls tough and hard.
2. They provide strong support to mature regions of plants.
3. Most cells of this type are non-living.
4. Sclerenchyma cells form fibers (used in linen and rope) and shorter sclereids (found in seed coats and nut shells).

D. Vascular Tissue

1. Xylem passively conducts water and mineral solutes upward through a plant from roots to leaves;
1. Xylem contains tracheids, vessel elements, and parenchyma cells that store various substances. (Fig. 36.6)
2. Tracheids (Fig. 36.6a)
1. Tracheids are hollow, thin, long non-living cells with tapered overlapping ends.
2. Water moves across end and sidewalls because of pits or depressions in secondary cell wall.
3. Vessel Elements
1. Vessel elements are hollow non-living cells lacking tapered ends.
2. They are larger than tracheids.
3. They lack transverse end walls.
4. They form a continuous pipeline for water and mineral transport.
4. Xylem also contains sclerenchyma cells to add support.
2. Phloem is vascular tissue that conducts the organic solutes in plants, from the leaves to the roots; it contains sieve-tube cells and companion cells. (Fig. 36.7)
1. Sieve-tube cells
1. Sieve-tube cells contain cytoplasm but no nucleus.
2. They are arranged end to end.
3. They have channels in their end walls (thus, the name "sieve-tube"), through which plasmodesmata extend from one cell to another.
2. Companion Cells
1. Companion cells are closely connected to sieve-tube cells by numerous plasmodesmata.
2. They are smaller and more generalized than sieve-tube cells.
3. They have a nucleus which may control and maintain the function of both cells.
4. They are also thought to be involved in the transport function of phloem.
3. Vascular tissue extends from root to leaves as vascular cylinder (roots), vascular bundles (stem) and leaf veins.

36.4. Organization of Roots (p. 644)

A. Dicot Root Tip

1. Dicot root tip, site of primary growth, is organized into zones of cells in various stages of differentiation. (Fig. 36.8a)
2. Cells are continuously added to a root cap below and zone of elongation above by contributions from the zone of cell division.
3. Root cap is a protective cover; its cells are replaced constantly because they are soon ground off.
4. Zone of elongation is above a zone of cell division where cells become longer and more specialized.
5. Zone of cell division contains meristematic tissue and adds cells to root tip and zone of elongation.
6. Zone of maturation is above zone of elongation; cells are mature and differentiated and it has root hairs.

B. Tissues of a Dicot Root (Fig. 36.8a)

1. Epidermis is a single layer of thin-walled, rectangular cells.
1. Epidermis forms protective outer layer of the root.
2. In region of maturation, there are many root hairs.
3. Root hairs project as far as 5-8 mm into the soil.
2. Cortex is a layer of large, thin-walled, irregularly shaped parenchyma cells.
1. These cells contain starch granules; function in food storage.
2. Cells are loosely packed; water and minerals can diffuse through cortex without entering cells.
3. Endodermis is layer of rectangular cells; forms boundary between cortex and inner vascular cylinder.
1. Its cells fit closely together and are bordered on four sides by the Casparian strip.
2. It regulates entrance of minerals into vascular cylinder.
3. Casparian strip is impermeable lignin and suberin layer that excludes water and mineral ions. (Fig. 36.8c)
4. The only access to the vascular bundle is through endodermal cells.
4. Vascular cylinder is an arrangement of vascular tissues as a cylinder. (Fig. 36.8b)
1. Pericycle is first layer of cells within vascular cylinder
1. Its cells have retained capacity to divide.
2. It can start development of branch or secondary roots. (Fig. 36.9)
2. Vascular tissue forms main portion of a vascular bundle; it is composed of
1. xylem, whose cells are arranged in a star-shaped pattern; and
2. phloem, whose cells are located in regions between arms of xylem.

C. Organization of Monocot Roots (Fig. 36.10)

1. Monocot roots do not undergo secondary growth.
2. Monocot root has a ring of vascular tissue; alternating bundles of xylem and phloem surround pith.
3. Monocot roots also have pericycle, endodermis, cortex, and epidermis.

D. Root Diversity

1. Roots have adaptations to help anchor plants, absorb water and minerals, and store carbohydrates.
2. There are three general root types:
1. Taproot is common in dicots; first or primary root grows straight down and remains dominant root of a plant; often fleshy and adapted to store food (e.g., carrots, beets). (Fig. 36.11a)
2. Fibrous root system of monocots is a mass of slender roots and lateral branches that hold soil. (Fig. 36.11b)
3. Adventitious roots develop from underground stems or from base of above-ground stems.
1. Prop roots main function is to anchor a plant (e.g. corn and mangrove plants). (Fig. 36.11c)
2. Pneumatophores of mangrove plants project above the water from roots to acquire oxygen.
3. Ivy has holdfast roots to anchor aerial shoots.
3. Haustoria are root-like projections of stems of parasitic plants (e.g., dodders and broomrapes).
1. They grow into the host plant.
2. They contact vascular tissue from which they extract water and nutrients.
4. Mycorrhizae are fungus roots.
1. In this mutualism, fungus receives sugars and amino acids from plant.
2. Plant receives water and minerals from the fungus.
5. Legumes (e.g., peas and beans) have root nodules containing nitrogen-fixing bacteria.
1. Bacteria extract nitrogen from air and reduce it to a form that can be used by plant tissues.
2. Legumes are often planted to bolster nitrogen supply of soil.

A. Primary Growth (p. 648)

1. Stem tip is site of primary growth where cell division extends length of stems or roots.
2. Shoot apical meristem produces new leaves and primary meristems; increases stem length. (Fig. 36.12)
3. Shoot apical meristem is protected within a terminal bud of leaf primordia (immature leaves).
4. Bud scales are scale-like coverings protecting terminal buds during winters when bud growth stops.
5. Three specialized types of primary meristem develop from shoot apical meristem. (Fig. 36.12b)
1. Protoderm is outermost primary meristem that gives rise to epidermis.
2. Ground meristem produces two tissues composed of parenchyma cells: pith and cortex.
3. Procambium is inner meristem that produces primary xylem and primary phloem.
6. Differentiation continues; cells become first tracheids or vessel elements within vascular bundle.
7. First sieve-tube cells are short-lived and do not have companion cells.
8. Mature phloem develops later after all surrounding cells have stopped expanding.

B. Herbaceous Stems

1. Herbaceous stems are mature nonwoody stems that exhibit only primary growth.
2. Outermost tissue of herbaceous stems is epidermis covered by waxy cuticle to prevent water loss. (Fig. 36.13)
3. Xylem and phloem are in distinctive vascular bundles.
1. In each bundle, xylem is found to inside of stem; phloem is found to outside. (Fig. 36.13b)
2. In dicot herbaceous stem, vascular bundles are arranged in a ring towards outside of the stem and separating cortex from central pith. (Fig. 36.13a)
3. In monocot stem, vascular bundles are scattered throughout the stem; there is no well-defined cortex or pith. (Fig. 36.14)
4. Cortex sometimes carries on photosynthesis; pith may function as a storage site.

C. Woody Stems (Fig. 36.15)

1. Woody plants have both primary and secondary tissues.
2. Primary tissues are new and form each year from primary meristem right behind apical meristem.
3. Secondary tissues develop from second year onward from growth of lateral meristem.
4. Primary growth increases length of a plant; secondary growth increases its girth. (Fig. 36.16)
5. As secondary growth continues, it is not possible to distinguish individual vascular bundles.
6. Woody dicot stem has a different organization with three distinct areas: bark, wood, and pith.
7. Rays are of living cells that allow materials to move laterally.
8. Bark of a tree contains cork, cork cambium, and phloem.
1. Secondary phloem is produced each year by vascular cambium but does not build up.
2. This phloem tissue is soft; therefore it is easy to remove the bark of a tree..
9. Cork cambium is a lateral meristem that produces new cork cells when needed.
1. Cork cambium begins to divide, producing cork that disrupts epidermis replacing it with cork cells.
2. Cork cells become impregnated with suberin, causing them to die but making them waterproof..
3. Consequently, cork forms an impervious barrier, even to gas exchange, except at lenticels,
10. First flowering plants were probably woody shrubs; herbaceous plants evolved later.
1. It is advantageous to be woody when there is adequate rainfall; woody plants can grow taller and have adequate tissue to support and service leaves.
2. It takes energy to support secondary growth and prepare plant for winter in temperate zones.
3. Long-lasting plants need more defense mechanisms against attack by herbivores and parasites.
4. Trees need years to mature before reproducing; they are more vulnerable to accident or disease.
11. Annual Rings
1. Vascular cambium is dormant during winter.
2. Spring wood is composed of wide xylem vessel elements with thin walls, necessary to conduct of sufficient water and nutrients to supply abundant growth that occurs during spring.
3. Summer wood forms when moisture is scarce; composed of a lower proportion of vessels, it contains thick-walled tracheids and numerous fibers.
4. Annual ring is one ring of spring wood followed by a ring of summer wood; equals one year's growth. (Fig. 36.15 and 36A)
5. Sapwood is outer annual rings where transport occurs. (Fig. 36.15b, c)
6. Heartwood is inner annual rings of older trees.
1. Vessels no longer function in transport; they become plugged with resins and gums that inhibit growth of bacteria and fungi.
2. Heartwood may help to support a tree.

D. Stem Diversity

1. Stolons are stems that grow along ground; new plants grow where nodes contact soil. (Fig. 36.17a)
2. Succulent stems of cacti are modified for water storage.
3. Tendrils of grapes and morning glories are stems adapted for wrapping around support structures.
4. Rhizomes are underground horizontal stems.
1. Rhizomes are long and thin in grasses and thick and fleshy in irises. (Fig. 36.17b)
2. Rhizomes survive winter and contribute to asexual reproduction because each node bears a bud.
3. Some rhizomes have tubers that function in food storage (e.g., potatoes). (Fig. 36.17c)
5. Corms are bulbous underground stems that lie dormant during winter, like rhizomes. (Fig. 36.17d)
6. Humans use stems: sugarcane is primary source of table sugar; cinnamon and quinine are from bark.

36.6. Organization of Leaves (p. 654)

A. Leaf Structure (Fig. 36.18)

1. Leaves are organs of photosynthesis in plants; they are made of a flattened blade and a petiole.
2. Leaf veins reveal the presence of vascular tissue within the leaves.
3. The vascular tissues of leaves transport water and nutrients.
4. Leaf veins have a net pattern in dicot leaves and a parallel pattern in monocot leaves. (Fig. 36.3)
5. A petiole is a stalk that attaches a leaf blade to the plant stem.
6. Epidermis is the layer of cells that covers the top and bottom sides of a leaf.
1. Epidermis often bears protective hairs or glands; epidermal glands produce irritating substances.
2. Epidermis is covered by a waxy cuticle that keeps the leaf from drying out.
3. Epidermis, particularly lower epidermis, contains stomates that allow gases to move into and out of leaf.
7. Mesophyll is the inner body of a leaf and the site of most of photosynthesis.
1. Palisade mesophyll is layer of mesophyll containing elongated parenchyma cells with many chloroplasts.
2. Spongy mesophyll contains loosely packed parenchyma cells that increase surface area for gas exchange.

B. Leaf Diversity (Fig. 36.19)

1. Simple leaves have margins not deeply lobed or divided into smaller leaflets.
2. Compound leaves are divided into smaller leaflets, and each leaflet may have its own stalk.
3. Leaves are variously modified.
1. Cactus spines are modified leaves; succulents have fleshy leaves to hold moisture. (Fig. 36.20a)
2. Onion bulbs have leaves surrounding a short stem.
3. Tendrils of peas and cucumbers are leaves. (Fig. 36.20b)
4. Venus's-flytrap has leaves to trap and digest insects. (Fig. 36.20c)