36.1. Plants Have Organs (p. 638)
A. Flowering Plant Structure
1. The structure of flowering plants is well-adapted to the land environment.
2. A flowering plant has three vegetative organs: the root, stem and leaf. (Figs. 36.1 and 36.2)
B. Roots Anchor
1. Although we speak of "a root," it is more appropriate to call it a root system.
2. The root system is the main root, together with any and all of its lateral (side) branches. (Fig. 36.2a)
3. Generally, it is equivalent in size and extent to the shoot system, the part above the ground.
4. Root systems have the following functions.
a. Roots anchor the plant in the soil and give it support.
b. Roots absorb water and minerals from the soil; root hairs are critical in this process.
1) Root hair cells are found in a special zone located near the root tip.
2) Root hairs are numerous to increase absorptive surface of the root.
3) Transplanting plants without preserving the root hairs is usually damaging to the plant.
4) The water and nutrients absorbed are then distributed to the rest of the plant
c. Perennials "die back" and regrow the next season; roots are site for stored food. (e.g., carrots, sweet potatoes).
C. Stems Support
1. A stem forms the main axis of a plant, along with its lateral branches. (Fig. 36.2b)
2. In vascular plants, the stem produces leaves and arrays them to be exposed to as much sun as possible..
3. The node is the location on a stem where a leaf attaches; the internode is the region on a stem between nodes, and presence of nodes and internodes identify a stem even if it happens to be underground. (Fig. 36.1)
4. Some stems function in storage; cactus stems store water; tubers are horizontal storage stems (e.g., potato).
D. Leaves Photosynthesize
1. The 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 a maximum surface area for absorption of CO2 and collection of solar energy.
4. The blade is the wide portion of a leaf where most photosynthetic tissue is located.
5. The petiole is a stalk that attaches a leaf blade to the plant stem.
6. The leaf axil is the upper acute angle between petiole and stem where an axillary (lateral) bud originates.
36.2. Monocot Versus Dicot Plants (p. 644)
A. Criteria for Monocots and Dicots
1. Flowering plants are divided into monocots and dicots based on the following criteria. (Fig. 36.3) [transp. 192]
| Monocots | Dicots | |
| 2. 1 a. Number of cotyledons in seed | one | two |
|
2. 1
b. Distribution of root xylem and 2. 1 phloem |
root xylem and phloem in a ring |
root phloem between arms of xylem |
| 2. 1 c. Distribution of vascular bundles | scattered in stem | arranged in a distinct ring |
| 2. 1 d. Pattern of leaf veins | form a parallel pattern | form a net pattern |
| 2. 1 e. Number of flower parts |
in threes and multiples of three |
in fours and fives and multiples of four or five |
|
2. 1
f. Number of apertures in pollen 2. 1 grains | usually one | usually three |
| 2. Representative members: | grasses, lilies, orchids |
dandelions to oak trees and palm trees |
3. Cotyledons are embryonic seed leaves; they provide nutrient molecules for a developing plant before its mature leaves begin to photosynthesize.
4. These characteristics represent an evolutionary path dating back to the origin of flowering plants.
36.3. Plant Organs Have Tissues (p. 641)
A. Meristem Produces Tissue
1. A plant grows its entire life because it has meristem (embryonic tissue) located in the stem and root apexes.
2. Three types of primary meristem continually produce three types of specialized tissue in the body of a plant.
a. Protoderm is the outermost primary meristem, which gives rise to epidermis.
b. Ground meristem is an inner meristem that produces ground tissue.
c. Procambium produces vascular tissue.
3. Three specialized tissues are produced.
a. Epidermal tissue form the outer protective covering of a plant.
b. Ground tissue fills the interior of a plant.
c. Vascular tissue transports water and nutrients in a plant and provides support.
B. Epidermal Tissue Protects
1. The epidermis is an outer protective covering tissue of plant roots, leaves, and stems of nonwoody plants.
2. It contains closely packed epidermal cells.
3. Walls of epidermal cells are covered with a waxy cuticle to minimize water loss and protect 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 the plant firmly in place. (Fig. 36.4a) [micro. slide 61]
5. Protective hairs of a different nature are produced by epidermal cells of stems and leaves.
6. Epidermal cells may be modified as glands that secrete protective substances of various types.
7. In leaves, particularly the lower epidermis, specialized epidermal cells (guard cells) form microscopic pores (stomates) and regulate gas exchange between interior and exterior of a leaf. (Fig. 36.4b) [micro. slide 62]
8. In older woody plants, the epidermis of the stem is replaced by cork tissue. (Fig. 36.4c) [micro. slide 63]
a. Cork is the outer covering of the bark of trees; composed of dead cork cells that may be sloughed off.
b. Cork cambium is a lateral meristem that produces new cork cells.
c. As cork cells mature, they encrust with the lipid suberin, which renders them waterproof and inert.
d. Cork protects the plant and makes it resistant to attack by fungi, bacteria, and animals.
C. Ground Tissue Fills
1. Ground tissue fills the interior of plants; it contains parenchyma, collenchyma and sclerenchyma.
2. Parenchyma are the least specialized of all plant cell types. (Fig. 36.5a) [micro. slide 64]
a. Cells of this type contain plastids (e.g., chloroplasts or colorless storage plastids).
b. They are found in all organs of a plant.
c. They divide and give rise to more specialized cells, as when roots develop from stem cuttings in water.
3. Collenchyma resemble parenchyma cells, but have thicker primary cell walls. (Fig. 36.5b) [micro. slide 65]
a. Collenchyma cells are uneven, usually in the corners of the cell.
b. They usually occur as bundles of cells just beneath the epidermis.
c. They give flexible support to immature regions of a plant body; a celery stalk is mostly collenchyma cells.
4. Sclerenchyma cells have thick secondary cell walls. (Fig. 36.5c) [micro. slide 66]
a. They are usually impregnated with lignin, which renders their walls tough and hard.
b. Their function is to provide strong support of mature regions of a plant.
c. Most cells of this type are nonliving.
d. Sclerenchyma cells are fibers (used in linen and rope) and sclereids (found in seed coats and nut shells).
D. Vascular Tissue Transports
1. Vascular tissue transports water and nutrients in a plant and provides support; there are two types.
a. Xylem passively conducts water and mineral solutes upward through a plant from roots to leaves; contains tracheids, vessel elements, and parenchyma cells that store various substances. (Fig. 36.6) [transp. 193]
1) Tracheids are hollow, thin and long nonliving cells with tapered overlapping ends; water moves across end and sidewalls because of pits or depressions in secondary cell wall. (Fig. 36.6a)
2) Vessel elements are hollow nonliving cells lacking tapered ends; larger than tracheids; lack transverse end walls; arranged to form a continuous pipeline for water and mineral transport. [micro. slide 67]
3) Xylem also contains sclerenchyma cells to add support.
b. 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) [transp. 194] [micro. slide 68]
1) Sieve-tube cells contain cytoplasm but no nucleus; arranged end to end; have channels in their end walls (thus, the name "sieve-tube"), through which plasmodesmata extend from one cell to another.
2) Companion cells are closely connected to sieve-tube cells by numerous plasmodesmata; smaller and more generalized than sieve-tube cells; they have a nucleus which may control and maintain the function of both cells; also thought to be involved in the transport function of phloem.
36.4. How Roots Are Organized (p. 644)
A. Dicot Root Tip
1. The dicot root tip, the site of primary growth of roots, is longitudinally organized into zones where cells are in various stages of differentiation. (Fig. 36.8a) [transp. 195] [micro. slide 69]
2. At root tip, 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 for a root tip; its cells are replaced constantly because they are soon ground off.
4. Zone of elongation is above the zone of cell division where cells become longer and more specialized.
5. The zone of cell division contains meristematic tissue and adds cells to the root tip and zone of elongation.
6. Zone of maturation is above zone of elongation where cells are mature and differentiated; has root hairs.
B. Tissues of a Dicot Root (Fig. 36.8a) [transp. 195] [micro. slide 69]
1. The epidermis is a single layer of thin-walled, rectangular cells.
a. The epidermis forms the protective outer layer of the root.
b. In the region of maturation, there are many root hairs.
c. Root hairs project as far as 5-8 mm into the soil particles.
2. The cortex is a loosely packed layer of large, thin-walled, irregularly shaped parenchyma cells.
a. These cells contain starch granules; function in storage of water and the products of photosynthesis.
b. The cells are loosely packed; water and minerals can diffuse through the cortex without entering the cells.
3. Endodermis is a layer of rectangular cells that forms boundary between cortex and inner vascular cylinder.
a. Its cells fit closely together and are bordered on four sides by the Casparian strip.
b. It regulates entrance of minerals into the vascular cylinder.
c. Casparian strip is an impermeable lignin and suberin that does not permit water and mineral ions to pass. (Fig. 36.8c)
d. Therefore, the only access to the vascular bundle is through the endodermal cells.
4. The vascular cylinder is an arrangement of vascular tissues as a cylinder. (Fig. 36.8b)
a. The pericycle is the first layer of cells within the vascular cylinder; its cells have retained the capacity to divide and can start the development of branch or secondary roots. (Fig. 36.9) [micro. slide 70]
b. Vascular tissue forms the main portion of the 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 the regions between the arms of the xylem.
C. How Monocot Roots Are Organized (Fig. 36.10) [micro. slide 71]
1. A monocot root has a ring of vascular tissue with alternating bundles of xylem and phloem surrounding pith.
2. Monocot roots also have pericycle, endodermis, cortex, and epidermis.
3. Typically, monocot roots do not undergo secondary growth.
D. How Roots Differ
1. Roots have adaptations to help anchor plants, absorb water and minerals, and store carbohydrates.
2. There are three general root types:
a. The taproot is characteristic of dicots; the first or primary root grows straight down and remains the dominant root of the plant; often fleshy and adapted to store food (e.g., carrots, beets). (Fig. 36.11a)
b. Fibrous root system of monocots is a mass of slender roots and lateral branches that hold soil. (Fig. 36.11b)
c. The adventitious roots develop from an underground stem or from the base of an above-ground stem.
1) Prop roots are adventitious roots whose main function is to help anchor the plant.
2) Corn and mangrove plants are examples of major prop roots. (Fig. 36.11c)
3) Pneumatophores project above the water surface from the prop roots of mangrove plants; acquire oxygen.
3. Haustoria are rootlike projections of the stems of parasitic plants (e.g., dodders and broomrapes) that grow into the host plant and make contact with vascular tissue from which they extract water and nutrients.
4. Mycorrhizae are fungus roots; a mutualistic association between roots and fungi, in which the fungus receives sugars and amino acids from the plant, and the plant receives water and minerals from the fungus.
5. Legumes (e.g., peas and beans) have root nodules containing nitrogen-fixing bacteria associated with plants.
a. The bacteria extract nitrogen from the air and reduce it to a form that can be used by the plant tissues.
b. Legumes are often planted to bolster the nitrogen supply of the soil.
36.5. How Stems Are Organized (p. 648)
A. Primary Growth
1. The stem tip is the site of primary growth where cell division extends the length of stems or roots.
2. Shoot apical meristem produces new leaves and primary meristems; increases stem length.
(Fig. 36.12) [micro. slide 72]
3. Shoot apical meristem is protected within a terminal bud where leaf primordia (immature leaves) envelop it.
4. Bud scales are scalelike coverings protecting terminal buds of plants during winters when bud growth stops.
5. Three specialized types of primary meristem develop from shoot apical meristem. (Fig. 36.12b)
a. The protoderm is the outermost primary meristem, which gives rise to epidermis.
b. Ground meristem produces two tissues composed of parenchyma cells, pith and cortex.
c. Procambium is an inner meristem that produces primary xylem and primary phloem. [micro. slide 68]
6. Differentiation continues with cells becoming the first tracheids or vessel elements within a vascular bundle.
7. The first sieve-tube cells are short-lived and do not have companion cells.
8. Mature phloem develops later, when all surrounding cells have stopped expanding.
B. Herbaceous Stems Are Nonwoody
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 desiccation. (Fig. 36.13) [micro. slide 73]
3. Xylem and phloem are found in distinctive vascular bundles.
a. In each bundle, xylem is found to inside of stem; phloem is found to outside. (Fig. 36.13b)
b. In the dicot herbaceous stem, the vascular bundles are typically arranged in a ring towards the outside of the stem and separating the cortex from the central pith. (Fig. 36.13a) [micro. slide 73]
c. In the monocot stem, the vascular bundles [micro. slide 75] are scattered throughout the stem, and there is no well-defined cortex or pith. (Fig. 36.14) [micro. slide 74, 76]
4. The cortex sometimes carries on photosynthesis, and the pith may function as a storage site.
C. How Stems Become Woody (Fig. 36.15) [transp. 196]
1. A woody plant (e.g., those of almost all trees and some shrubs) have both primary and secondary tissues.
2. Primary growth continues for only a short distance behind apical meristem; then secondary growth begins.
3. Primary growth increases the length of the plant and secondary growth increases its girth. (Fig. 36.16)
4. A woody stem, therefore, has a totally different organization with three distinct areas: bark, wood, and pith.
5. Secondary growth of woody stems is due to a change in vascular cambium.
a. Vascular cambium of woody plants forms a ring of meristem that divides parallel to surface of the plant.
b. This lateral meristem produces secondary phloem and secondary xylem that add to the girth of the stem.
c. In trees that have a growing season, vascular cambium is dormant during the winter; consequently, the wood is composed only of heavy fibers with thick secondary walls.
d. Spring wood is composed of wide vessel elements with thin walls, necessary for the conduction of sufficient water and nutrients to supply the abundant growth that occurs during the spring.
e. Summer wood forms when moisture is scarce; composed of a lower proportion of vessels, it contains thick-walled tracheids surrounded by numerous fibers.
f. 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)
g. Sapwood is the outer annual rings, where transport occurs. (Fig. 36.15b, c)
h. Heartwood is the inner annual rings of older trees; vessels no longer function in transport because they become plugged with resins and gums that inhibit growth of bacteria and fungi; may help to support a tree.
i. Cork cambium is a lateral meristem that produces new cork cells when needed.
1) As cork cambium begins to divide, it produces cork that disrupts epidermis, replaces 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 lenticles, which are pockets of loosely arranged cork cells not impregnated with suberin.
6. The first flowering plants were probably woody shrubs; herbaceous plants evolved later.
a. 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.
b. However, it takes energy to prepare the plant body for winter in temperate zones.
c. A long-lasting plant needs more defense mechanisms against attack by herbivores and parasites.
d. Trees need many years to mature before reproducing; they are more vulnerable to accident or disease.
D. How Stems Differ
1. Stolons are stems that grow along ground and establish plants as they contact soil. (Fig. 36.17a)
2. Rhizomes are underground horizontal stems.
a. Rhizomes may be long and thin (e.g., those of grasses), or thick and fleshy (e.g., irises). (Fig. 36.17b)
b. Rhizomes survive the winter and contribute to asexual reproduction because each node bears a bud.
c. Some rhizomes have tubers that function in food storage (e.g., potatoes). (Fig. 36.17c)
3. Corms are bulbous underground stems that lie dormant during the winter, like rhizomes. (Fig. 36.17d)
4. The succulent stems of cacti are modified for water storage.
5. Tendrils of grapes are modified stems adapted for wrapping around support structures.
6. Plant stems have numerous applications for humans, including the stem of sugarcane as the primary source of table sugar, and cinnamon and the drug quinine derived from the bark of different plants.
E. Humans Utilize Wood
1. Much wood is harvested for lumber; the waste produced is used for particleboard, pulp, etc.
2. Paper is made from wood pulp. (Fig. 36B)
3. Much wood is used in laminated form for plywood, surfaced furniture, boats, helicopter blades, etc.
4. Particleboard, made from gluing wood fiber together, has widespread use in cabinets, toys, car parts, etc.
5. Poles are vital in mine timbers, telephone poles, wharf and building foundations, etc.
6. Wood is important for bowling pins, baseball bats, oars and musical instruments.
7. Nonindustrialized countries still use wood as fuel; coal also was once wood.
36.6. How Leaves Are Organized (p. 654)
A. Leaf Structure (Fig. 36.18) [transp. 197] [micro. slide 77]
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.
a. Epidermis often bears protective hairs or glands; epidermal glands produce irritating substances.
b. Epidermis is covered by a waxy cuticle that keeps the leaf from drying out.
c. 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.
a. Palisade mesophyll is layer of mesophyll containing elongated parenchyma cells with many chloroplasts.
b. Spongy mesophyll contains loosely packed parenchyma cells that increase surface area for gas exchange.
B. How Leaves Differ (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.
a. Spines of cactus are modified leaves; other succulents have fleshy leaves to hold moisture. (Fig. 36.20a)
b. An onion bulb is leaves surrounding a short stem.
c. The tendrils of peas and cucumbers are leaves. (Fig. 36.20b)
d. The Venus's-flytrap has leaves that trap and digest insects. (Fig. 36.20c)