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Chapter 6: Plant anatomy and morphology.

Key Concepts

* Monocots such as corn and wheat, and dicots such as soybean and cotton are two important groups of crop plants.

* Plants have four organs: roots, stems, leaves, and flowers. The form and function of these organs can vary among plants.

* Within plants' four organs, the tissues are vascular, meristem, epidermis, and ground tissue.

* Seeds are the fertilized ovules of the flower. They emerge from the soil and develop into mature plants.

Key Terms

adventitious root

anatomy

apical meristem

axillary meristem

blade

bulb

clone-former

collar

collenchyma

complete flower

compound leaves

corm

crown-former

dicot

dioecious plant

epidermis

epigeal

fibrous root system

flower

fruit

germination

ground tissue

hypogeal

imperfect flower

incomplete flower

inflorescence

intercalary meristem

internode

leaves

meristem

monoecious plant

monocot

morphology

node

parenchyma

perfect flower

petal

petiole

phloem

pistil

rhizome

root

sclerenchyma

seed

sepal

sheath

simple leaves

stamen

stem

stolon

stomata

taproot

tiller

tuber

vascular cambium

venation

xylem

Introduction

Plants are highly organized and contain many specialized organs and tissues. Understanding the anatomy and morphology of plants provides a foundation for understanding how plants look, grow, and ultimately produce food. It is also important in developing plant management and pest control strategies for optimizing crop yield. Morphology deals with the form and structure of plants. The plant morphological features we typically see are the flowers, roots, stems, and leaves (Figures 6-1 and 6-2). Plant anatomy is the study of the internal tissues and cells of plants. The epidermis, vascular system, meristems, and ground tissues are important types of tissues that are constructed of individual specialized cells.

Monocots

Monocots and dicots are two important groups of crop plants. The names monocot (one) and dicot (two) are derived from the numbers of cotyledons a plant has. Cotyledons are the first leaves formed in a seed. Though monocots and dicots have many of the same plant tissues, their anatomy and morphology differ (Table 6-1).

Plant Tissues

The tissues of a plant include the epidermis, xylem, phloem, ground tissue, and meristems. We discuss the different types of tissues in this chapter.

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Epidermis

The epidermis is the outer, protective cell layer on the stem and leaves. The epidermis is frequently covered with cuticle, a waxy covering that reduces water loss and protects the tissues from attacks by parasites and predators such as fungi and insects (Figure 6-3). Many leaves and stems are covered with hairs or spines that offer additional protection for the plant and sometimes secrete chemicals to repel insects. Stomata are very small openings in the epidermis of the leaf and stem that aid in the exchange of water vapor, carbon dioxide, and oxygen (Figure 6-4). Stomata are more numerous on the lower epidermis (or the underside of the leaf). The lower sides of corn and soybean leaves have more than 30,000 stomata per square inch.

Xylem and Phloem

Xylem and phloem make up the vascular bundles, or the conducting tissue, in the stems, leaves, and roots of plants. The xylem and phloem form a tunnel system throughout the plant for the movement of water and nutrients. The xylem transports water and minerals from the root system to the other plant parts. Xylem tissue also provides structural support for the plant as a result of secondary cell wall formation and lignification (a process which makes the cell wall harder through the deposition of lignin). The phloem transports sucrose and other organic compounds throughout the plant. In a dicot plant stem, the vascular bundles are arranged in a ring that separates the cortex and pith (Figure 6-5). In monocot plants, the vascular bundles are scattered throughout the pith of the stem.

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Ground Tissue

The ground tissue, such as the pith and cortex, is the tissue other than epidermal or vascular. It composes the bulk of the plant. It consists of parenchyma, collenchyma, and sclerenchyma cells. Of these three, the parenchyma cells are found in all plant organs. In leaves, parenchyma cells contain chlorophyll for photosynthesis. In stems and roots, they have potential to store energy. Collenchyma cells, found in the stem, and sclerenchyma cells have thickened cell walls and provide mostly structural support for the plant. These contain cellulose and lignin, which constitute the fiber described in Chapter 5.

Meristems

Meristems are areas of actively dividing cells in plants. Meristems produce cells that can differentiate and form other tissue such as epidermis, and ultimately leaves and stems (Figure 6-6). There are several different types of meristems.

* Apical meristems are located at the tip or apex of a shoot or root. Primary growth usually occurs in the apical meristems. Apical dominance occurs when the apical meristem produces compounds that prevent the axillary meristems (see the discussion later in this section) from developing.

* Intercalary meristems are found in grasses and are independent from the apical meristems. They are responsible for the growth of the stem internodes and leaves. Internodes are the portions of the stem between the nodes. Intercalary meristems are located at the base of internodes, at the base of the leaf sheath, and at the base of the leaf.

* Axillary meristems are responsible for the development of buds for branches or flowers. They are located at the nodes where the leaves are attached to the stems. Nodes are enlarged portions of the stem to which leaves are attached and where vegetative and floral buds develop. In soybeans, axillary buds develop into flowers that, when fertilized, become pods containing seeds.

* Vascular cambium is found in the stems and roots of dicot plants and is a type of meristem that provides for increase in stem diameter. Vascular cambium activity's effect on stem diameter growth is most evident in the yearly rings that form in tree trunks.

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Plant Organs

A plant body has three vegetative organs: roots, stems, and leaves. Flowers are modified leaves containing the sexual reproductive organs necessary for seed production. Each plant organ has its own morphology and performs a specific function within the plant.

Roots

Roots serve several important functions for the plant. They anchor the shoot into the soil and support the upright growth of stems, they absorb minerals and water from the soil, and they provide storage of energy reserves. Storage of energy reserves is especially important in overwintering biennial and perennial plants such as sweetclover, carrot, sugarbeet, dandelion, and alfalfa. Some annual plants such as sweet potato have modified roots for extensive food storage.

Root Anatomy

The anatomy of the dicot and monocot roots is shown in Figure 6-7. Both monocots and dicots have xylem and phloem for conducting water and nutrients to the stem. In the dicot, the xylem cells are located in the center of the root in a star arrangement surrounded by the phloem. A layer of vascular cambium cells responsible for root diameter growth separates the phloem and xylem. A ring of endodermis and pericycle cells surrounds these vascular tissues. Branch roots arise in the pericycle and grow outward through the endodermis. The other major portion of the root is the cortex (made primarily of parenchyma tissue for energy storage) and the epidermis or covering of the root. In the monocot root, alternating xylem and phloem cells encircle a central core of pith. A layer of cortex surrounds the vascular ring. Both monocots and dicots have numerous root hairs. They are extensions of the epidermis that significantly increase the root surface area.

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Root Morphology

Most plant species can be categorized as having either a taproot or fibrous root system, although many species may have a combination of both root system types (Figure 6-8). A taproot system usually consists of a large main root with small lateral roots. Alfalfa, carrots, and dandelions are examples of tap-rooted plants. A fibrous root system consists of several main roots that each branch and develop many lateral roots to form an interwoven mass. Grasses typically have highly branched fibrous root systems.

Root systems are most extensive in perennial plants and often have belowground mass and length equal to the aboveground plant tissue. For most plant species, the greatest mass and length of the root system can be found in the top foot (0.3 meter) of the soil. Some taprooted species such as alfalfa and trees have root systems with the potential to grow 30 feet (9 meters) into the soil.

Plants often have adventitious roots or roots that arise from atypical places such as nodes on stems. Examples of adventitious roots include brace roots that develop from the nodes on the base of a corn stalk (Figure 6-9) and roots that develop on the horizontal stems of white clover.

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Rhizobia and Mycorrhizae

Many plants have beneficial associations with soil microorganisms. Plants in the legume family (e.g., soybean) and alfalfa form symbiotic (mutually beneficial) relationships with indigenous soil bacteria called Rhizobia. These bacteria invade the plant root hairs and form nodules that are connected to the vascular system of the plant. The bacteria receive energy and other nutrients from the plant and in return fix atmospheric nitrogen for the plant to use. The nature of this relationship is described in the next chapter. Many other plants form a symbiotic relationship with soil mycorrhizae or fungi. The fungi grow from the cortex of the plant root into the soil in threads. In exchange for energy from the plant, the fungus absorbs water and nutrients, especially phosphorous, which is immobile in the soil. Mycorrhizal fungi are critical for the growth of many native prairie grasses and legumes.

Stems

The stem is the primary supporting structure for the leaves, flowers, and fruits of a plant and is the conduit for the movement of water, nutrients, and products of photosynthesis between the crown of the plant and the roots. Stems can also conduct photosynthesis and store energy.

Stems can be distinguished from other plant parts by the presence of nodes and internodes. Any leaf, vegetative bud, or floral bud that arises from the node at the leaf axil is called an axillary bud, whereas the one that arises from the top is the apical bud. Stems are typically terminated by meristems that can produce additional vegetative or floral growth.

Grasses have tillers, or shoots (Figure 6-2), which arise from the crown and consist of stem and leaf parts and sometimes reproductive parts. These additional upright stems are common in perennial grasses, as well as small grains. Species with tillers include wheat, barley, and oats.

Most stems have a vertical structure, but some plants have modified stems that enable them to spread horizontally, reproduce asexually, and store energy (food) for survival (Figure 6-10). Examples of these modified stems include

* Rhizomes, which are horizontal, underground stems that are important for asexual reproduction in plants. Plants with rhizomes include Kentucky bluegrass and Canada thistle.

* Stolons, which are horizontal, aboveground stems. These stems are also called runners. Plants with stolons include white clover, strawberry, and Bermudagrass.

* Tubers, which are enlarged, fleshy stems containing a significant amount of storage carbohydrates. The potato is a tuber, and the eyes of the potato are buds.

* Bulbs, which have a narrow stem surrounded by layers of concentric, fleshy rings. You can see these rings by cutting an onion or tulip bulb.

* Corms, which are compressed, solid, fleshy portions of an underground stem. Timothy and crocus have corms.

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Classification Based on Growth Habit

Perennials have been further characterized as crown-formers or clone-formers (Figure 6-11). Parent plants of crown-forming perennials often have deep tap roots and can survive for several years. Crown-formers depend on seed production for long-term persistence of the genotype. Alfalfa and dandelion are examples of crown-forming perennials.

In clone-forming perennials, the original parent plants are replaced by new plants established asexually by rhizomes, stolons, or adventitious roots. Because of their "mobility," clone-formers can exploit new environments. Species that send out rhizomes are especially tolerant to climate extremes. Quackgrass, Jerusalem artichoke, and big bluestem are rhizome-forming plants. Bermudagrass and white clover are stolon-forming plants.

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Leaves

Leaves are the primary organs where the essential processes of photosynthesis and transpiration occur. Light energy is captured in the chlorophyll of the leaf mesophyll cells, and gases such as carbon dioxide (C[O.sub.2]), oxygen ([O.sub.2]), and water ([H.sup.2]O) are exchanged through stomata on the leaves. Stomata are composed of two guard cells whose turgor pressures open or close the pore opening. All leaves have veins, or venation, that are part of the vascular system of the plant for the movement of water and nutrients.

The leaves of monocot plants (grasses) consist of a sheath, blade, and collar (Figure 6-2). The blade is the flat surface of a leaf that is exposed for maximum interception of sunlight. The sheath is the lower portion of the leaf that surrounds the stem. The collar of a leaf, located at the union of the blade and sheath, has appendages such as auricles and ligules that can be used to distinguish species (Figure 6-12). Monocot leaves have parallel venation.

Dicot plants have either simple or compound leaves (Figure 6-13). Simple leaves are composed of a blade and a petiole that attaches the blade to the stem. Sugar beets and canola have simple leaves. Compound leaves have a blade made up of a number of leaflets that are attached to the petiole. These leaflets can have either a palmate or pinnate arrangement. Soybeans have pinnately compound leaves. At the base of the petiole, some dicot species also have small attachments called stipules. Dicot leaves have netted venation.

Some plants have other modified leaves such as tendrils that help the plant to climb. In peas, pumpkin, and vetch, tendrils wrap around other plants or structures and provide support for the stem of the plant.

Flowers

The process of sexual reproduction in plants begins in flowers. Flowers are modified leaves that may retain some leaf-like characteristics. Flower forms vary among plant families, but a general flower consists of four parts: stamens, pistil, petals, and sepals (Figure 6-14).

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Stamens are the male component of the flower. Each stamen has a filament or stalk that supports the anther. The anther produces the pollen that contains sperm needed for sexual reproduction. Most flowers have several stamens.

The pistil is the female reproductive portion of the flower. The pistil is composed of the stigma, style, and ovary. During pollination, pollen attaches to the stigma and produces a pollen tube, which the sperm travels through in the style to the ovary. The ovary contains the ovules which contain an egg. Fertilization occurs when the male sperm and female ovules unite. These fertilized eggs within the ovary develop into seeds (Figure 6-15).

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Petals are showy, modified leaves that are important for attracting pollinators. They also provide some protection to the immature stamens and pistils. Following fertilization, the petals often die. Located at the base of the petals, sepals are the green, leaf-like structures. In some plants such as cotton and alfalfa, the sepals are large and cover the entire flower bud before it opens. After the flowers open, sepals become relatively small.

Classes of Flowers

Flowers can be categorized by whether all the flower parts are present (complete or incomplete) or by whether all the reproductive parts are present (perfect or imperfect).

Complete flowers are composed of stamens, pistils, petals, and sepals. Many plants that depend on cross-pollination (the movement of pollen from one plant to another) have complete flowers. Legumes are an example of plants that have complete flowers.

Incomplete flowers lack one or more of the four flower parts. Grass flowers lack both sepals and petals and instead have leaf-like coverings (lemmas and paleas) around the stamens and pistils, with glumes providing an additional covering (Figure 6-16). Awns are long appendages on the lemmas that are present in some species.

Perfect flowers have both stamens and pistils present. Soybean and alfalfa have perfect flowers.

Imperfect flowers are incomplete flowers that lack either the stamen or pistil. Corn and squash are examples of plants with imperfect flowers. There are two categories of plants with imperfect flowers: monoecious and dioecious plants.

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* Monoecious plants have separate male and female flowers on the same plant. Corn is an example of a monoecious plant. The corn tassel is the male organ and is composed of only stamens, whereas the ear, or female organ, is composed of only pistillate flowers. Corn silks are elongated styles with a sticky stigma attached on the end.

* Dioecious plants have separate male and female plants. Therefore, if fertilization is to occur, both male and female plants must be present. Hops, hemp, and buffalograss are examples of dioecious plants.

Inflorescences

Some crops such as cotton have flowers that occur singly, but many crops have flowers clustered together on branches or a system of branches. These arrangements of flowers are called inflorescences. In inflorescences, individual flowers (also called florets) are sometimes borne on a stalk called a pedicel (Figure 6-17). The flowers of legumes and grasses generally occur in inflorescences. There are several different types of inflorescences.

* The spike has flowers born directly on the main stalk of the inflorescence. Examples include wheat, rye, and barley.

* The panicle and raceme inflorescences have flowers in a branched pattern. Grasses such as oat and smooth brome have highly branched panicle inflorescences, whereas alfalfa has a raceme.

* The umbel is an inflorescence with all the flower pedicels originating from a single spot. Birdsfoot trefoil and carrot have umbel inflorescences.

* The head is an inflorescence with disk and ray flowers. Disk flowers are in the center of the head and develop into the seed, whereas ray flowers are colorful to attract insects. Examples of this type of inflorescence are sunflower and Canada thistle.

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Fruits

The ovary of a plant is the enlarged, basal portion of the pistil. After fertilization, a ripened or mature ovary containing the seed or seeds is called a fruit. A fruit may be classified as either fleshy or dried. Fleshy fruits have a soft or "fleshy" consistency and may be consumed by humans or other animals as food. This consumption of the fruit is important for dispersal of the seed. Examples of fleshy agronomic fruits include pumpkin and squash. Apple and tomato are fleshy fruits of horticultural significance.

Dry fruits are those that have hard seeds contained within the fruit wall. Dry fruits have either ovaries that split open at maturity and release their seeds (dehiscent) or ovaries that do not split open when mature (indehiscent). Examples of dehiscent fruits (generally called pods) include soybean, alfalfa, canola, and milkweed.

All legumes have pods. Indehiscent fruits include sunflowers and grasses. The seed of grasses such as wheat and corn is actually a caryopsis, another type of fruit, that has the ovary wall (pericarp) attached to the seed.

Seeds

In plants, seeds are the mature, fertilized ovules within the ovary. Each seed contains an embryo (an immature plant) that has the genetic material essential for production of future generations of the species. Seeds also have food storage reserves and are protected by a seed coat. The food storage tissues of a seed are either cotyledons or endosperm. Seeds and plants are often described as monocots or dicots, the distinction being the number of cotyledons (seed leaves) they have (Figures 6-18 and 6-19). Monocots store their principal food reserves in the endosperm. Dicots store all their nutrients within their two cotyledons.

Seed Germination

Seeds are amazing structures that can remain viable for extended periods of time. Germination is the first step in the development of a plant from seed. To begin the process of germination, seed dormancy needs to be broken. Factors required to break dormancy include the correct environment (temperatures, moisture, light) and the decay of the seed coat by either natural or mechanical means. When these conditions have been met, germination begins.

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The growth of the radicle (or embryonic root) is the first stage in germination. The radicle absorbs water and swells, causing the seed coat to split. The embryonic plant begins using its food reserves from either the cotyledons or endosperm to continue growing toward the soil surface.

There are two primary types of plant emergence from the soil: hypogeal and epigeal. In epigeal emergence, the hypocotyl, the portion of the embryonic plant between the radicle and the cotyledons, lengthens and carries the cotyledons above the soil surface. After the cotyledons emerge, the first true leaves of the plants develop, and the cotyledons begin to wither. The cotyledons develop chlorophyll and for a short time until true leaves develop, they conduct photosynthesis. This type of emergence is typical of dicots such as soybean and cotton (Figure 6-18). In hypogeal emergence, the cotyledons remain beneath the soil surface and the epicotyl emerges from the soil. Once the epicotyl breaks the soil surface, the first true leaves will develop and grow (Figure 6-19). This type of emergence is typical of grasses such as corn, wheat, and smooth bromegrass.

Review Questions

1. Describe two important differences between monocot and dicot plants.

2. What are the four basic types of plant tissues, and what are they composed of?

3. Define the term meristematic tissue. Describe the different types of meristems found in plants.

4. Describe the functional differences between the vascular tissues, xylem, and phloem.

5. What are the four organs that most plants have?

6. What are the main functions of roots? Name two types of root systems.

7. Describe the main functions of the stem.

8. Name and describe three types of specialized stems.

9. What are the main functions of leaves? Compare the major leaf parts of monocots and dicots.

10. What are the differences between complete and incomplete flowers?

11. What are the differences between perfect and imperfect flowers?

12. Explain the difference between monoecious and dioecious plants.

13. Describe three types of inflorescences.

14. How are seeds produced, and what parts do they contain?

15. What are the two types of germination? Describe each.

References

Capon, B. (2005). Botany for gardeners. Portland, OR: Timber Press, Inc.

Esau, K. (1960). Anatomy of seed plants. New York: John Wiley & Sons, Inc.

Levetin, E., & McMahon, K. (1996). Plants and society. Dubuque, IA: William C. Brown Publishers.

Moore, R. W., Clark, D., & Stern, K. R. (1995). Botany. Dubuque, IA: William C. Brown Publishers.

Rost, T. L., Barbour, M. G., Thorton, R. M., Weier, T. E., & Stocking, C. R. (1979). Botany: A brief introduction to plant biology. New York: John Wiley & Sons, Inc.

Stern, K. R. Plant biology. Dubuque, IA: William. C. Brown Publishers.
Table 6-1
Monocot and dicot characteristics.

Monocots (1 cotyledon)               Dicots (2 cotyledons)

Usually herbaceous                   Herbaceous or woody
Linear leaves with parallel veins    Leaves with netted venation
Vascular bundles scattered in stem   Vascular bundles in a ring
Fibrous root system                  Taproot system
Hypogeal germination                 Usually epigeal germination
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Publication:Introduction to Agronomy, Food, Crops, and Environment
Geographic Code:1USA
Date:Jan 1, 2009
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Previous Article:Chapter 5: Chemistry of food and plants.
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