Exercise A Roots to Anchor and Absorb

The typical plant root is a vertical, underground axis, and as such it provides the plant with a firm footing in the soil. Roots also function as the primary source of water and mineral absorption. Additionally, roots are often storage sites for plants. In arid regions, the root may store water. Roots may also be enlarged for the storage of starch, an energy reserve.

The root systems are organized as either tap or fibrous. Common to many dicots, a tap root system has a single main root, enlarged for water and/or food storage, and many smaller branch roots. The fibrous root system is typical of monocots in which there are several main roots of equal size.

There arc several functional regions at the tips of roots. The root cap is the thimble-shaped region of short-lived cclls at die very end of a root. It protects underlying tissues as the root grows through the soil. It is also the part of the root that detects direction so that roots grow down, in the direction of gravity. Immediately above the root cap lies die apical mcristem or zone of cell division, a site of actively dividing cells that accounts tor the growth of roots and replaces cells in the root cap as they are worn off. Moving upwards, the next region is the zone of elongation, an area of rapid root growth as cclls elongate. The zone of maturation is the final region in which embryonic cclls differ entiatc into specialized tissues that make up die anatomy of a root. It is marked by the appearancc of root hairs, fine extensions of the epidermis. Root hairs arc the sites where water and minerals are first absorbed into the root.

In a cross section, the root tissuo arc arranged into two distinct regions. A larger outer region, called the cortex, surrounds the central corc or vascular cylinder. The cortex is adapted for storage and the passage of water and minerals to the vascular cvlindcr. It is in the m vascular cylinder that die conducting tissues of xylcm and phloem are located.

Materials Needed for Exercise A

Compound light microscope

Dissecting microscope

Dropper botde of distilled water

Dropper bottle of iodine solution (I2KJ

Dropper bottle of phloroglucinol

Elodca specimen

Epiphytic orchid specimen

Fibrous root example—grass

Filter paper to line petri dishes

Glass slides

Parsnips

Petri dish

Prepared slide of buttercup (Ranunculus) root cross section

Prepared slide of Elodca root cross scction Prepared slide of orchid root cross scction Radish seedlings Razor blades, single-edged Taproot example—dandelion

Procedure for Exercise A

1. Observe the display of the root systems of a grass and a dandelion (fig. 4.1). How would you describe the differences between these two systems? Which has a single main root? In which are there several roots of equal size? Which is an example of a taproot system? Which is a fibrous root system?

Taproots can extend to significant depths in some species; in mcsquite, the taproot reaches a depth of 32 m <96 ft). Taproots arc often greatly enlarged for food and/or water storage, 'l hc fibrous root, by contrast, is shallow but spreading. Flow can these two different root system designs both achieve the same result of securing a water supply? What arc their alternative strategies?

Weeds With Tap Fibrous Roots

FIGURE 4.1 ROOT SYSTEMS. (A) THE FIBROUS ROOT SYSTEM OF A GRASS. (8) THE TAPROOT SYSTEM OF A DANDELION.

Vascular Stele

Ap<cai meristem

Root cap

Root cap

Ap<cai meristem

The ccntral core is the vascular cylinder or stele. Add a few drops of phloroglucinol to the stele of the parsnip section. What color change do you notice? What type of tissue is found in the stele? (Refer to Laboratory Topic 3.

7rg caiote

FIGURE 4.2 ROOT HAIRS AND ZONES IN A RADISH SEEDLING.

  1. To view the zones of a root tip, obtain a young radish seedling and quickly transfer it to a petri dish kept humid with a lining of moist filter paper. Examine the seedling through the petri dish lid under the dissecting microscope. Locate the thimble shaped root cap (fig. 4.2) at the very tip of the root. Immediately behind die root cap lies the zone of cell division. Next locate the zone of elongation. The zones of cell division and elongation are the sites of root growth. Still further up, the zone of maturation is indicated by the appearance of root hairs. Each root hair is an extension of a single epidermal cell and extremely fragile.
  2. To examine the anatomy of a mature root, obtain a fresh parsnip. Make a thin cross section with a razor blade at the smaller end of the root. Place your thin section on a glass slide and view under scanning (4 x objective) or low (10 x objective) power of the compound light microscope. Do not cover with a cov-erslip. Observe the large, diin-walled cells of the cortex, the outer region of the root. Add a drop of iodine, the indicator test for starch. What color change do you notice? What type of tissue comprises the root cortex? What is stored here?

Let's take a more detailed look at the internal anatomy of a root by viewing the prepared slide of the buttercup root.

Obtain a compound light microscope and a prepared slide of a cross section of buttercup • Ranunculus) root. The prepared slide has been sliced very thin, making it easier to observe the fine details.

Again, observe the slide first under scanning <4 x objective) or low power 10 x objective) to sec the overall organization of the root (fig. 4.3). From the exterior, note the single layer of epidermis. Immediately inside the epidermis and extending to the vascular cylinder is the cortex. As noted previously, the cortcx consists of parenchyma cells, many of which contain stained starch grains. The innermost layer of cortex is the cndodcrmis. The endo-dcrmis has been stained red. A waxy material, the Casparian strip, encircles each cndodcrmal cell wall. Only the faces of the cell walls adjoining the cortex and vascular cylinder lack a Casparian strip. Because of this strip, water and minerals pass through the endodermal cells, not between them.

Within die vascular cylinder, note the red-stained cross or star of xylcm. The thin walled phloem is located between the xylcm points. This pattern of xylem and phloem within the vascular cylinder is typical of dicot roots.

Root anatomv can be modified bv the environment.

Examine a living specimen of Elodea, an aquatic angiosperm. Elodea is a hydrophyte, a plant adapted to a very wet environment—in this case, ponds and streams. Elodea is common in freshwater svstems

throughout North America, and you may recognize it as a popular aquarium plant. What observations can you make about the roots of the living Elodea spec imcn? How do they differ in appearance from the root system of a typical land plant of grass or dan deliqn that you examined earlier?

Dicot Root Vascular Cylinder

Vascular cylinder

Cortex

Endoderms Phloem

FIGURE 4.3 ROOT ANATOMY (A) DICOT ROOT (BUTTERCUP) IN CROSS SECTION. (8) THE VASCULAR CYLINDER TYPICALLY SHOWS A STAR OR CROSS PATTERN.

Epiphytic Orchid Cross SectionMicroscopic Morphology Buttercup RootRoot Buttercup

Now examine the prepared cross section of the Elodca root (fig. 4.4/r and b). How does the anatomy of the Elodca root differ from that of the buttercup root viewed earlier? Why do you think these modifications in the Elodca root evolved? (Hint: Examine the vascular cylinder closely!)

Vascular Cortex cylinder Epidermis

6. Many tropical orchids are epiphytes, or aerial plants. These are nonparasitic plants that rest upon the branches of other plants. Picturc a gigantic tropical tree festooned with hundreds of orchids and vines. Observe the epiphytic orchid on display. Locatc the roots. Unlike the roots you may typically think of, these epiphytic orchids have aerial roots, roots growing not in the soil but out into the air. Noticc that the tips of the aerial roots are green. What is present that imparts this green color? What

Vascular

Velamen cyl nder Pith Cortex

FIGURE 4.4 ROOT ADAPTATIONS. (A) THE ANATOMY OF AN ELODEA ROOT SHOWS ADAPTATIONS TO AN AQUATIC ENVIRONMENT. (3) CLOSE-UP OF VASCULAR CYLINDER OF ELODEA ROOT. (C) THE VASCULAR CYLINDER OF A MONOCOT ROOT TYPICALLY SURROUNDS A PITH. NOTE THE VELAMEN. A CHARACTERISTIC OF EPIPHYTIC ORCHID ROOTS.

function is indicated by the green pigment diat we do not usually associate with roots?

Note that away from the root's growing dp, the root is no longer green, and is covered with a white tissue. This is vclamcn.

Obtain a slide of an orchid root cross section, and locate the epidermis, cortex, cndodcrmis, and vascular cylinder (fig. 4Ac). Note the different organization of the xylem and phloem in the vascular cylinder of the monocot root. Unlike the dicot root, monocot roots have a center of parenchyma tissue, or pith, in the vascular cylinder. Sketch the stele of a monocot root.

Locate the velamen on die periphery of the cross section. What tissue appears to give rise to vclamcn? What function do you think velamen serves in the aerial root of an orchid?

Record your findings about root structure of monocots and dicots in worksheet 4-1 at the end of this laboratory topic.

EXERCISE B: The Nuts and Bolts of Stem Anatomy

When you think of stems, chances are you picturc an upright axis of green that supports leaves. But in addition to support, the stem is the conduit for water and minerals up from the soil and roots as well as for transporting the organic products of photosynthesis (sugars) to the nonphotosynthctic, grow ing, and storage regions of the plant. Some stems are enlarged for storage of cither water or starch. In this exercise, you will use a nut-and-bolt microtome to thin-section monocot and dicot stems in order to get a close-up look at their anatomy.

Materials Needed for Exercise B

Asparagus stem Beakers, 30 ml Compound light microscope Covcrslips

Creeping charlic {Plcctranthiis atistralis) Dissecting needles Dropper botdc of distilled water Dropper botdc of mediylene blue Glass slides

Nut-and-bolt microtomes Paraffin wax, melted Petri dish lid

Razor blade, single-edged

Procedure for Exercise B

1. Obtain a small beaker and add a small volume (about 0.5 cm depth) of methylene blue. Using a razor blade, cut ofYa 5 cm section of the stem of creeping charlic and immediately placc in the beaker, making sure the base of the stem is in contact with the dye. Allow the stem to set in the dye for about 10 minutes to give enough time for the methylene blue to be transported up the xylem.

As you wait, prepare as above a section of asparagus stem to placc in methylene blue.

  1. Obtain two nut-and-bolt microtomes. A microtome is a device that is used to cut very thin sections for viewing under a microscope. Screw the nut so that it is on the last few rungs of the bolt. This should make a deep well.
  2. Cut off about a 3 cm section of creeping charlic, a member of the dicot mint family. Trim the stem so that it fits into the well crcatcd b\ the nut and bolt and

extends slightly above it. Obtain the melted paraffin from die warming tray. Position die stem section in the center of die well and nil the w ell with paraffin. Allow a cap of paraffin to top the stem. Set the nut and bolt in an undisturbed placc to allow the paraffin to harden. Tliis will take 10-15 minutes. As you wait, prepare the stem of asparagus, a member of the monocot lily family, in die same way.

4. When the paraffin has completely hardened, use a razor blade to trim off the top by cutting level with

Making Microtome

FIGURE 4.5 NUT-AND-BOLT MICROTOME READY FOR SECITONING.

Note the outermost single layer of cells, the epidermis (fig. 4.6). Note the large ccnter of parenchyma tissue. As in roots, a ccnter of parenchyma tissue is a pidi. What would the region around the pith he called?

FIGURE 4.5 NUT-AND-BOLT MICROTOME READY FOR SECITONING.

Observe die interrupted ring of vascular bundles, the xylem stained by methylene blue. In stems, the xylem and phloem arc organized into bundles rather than a vascular cylinder. A ring of vascular bundles is characteristic of the herbaceous dicot stem.

7. Now examine die asparagus cross section you made. Again, what tissue comprises the outermost single layer in this herbaceous stem?

Epidermis Pith Vascular bundles

Epidermis Fibers Phloem Xylem Cortex

Herbaceous Dicot Stem

FIGURE 4.6 (A) DICOT STEMS HAVE VASCULAR BUNDLES IN A RING. (8) CLOSE-UP OF VASCULAR BUNDLES IN A DICOT STEM.

the top of the nut (fig. 4.5). Then twist die bolt so that the paraffin-embedded stem section is just a few millimeters above the nut. Shave off a section, and place it in the lid of a petri dish. Repeat the same procedure with the asparagus stem. Try to make your stem sections as thin as possible.

  1. Place sections in a petri dish lid. Use dissection needles to gently remove the wax from the stem section.
  2. Place a stem section on a glass slide, add a drop or two of distilled water, and cover it with a coverslip. Observe under the compound light microscopc.
  3. Examine the creeping charlic stem first under scanning (4 x objective) or low (10 x objective) power.

Epidermis Pith Vascular bundles

Locate the methylene blue stained vascular bun dies in asparagus. Do they form a ring as they did in the creeping chariie? Is a pith observable? How would you describe the pattern of vascular bundles in the asparagus, a monocot stem (fig. 4.7/7.?

Epidermis Fibers Phloem Xylem Cortex

FIGURE 4.6 (A) DICOT STEMS HAVE VASCULAR BUNDLES IN A RING. (8) CLOSE-UP OF VASCULAR BUNDLES IN A DICOT STEM.

Plash Arc hi i h< ture 43

Vasoj'ar bundies

Vasoj'ar bundies

W&M

Companion cell

S eve tut>e member

Phloem a)

Companion cell

S eve tut>e member

Phloem

Grape Phlome Structure

FIGURE 4.7 (A) IN MONOCOT STEMS. THE VASCULAR BUNDLES ARE SCATTERED. (8) CLOSE-UP OF A VASCULAR BUNDLE IN CORN. A MONOCOT STEM.

S. To see fine details, obtain a prepared slide of a cross section of a corn stem corn (Zta mays). Zoom in with 40 x high power on one of the vascular bundles. The entire bundle has been compared to a monkey face in appearance (fig. 4.7 b). What type of cells or tissues would the rwo large« red "eyes" be?

The uforehead" of the "monkey face" is phloem tissue, and you may actually see a pattern of small square companion cells alternating with the larger sieve-tube members. The "mouth" is an air space. The entire ~facc>" or vascular bundle, is surrounded by thick-walled fibers.

Record your findings about stem structure in monocots and dicots in worksheet 4 I at the end of this laboratory topic.

EXERCISE C: Leaves for Identification

Leaves arc the primary photosynthctic organs tor most plants. As with stems and roots, the leaves of some species have been adapted to store water. Think of succulent plants like the jade plant or agave. In other species, leaves may be starch storing. Leaves also are important players in gas exchange and in the water stream that flows upwards from the soil and transpires from the leaves. These functions of leaves arc also discussed in Laboratory Topic 5.

Leaves arc composed of three parts: a blade, a petiole, and a pair of stipules (tig. 4.8/1). The blade is the flat.

expanded portion of the leaf. The petiole is the stalk that supports the blade; in some plants, the petiole is absent. When this happens, the leaf is said to be sessile. Stipules are paired structures that may be found at the base of leaves. Their presence in many specics is temporary sincc they arc often shed early in the growing season. Stipules vary gready in appearance. Some are like the spines, commonly called thorns, at die base of a rose leaf. Odiers, such as those of the sycamore tree, look like miniature versions of the leaves.

Leav es are further classified on the basis of composition, arrangement, and venation {iig. 4.Sa). The appcarance of the blade is important in determining leaf composition. If a blade is undivided, the leaf is classified as simple. However, if the blade is divided into separate pieces or leaflets, the composition of the leaf is said to be compound. In some leaves, even the leaflets are subdivided!

Often it may be difficult to tell whether you are looking at a simple leaf or a leaflet. Luckily, the position of the axillary bud (fig. 4.8/*) is a good clue as to whether the leaf is simple or compound. The upper angle that forms between the top surface of a leaf and the stem iscallcd the axile, and it is here that the axillary bud can be found. Axillary buds arc onlv found in the axile of a leaf. Leaflets

do not have axillary buds.

There arc two types of compound leaves. When the leaflets occur in a fcatherlike pattern, it is a pinnately compound leaf. If all die leaflets arise from a single point, it is a palmatcly compound leaf (fig. 4.cS/i).

Arrangement pertains to how leaves arc ordered on a stem. Nodes arc areas on a stem that give rise to leaves or branches. The areas of stem between nodes are caHcd in tern odes. If there is only one leaf at a node, the leaf arrangement is called alternate. If the number of leaves

Bud And Axillary Nodes

Axillary bud

Simple

Axillary bud

Simple

Blade

Blade

Palmately Pinnateîy compound compound

(a) Composition

Palmately Pinnateîy compound compound

Parallel Venation Patterns
  • c) Venation Parallel
  • a) Composition
Leaf Venation Pattern

Alternate (b) Arrangement

Node

Opposne Wnorfed

Alternate (b) Arrangement

(c) Venation Parallel

Node

Opposne Wnorfed

FIGURE 4.8 LEAF MORPHOLOGY. (A) A LEAF HAS A BLADE. A PETIOLE, AND A STIPULE PAIR. AXILLARY BUDS ARE FOUND IN THE AXILE. IN LEAF COMPOSITION. LEAVES MAY BE SIMPLE. CONSISTING OF A SINGLE BLADE. OR COMPOUND. IN WHICH THE BLADE IS SUBDIVIDED INTO LEAFLETS. (8) LEAF ARRANGEMENT. ALTERNATE. OPPOSITE. OR WHORLED INDICATES THE NUMBER OF LEAVES COMING OFF A NODE. (C) LEAF VENATION. THE VENATION PATTERN IS COMMONLY PARALLEL IN MONOCOT LEAVES AND NET IN DICOT LEAVES.

at a node is upped to two, the arrangement is opposite. With more than two leaves at a node, the arrangement is whorlcd (fig.

Leaf venation is another easily recognizable characteristic. Leaf veins are continuations of the vascular bundles, you saw earlier in the stem. Venation refers to the pattern of the veins, and once jgaiih it divides along monocot and dicot class lines. In most monocot leaves, all of the major veins arc parallel. In dicot leaves, the veins branch into ever smaller veins to forming an overall lacy appearance. This type of vein arrangement is callcd net (fig. 4.8 c).

In this exercise, you will learn how a diagnostic key based on the characteristics of leaves can be helpful in tree identification. You will also learn the components of leaf anatomy and how the anatomical design of leaves can be modified bv environmental selection.

Materials Needed for Exercise C

Branch of living privet

Compound light microscope

I^rafv branches for tree leaf key

Leaves to demonstrate composition, venation, and arrangement

Living Aloe vera plant

Prepared slide of aloe (Aloe) leaf cross section Prepared slide tof privet (Ugustrum) leaf cross section

TABLE 4.1 A LEAF KEY TO COMMON TREES

a. Leaves simple b b. Margin entire c c. Leaves opposite or whorled . . . d d. Leaves opposite ... dogwood d. Leaves whorled ... .eatalpa c. Leaves alternate . — redbud b. Margin lobed . .. . e e. Leaves alternate oak e. Leaves opposite maple a. Leaves compound . . . f f. Pinnarely compound g g. Leaves alternate black walnut g. Leaves opposite ... ash f. Palmatcly compound buckeye

Building Your Own Greenhouse

Building Your Own Greenhouse

You Might Just End Up Spending More Time In Planning Your Greenhouse Than Your Home Don’t Blame Us If Your Wife Gets Mad. Don't Be A Conventional Greenhouse Dreamer! Come Out Of The Mould, Build Your Own And Let Your Greenhouse Give A Better Yield Than Any Other In Town! Discover How You Can Start Your Own Greenhouse With Healthier Plants… Anytime Of The Year!

Get My Free Ebook


Responses

  • Philipp
    What type of tissues appears to give rise to velamen?
    6 years ago
  • sandra
    How do roots grow when the direction of gravity changes pictures?
    6 years ago
  • ARJA
    Do monocots have a tap root or fiberous roots?
    6 years ago
  • michael
    Where is a sieve cell in comparison to the cortex?
    6 years ago
  • Eerik
    Why is the root not completely worn off as the root penetrates the soil?
    4 years ago
  • welde
    Why are there so many roots hairs on the radish seedling roots?
    4 years ago
  • Haben
    What is phloroglucinol in botany?
    4 years ago
  • Awet
    How roots absorbed arrchor?
    4 years ago
  • viljo
    What type of tissue comorises the root coetex of parsnip?
    2 years ago
  • William
    How are the vascular bundies in the stem of corn arranged?
    2 years ago

Post a comment