Cell Structure Through Macro-Morphology

Lab 1: Cell Structure Through Macro-Morphology

Objectives

By the end of this laboratory exercise, you will be able to:

• Infer cellular organization and tissue architecture from macroscopic plant structures

• Identify and distinguish between major plant tissue types through visual and tactile examination

• Connect microscopic cellular features to observable plant characteristics at the macro scale

• Apply knowledge of cell types (parenchyma, collenchyma, sclerenchyma, xylem, phloem) to real plant specimens

• Develop skills in botanical illustration and comparative analysis

• Practice scientific observation, documentation, and peer review

Important Terms

Plants are composed of three primary tissue systems: dermal (protective outer layers), ground (bulk of the plant body), and vascular (transport tissues). Each system contains specialized cell types that can be inferred from macroscopic characteristics:

Cell Types You'll Identify:

Parenchyma cells: Thin-walled, living cells that appear soft and spongy; often store water and nutrients

• Think of these like water balloons packed together - soft and squishy

Collenchyma cells: Unevenly thickened walls providing flexible support; appears stringy or fibrous

• Like the strings in celery - flexible but supportive

Sclerenchyma cells: Thick-walled, often dead at maturity; provides rigid support and protection; feels hard or gritty

• Think of wood - hard, rigid, and tough

Xylem tissue: Dead cells forming tubes for water transport; appears as organized channels or rings in cross-section

• Like tiny straws bundled together carrying water upward

Phloem tissue: Living cells transporting sugars; typically found near xylem but harder to distinguish macroscopically

• The food delivery system of the plant

Even without a microscope, careful observation of texture, structure, color, and function can reveal the underlying cellular organization of plant tissues.

Materials

Plant Specimens (5 types required)

Modified for Mohave County, Arizona: Select locally available plants from your area. Here are excellent options that grow well in the region:

1. Succulent stem - demonstrates water storage parenchyma

   • Best local options: Prickly pear cactus pad (Opuntia), aloe vera, or jade plant (common in yards)

   • Why it works: Desert succulents have lots of water-storing parenchyma

2. Woody stem - shows secondary growth and extensive sclerenchyma

1. • Best local options: Mesquite, creosote bush, or tamarisk branch (1-2 cm diameter)

   • Why it works: Desert woody plants show clear growth rings and tough tissues

3. Herbaceous dicot stem - displays primary growth with distinct vascular bundles

1. • Best local options: Sunflower, tomato plant (from garden), or celery from store

   • Why it works: Soft stems with visible vascular bundles in a ring pattern

4. Monocot stem - exhibits scattered vascular bundles

1. • Best local options: Corn stalk (from local farm or garden), or yucca stem

   • Why it works: Vascular bundles scattered throughout (not in a ring)

5. Modified stem - demonstrates specialized storage structures

1. • Best local options: Potato (from store), onion bulb, or ginger root

   • Why it works: Modified for storage, shows mostly parenchyma tissue

Equipment and Supplies

• Knife or razor blade (sharp!)

• Metric ruler with millimeter markings

• Camera with zoom (phone camera works great)

• White paper

• Paper towels

• Three clear cups

• Food coloring (red or blue works best)

Pre-Lab Preparation

1. Specimen collection: Gather all five plant types at least 24 hours before the lab. Store in plastic bags in refrigerator if not using immediately to maintain freshness.

   • Desert tip: Collect early morning when plants are most hydrated

2. Review diagrams: Study cross-sectional diagrams of plant stems

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   • Notice the three layers: outer protective layer, middle bulk tissue, and central region with vascular bundles

3. Prepare colored water: Mix 10-15 drops of food coloring per 100 mL of water in each cup. Prepare one solution for each specimen you'll use in Part 3.

Procedure

Part 1: Cross-Section Analysis

Step 1: Creating cross-sections

For each of the five plant specimens:

a. Select a region of the stem that appears healthy and representative (avoid damaged, diseased, or flowering portions).

b. Using the ruler, measure 3-5 cm from one end and mark this location lightly with a pencil or fingernail.

c. Place the specimen on the cutting board with the marked area overhanging slightly.

d. With one smooth, firm motion, cut straight through the stem perpendicular to its length. Avoid sawing motions, which distort tissues.

• Think of it like slicing a loaf of bread - one smooth cut works better than sawing

• If the specimen is hard (woody stem), you may need to use a sawing motion, but work slowly and carefully.

e. Examine the cut surface immediately. If it appears crushed or uneven, make a fresh cut 2-3 mm closer to the end to reveal undamaged tissue.

f. Create 2-3 additional slices from each specimen, each 1-2 cm thick. These will be used for different observation activities.

g. Pat the cut surface gently with a paper towel to remove excess moisture, but do not allow it to dry out completely.

Step 2: Initial observations

For each cross-section, immediately observe and record:

a. Overall shape: Is the cross-section circular, oval, square, star-shaped, or irregular?

b. Surface moisture: Is the cut surface very wet (succulent), moderately moist (herbaceous), or relatively dry (woody)? Gently touch the surface to assess.

c. Color patterns: Note any color differences between regions. Common patterns include:

• Green outer layers

• White or cream inner tissues

• Brown or red vascular tissues

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d. Structural features: Can you see any of the following?

• Distinct outer "skin" or rind

• Ring patterns (growth rings in woody stems)

• Dots or clusters arranged in patterns (vascular bundles)

• Hollow center or pith cavity

• Radiating lines or rays

• Fibrous threads or strands

e. Odor: Note any distinctive smells when freshly cut

• Desert plants like creosote have very distinctive odors!

Step 3: Photography

Proper documentation is essential for later analysis and comparison.

a. Place each cross-section on a white background with adequate lighting. Natural indirect light or a bright LED lamp works best.

b. Place a metric ruler next to (not on top of) the specimen for scale reference.

• The ruler shows how big things really are in your photos

c. Take multiple photos of each specimen:

• Overview shot showing entire cross-section

• Close-up of center region

• Close-up of outer/edge region

• Close-up of any distinctive features (vascular bundles, rings, rays)

d. For better detail, use your phone's macro mode if available, or use a hand lens held between the camera and specimen.

e. Ensure photos are in focus and well-lit. Take multiple shots of each view to ensure at least one high-quality image.

Step 4: Regional identification

For each specimen, identify and label three major regions:

a. Outer protective layer (dermal tissue system):

• Usually thin, may be green, brown, or corky in appearance

• Often tougher or more resistant to cutting than inner tissues

• May peel away as a distinct layer

• Record the thickness in millimeters

b. Middle tissue region (ground tissue system):

• Usually the bulk of the stem

• May appear uniform or divided into distinct zones

• Texture varies: firm, spongy, stringy, or chambered

• Record the diameter or width of this region

c. Central region (may include vascular tissue):

• May appear as a solid core, hollow cavity, distinct bundles, or ring pattern

• Often different in color or texture from surrounding tissue

• May contain obvious channels or strands

• Record the diameter of this region

Step 5: Creating scale drawings

For each specimen:

a. Measure the exact diameter (or width and height if not circular) of your cross-section to the nearest millimeter.

b. On graph paper, create a 1:1 scale drawing (actual size) of the cross-section outline.

• 1:1 scale means if your specimen is 20mm wide, your drawing should be 20mm wide

c. Alternatively, trace around the specimen directly on white paper if the surface is flat enough.

d. Within this outline, sketch the boundaries between different tissue regions as accurately as possible.

Step 6: Texture annotation

Using close examination:

a. Examine each tissue region carefully and describe its texture using specific terms:

• Soft and wet: Likely water-storing parenchyma (common in succulents, pith)

• Spongy or foam-like: Likely air-filled parenchyma (common in herbaceous stems)

• Stringy or fibrous: Likely collenchyma or phloem fibers (found in celery, beneath epidermis)

• Hard or gritty: Likely sclerenchyma or heavily lignified cells (found in woody stems, seed coats)

• Hollow or chambered: Specialized parenchyma or aerenchyma (common in monocots)

• Organized channels or dots: Likely xylem vessels (visible as small holes or clusters)

b. Label each region on your drawing with its texture description.

c. Use different colors to shade different regions, creating a texture map:

• Light green for soft parenchyma

• Darker green for firmer tissues

• Brown for woody/hard tissues

• Yellow or white for hollow regions

• Red dots for vascular bundles

d. Create a key explaining your color coding and texture categories.

Step 7: Quantitative measurements

Measure and record the following for each specimen:

a. Total stem diameter: _____ mm

b. Thickness of outer protective layer (epidermis/periderm): _____ mm

c. Width of middle tissue region: _____ mm

d. Diameter of central region: _____ mm

e. If vascular bundles are visible as distinct units:

• Number of bundles visible: _____

• Diameter of individual bundles: _____ mm

• Pattern of arrangement (ring, scattered, other): _____

f. Calculate proportions:

• % of cross-section occupied by outer layer: _____

• % occupied by middle region: _____

• % occupied by central region: _____

Tip: To calculate percentage, divide the region's width by total diameter, then multiply by 100

Step 8: Setting up the transport experiment

a. Select one fresh specimen from each plant type.

b. Make a fresh, clean cut at the base of each specimen under water if possible (this prevents air bubbles from blocking xylem vessels).

• Air bubbles can block the tiny tubes - cutting underwater prevents this

c. Immediately place each specimen into a cup containing colored water. Ensure the cut end is submerged at least 1-2 cm deep.

d. Record the start time: _____

e. Place cups in a bright location (near a window) to encourage transpiration, which pulls water upward.

• In Mohave County's dry climate, transpiration happens quickly - great for this experiment!

f. Photograph the initial setup.

g. Wait for 2 hours. Leave specimens for 3-4 hours or overnight, though 2 hours is usually sufficient to see initial results in dry Arizona air.

Step 9: Observing dye distribution

After the waiting period:

a. Remove specimens from colored water and photograph them externally. Can you see colored water in leaves or along the stem exterior?

b. Pat specimens dry with paper towels.

c. Make a fresh cross-sectional cut 2-3 cm above the original cut (the portion that was submerged).

d. Immediately observe and photograph the cross-section. Do not let it dry out.

e. Document the dye distribution pattern:

• Is dye concentrated in specific regions or throughout the tissue?

• Are there distinct colored dots, rings, or sectors?

• What is the pattern of distribution (ring, scattered bundles, rays)?

• Which regions show NO dye uptake (these are not active in water transport)?

Important interpretation:

• The regions that stained with dye contain functional xylem tissue (water-conducting cells)

• Regions that did not take up dye may include:

  • Phloem tissue (transports sugars, not water with dissolved dyes)

  • Parenchyma, collenchyma, or sclerenchyma (ground tissues)

  • Dead or non-functional tissues

f. Make 2-3 additional cuts at different heights along the stem (every 1-2 cm) to see how far the dye traveled and how the pattern changes with height.

g. If possible, examine leaves or flowers that were attached. Are veins visible in color?

Data Collection and Organization

Create a comprehensive comparative table with the following information for each of the five specimens. You may use the template below or create your own in a spreadsheet program.

Comparative Data Table