Seed Plant Diversity - Gymnosperms & Angiosperms

Lab 11: Seed Plant Diversity - Gymnosperms & Angiosperms

Objective

By the end of this lab, you'll understand:

• The major innovation of seeds and why they revolutionized plant evolution

• Key differences between gymnosperms (conifers) and angiosperms (flowering plants)

• The incredible diversity of fruits and their dispersal strategies

• How seed plants dominate different habitats

• Why angiosperms are now the most diverse and successful plant group on Earth

Materials

Equipment

• Resealable plastic bags

• Measuring tape

• Knife

• Camera

• Stopwatch

• String

• Field notebook

Study Sites

• Conifer trees (pinyon pines, junipers, Arizona cypress)

• Diverse flowering plants

• Different habitat types (desert floor, canyon, riparian areas, high elevation)

EVOLUTIONARY BACKGROUND

The Seed Revolution

Seeds = The game changer for land plants!

A seed is a complete baby plant (embryo) packaged with food (endosperm) and wrapped in a protective coat. This innovation freed plants from needing water for reproduction - no more swimming sperm!

Think of it like packing a lunch box for a long journey. The seed contains everything the baby plant needs to survive until it can make its own food. The protective coat is like waterproof packaging that keeps everything safe.

Two Groups of Seed Plants

GYMNOSPERMS ("naked seeds")

• Seeds exposed on cone scales (not enclosed)

• Mostly conifers (pines, spruces, firs)

• Also includes cycads, ginkgo, and gnetophytes

• About 1,000 species worldwide

• Mostly trees and shrubs

• Wind-pollinated

• Examples: Pine trees, redwoods, junipers

"Gymnosperm" literally means "naked seed" - you can see the seeds sitting exposed on cone scales, like groceries in an open basket rather than sealed in a bag.

ANGIOSPERMS ("enclosed seeds")

• Seeds enclosed inside fruits (ovary becomes fruit)

• ALL flowering plants

• About 300,000+ species worldwide

• Incredible diversity: grasses to trees, cacti to orchids

• Many pollination strategies (wind, insects, birds, bats)

• Examples: Oak trees, roses, grasses, vegetables, most crops

"Angiosperm" means "vessel seed" - seeds are packaged inside fruits. Every fruit you eat (apple, cherry, acorn, even a maple "helicopter") is actually a protective package around angiosperm seeds.

Gymnosperms dominated Earth for millions of years. Then angiosperms evolved flowers and fruits and exploded in diversity.

Part 1: Gymnosperm Survey & Cone Analysis

Conifers (most common gymnosperms) have two types of cones on the same tree:

• Seed cones (female): Large, woody, take 2+ years to mature

• Pollen cones (male): Small, soft, release pollen and fall off quickly

It's like having two separate factories on one tree - one makes pollen (male), the other makes seeds (female). The male cones release clouds of yellow pollen in spring, then fall off. The female cones catch the pollen, then spend years developing seeds.

Step 1: Find Gymnosperm Species

Locate at least 3-5 different conifer species (modified for local availability).

Common gymnosperms in Mohave County to find:

• Pinyon Pine (Pinus edulis or P. monophylla)

  • Needles in bundles of 1-2

  • Small cones with edible seeds

  • Common in high desert/foothills

• Utah Juniper (Juniperus osteosperma)

  • Scale-like leaves

  • Blue berry-like "cones"

• Arizona Cypress (Cupressus arizonica)

  • Scale-like leaves

  • Round, woody cones

  • Found in canyons and higher elevations

• Ponderosa Pine (Pinus ponderosa)

  • Needles in bundles of 3

  • Large cones (3-5 inches)

  • Higher elevations in Hualapai Mountains

In lower desert areas, you may only find junipers and occasional pinyon pines. This is normal! Focus on what's available, and if needed, visit higher elevations (Mt. Tipton, Hualapai Mountains) to see more diversity.

Step 2: Identify Each Species

Use these features:

1. Needle/Leaf Type

• Bundles of 1-2 = Pinyon pine

• Bundles of 3 = Ponderosa pine

• Scale-like = Juniper or Arizona cypress

• Awl-shaped (short, sharp) = Young juniper

2. Cone Appearance

• Small (0.5-2 inches), rounded, woody = Pinyon

• Medium to large (3-5 inches), thick scales = Ponderosa

• Small, berry-like, blue = Juniper (technically a modified cone!)

• Round, woody, with points = Arizona cypress

Juniper "berries" aren't true berries - they're actually seed cones with fleshy, merged scales. This tricks animals into eating them and dispersing the seeds!

3. Bark Texture and Color

• Gray, shreddy strips = Juniper

• Reddish-brown, puzzle-piece plates = Ponderosa (smells like vanilla!)

• Gray-brown, fibrous = Cypress

Take photos and use a field guide or ID app!

Step 3: Collect or Observe Cones

For each species:

A. Find Seed Cones (Female)

Look for:

• Woody cones on branches or ground

• Various ages: Fresh (green), mature (brown), old (weathered)

• Collect fallen cones OR photograph cones on tree

What to look for:

• Open cones: Scales spread apart, seeds already released

• Closed cones: Scales tight, seeds still inside

• Partially open: In process of releasing seeds

Desert heat can cause cones to open quickly. Many cones found on the ground will already be open and empty. Look under trees for recently fallen cones that still have seeds.

B. Find Pollen Cones (Male)

Look for:

• Small, soft cones in clusters

• Usually on lower branches

• Often yellow or brown

• Release clouds of pollen when tapped (in spring: March-May)

• Quickly fall off after releasing pollen

Male pollen cones are like temporary pollen factories. They grow in spring, release their pollen, then fall off and disappear. That's why they're harder to find than seed cones!

Step 4: Measure Cone Dimensions

For seed cones:

A. Length

• Measure from base to tip

• Record in centimeters

B. Diameter

• Measure at widest point

• Record in centimeters

C. Scale Thickness

• Measure thickness of mature scale at thick end

• Record in millimeters

D. Scale Number

• Pick one complete whorl (ring) around the cone

• Count scales in that whorl

• This gives you an idea of cone complexity

A "whorl" is one ring of scales around the cone, like one layer of a layered cake. Cones grow by adding whorls over time.

Step 5: Examine Seeds

If you have open cones with seeds still present:

A. Seed Size and Shape

• Measure length and width (mm)

• Describe shape: oval, triangular, elongated

Pinyon pine seeds are prized as food by humans, birds, and rodents. They're larger than most pine seeds (10-20 mm) and have little to no wing.

B. Wing Presence

• Does seed have a wing for wind dispersal?

• If yes, measure wing dimensions:

  • Length (mm)

  • Width (mm)

  • Is it attached firmly or papery-thin?

Seed wings work like helicopter blades - they spin as they fall, slowing descent and allowing wind to carry them away from the parent tree.

C. Seeds Per Scale

• Carefully peel back one scale

• Count seeds (usually 2 per scale)

• Some may have fallen out

D. Seed Weight (relative)

• Compare by feel: Heavy or light?

• If scale available, weigh

• Pine seeds typically weigh 5-50 mg each

• Pinyon seeds: 150-250 mg (much heavier!)

Step 6: Count Total Seeds Per Cone

Method 1 (if cone is open):

• Count number of scales around cone

• Multiply by number of whorls

• Multiply by 2 (usually 2 seeds per scale)

• Example: 8 scales × 10 whorls × 2 = 160 seeds

Method 2 (if cone is closed and you can dissect it):

• Carefully pull apart cone

• Count actual seeds

• Note how many are filled vs. empty

Pinyon cones produce fewer seeds (10-30) but each seed is large and nutritious. Ponderosa cones produce many more small seeds (50-100+) with wings.

Step 7: Estimate Cone Age

Rate each cone:

• Fresh: Green or light brown, closed, resinous smell

• One-year: Brown, opening, some seeds present

• Old: Gray/weathered, fully open, no seeds

Seed cones stay on the tree for years! Fresh cones are still developing seeds. One-year cones are releasing seeds. Old cones have been emptied for months or years but haven't fallen yet.

Step 8: Note Cone Position on Tree

• High or low on tree?

• Branch tips (exposed) or inner branches (protected)?

• Pattern: Seed cones often higher, pollen cones lower (wind carries pollen up!)

Step 9: Analyze Foliage

For each species:

A. Leaf Type

• Needle (long and thin)

• Scale (small, overlapping like fish scales)

• Awl (short and sharp, triangular)

B. Measure Leaf Length

• For needles: Measure 5 needles, average

• For scales: Measure length of scale

C. Count Leaves Per Bundle (for pines)

Pull out one fascicle (bundle) and count needles.

This is KEY for pine ID!

• 1-2 needles = Pinyon pine (Mohave County!)

• 3 needles = Ponderosa pine

• 5 needles = White pine, Sugar pine (rare locally)

D. Leaf Arrangement

• Spiral around stem?

• Opposite pairs?

• Whorled?

E. Leaf Persistence

• Evergreen (keeps leaves year-round) - most conifers

• Deciduous (drops leaves seasonally) - rare, but some junipers can look brownish in winter

F. Test for Resin

• Gently crush or scratch needle

• Smell: Strong piney/resinous smell?

• Feel: Sticky resin on fingers?

Desert conifers like junipers often have strong-smelling resin to deter herbivores and reduce water loss. The smell is especially noticeable when you crush the foliage!

Step 10: Data Table for Gymnosperms

Part 2: Angiosperm Reproductive Structure Survey

Angiosperms = Flowering plants

Their key innovation: Flowers (for pollination) become Fruits (for seed dispersal)

You'll examine flowers (if present) and especially the incredible diversity of fruits.

The flower is the reproductive structure. After pollination, the ovary of the flower swells and becomes a fruit containing seeds. So every fruit started as a flower!

Step 1: Find Diverse Angiosperm Species

Survey 10-15 different flowering plant species showing variety (modified for desert availability).

Look for diversity in:

• Trees vs. shrubs vs. herbs

• Fleshy fruits vs. dry fruits

• Large seeds vs. tiny seeds

• Different dispersal strategies

Common examples to find in Mohave County:

Trees & Shrubs:

• Fremont Cottonwood (Populus fremontii) - fluffy seeds with parachutes, found along washes

• Mesquite (Prosopis spp.) - long seed pods, very common

• Catclaw Acacia (Senegalia greggii) - coiled seed pods with hooks

• Desert Willow (Chilopsis linearis) - long capsules with winged seeds

• Creosote Bush (Larrea tridentata) - small fuzzy fruits, ubiquitous

Herbaceous Plants:

• Russian Thistle / Tumbleweed (Salsola spp.) - entire plant is seed dispersal!

• Desert Lupine (Lupinus sparsiflorus) - legume pods

• Brittlebush (Encelia farinosa) - achenes (sunflower family)

• Desert Marigold (Baileya multiradiata) - achenes

• Cheatgrass (Bromus tectorum) - grain fruits

Cacti (if season allows):

• Cholla (Cylindropuntia spp.) - fleshy fruits with spines

• Prickly Pear (Opuntia spp.) - large red fleshy fruits (tunas)

• Barrel Cactus (Ferocactus spp.) - yellow fleshy fruits

Step 2: Examine Flowers (if present)

If flowering season:

A. Identify as Monocot or Eudicot

Monocot characteristics:

• Flower parts in 3s (3 petals, 6 stamens, etc.)

• Parallel leaf veins

• Examples: Yucca, grasses, desert lilies

Eudicot characteristics:

• Flower parts in 4s or 5s

• Net-like leaf veins

• Examples: Most desert shrubs, cacti, mesquite

B. Identify Pollination Syndrome

Look at flower features to guess pollinator:

Wind-pollinated:

• Small, dull flowers

• No petals or small petals

• Lots of pollen

• Feathery stigmas

• Local examples: Grasses, Russian thistle

Insect-pollinated:

• Colorful petals

• Sweet smell

• Nectar guides (lines pointing to nectar)

• Landing platforms

• Local examples: Desert marigold, brittlebush, desert lupine

Bird-pollinated:

• Red/orange color

• Tubular shape

• No scent (birds have poor smell)

• Lots of nectar

• Local examples: Chuparosa, ocotillo, penstemon

Bird-pollinated flowers are red because birds see red well, but bees don't! The tubular shape fits a bird's beak perfectly but keeps insects out.

Bat-pollinated:

• Open at night

• White or pale

• Strong musty smell

• Large sturdy flowers

• Local examples: Saguaro (in nearby areas), some agaves

Moth/Beetle-pollinated:

• White or pale

• Strong fragrance at night

• Local examples: Sacred datura, evening primrose

Step 3: Collect and Classify Fruits

For each species:

A. Basic Fruit Classification

Simple, Aggregate, or Multiple?

• Simple: One flower → One fruit (most fruits!)

  • Examples: Cherry, acorn, apple, mesquite pod

• Aggregate: One flower with many pistils → Cluster of fruits

  • Examples: Raspberry, blackberry (Note: rare in Mohave County)

• Multiple: Many flowers → One compound fruit

  • Examples: Pineapple, mulberry (not native locally)

Most desert plants produce simple fruits - each flower makes one fruit. Aggregate and multiple fruits are rare in arid environments.

B. Fleshy or Dry?

Fleshy fruits (soft, juicy):

• Berry: Seeds throughout flesh (cactus fruits, grapes)

• Drupe: One seed inside hard pit (rare locally - maybe hackberry)

• Pome: Seeds in core (rare locally)

Dry fruits (hard when mature):

Dehiscent (split open to release seeds):

• Legume: Splits along two seams (mesquite, acacia, palo verde)

• Capsule: Splits in various ways (desert willow, yucca)

• Follicle: Splits along one seam (milkweed if present)

Indehiscent (don't split open):

• Achene: Single seed in thin coat (sunflower-family plants - very common!)

• Nut: Single seed in hard shell (less common locally)

• Samara: Winged achene (rare in desert, but present in cottonwood areas)

• Grain: Seed coat fused to fruit wall (all grasses - invasive cheatgrass abundant)

"Dehiscent" means "splits open." Think of a pea pod popping open. "Indehiscent" means sealed shut - like a sunflower seed that you have to crack open.

Achenes (sunflower family) and legumes (pea family) dominate desert angiosperms!

C. Specific Fruit Type

Use the classification above to name the fruit type as specifically as possible.

Step 4: Measure Fruit and Seed Characteristics

For each species:

A. Fruit Dimensions

• Length (mm or cm)

• Width (mm or cm)

• Note if fruit is still developing or fully mature

B. Seeds Per Fruit

• Open or cut open fruit (carefully!)

• Count all seeds

• Note: 1, few (2-10), many (10+), or numerous (100+)

Mesquite pods contain 10-20 seeds per pod. Brittlebush achenes contain just 1 seed.

C. Seed Dimensions

• Measure length and width (mm)

• Measure 3-5 seeds if variable

• Calculate average

D. Dispersal Structures

Look for special features that help dispersal:

Wind dispersal:

• Wings (rare locally)

• Parachutes/plumes (cottonwood)

• Light weight

Animal dispersal:

• Fleshy, edible tissue (cactus fruits, hackberry)

• Bright colors

• Hooks or barbs (catclaw acacia, bur sage)

• Hard seed coat (survives digestion)

Water dispersal:

• Floats (cottonwood - moves along washes)

• Waterproof coating

Ballistic/mechanical:

• Explosive pods (rare locally, but some legumes)

• Spring mechanisms

Russian thistle (tumbleweed) is the ultimate seed dispersal machine - when seeds are ripe, the entire plant dies, breaks off, and tumbles across the desert scattering up to 250,000 seeds!

Step 5: Describe Fruit Details

A. Fruit Color and Texture

Color:

• Green (usually immature)

• Red, orange, purple, black (ripe, attractive to animals)

• Brown (dry, mature)

• Yellow/tan (many desert fruits)

Texture:

• Smooth or rough

• Hairy or fuzzy (creosote bush)

• Sticky or dry

• Hard or soft

• Spiny (cholla, prickly pear)

Many desert fruits have protective features like spines, thick coats, or bitter chemicals to discourage seed predators while allowing some dispersers through.

B. Seed Coat Characteristics

• Smooth or textured

• Thick (hard to break) or thin

• Color

• Any special features (arils, appendages)

Many desert seeds have extremely hard coats that can survive years of harsh conditions, waiting for the right rainfall to germinate!

C. Dispersal Mechanism

Based on all features, determine primary dispersal mode:

• Wind

• Animals (internal - eaten)

• Animals (external - hooked on fur)

• Water

• Ballistic (mechanical ejection)

• Gravity (just falls)

• Tumble (unique to desert!)

Step 6: Data Table for Angiosperms

Part 3: Seed Dispersal Analysis

Test how well different seed dispersal mechanisms work by conducting simple experiments!

Experiment A: Wind-Dispersed Seeds

Step 1: Collect Wind-Dispersed Seeds

Find seeds with:

• Wings (rare locally - maybe tree of heaven if present)

• Parachutes (cottonwood, Russian thistle)

• Light, fluffy structures

Arizona options:

• Cottonwood fluff (spring - May/June along washes)

• Russian thistle seeds with papery wings

• Desert willow seeds with wings (summer)

• Brittlebush achenes with papery scales

Collect 3-5 different types.

Step 2: Drop Test

Setup:

1. Find a high point: 2nd story window, balcony, bleachers, or tall ladder

2. Measure exact height: _____ meters

3. Pick a calm morning (less wind) OR note wind conditions

   • Avoid midday when thermal winds are strong!

For each seed type:

1. Hold seed at release height

   • Don't throw - just release gently

2. Start timer when released

3. Stop timer when seed hits ground

   • Record time in seconds

4. Measure horizontal distance traveled

   • From drop point to landing spot

   • Record in meters or centimeters

5. Repeat 3 times per seed type

   • Average the results

Have a partner help spot where seeds land - they can drift far in even light wind!

Step 3: Calculate Descent Rate

• Descent rate = Height ÷ Time

• Example: 2 meters ÷ 4 seconds = 0.5 m/sec

• Slower descent = more time for wind to carry seed away

A seed falling at 0.5 m/sec takes twice as long to hit the ground as one falling at 1.0 m/sec. That extra time means the wind can carry it much farther from the parent plant!

Experiment B: Animal-Dispersed Fruits

Step 1: Rate Attractiveness to Animals

For fleshy fruits, rate on 1-5 scale:

Color attractiveness:

• 1 = Dull, green, cryptic

• 5 = Bright red, purple, orange (high visibility)

Odor:

• 1 = No smell

• 5 = Strong, sweet smell

Accessibility:

• 1 = Hidden, hard to reach

• 5 = Exposed, easy to reach

Examples:

• Prickly pear fruits (tunas): Color=5, Odor=3, Accessibility=3 (spines!)

• Cholla fruits: Color=3, Odor=2, Accessibility=2 (spines!)

• Mesquite pods: Color=3, Odor=4 (sweet!), Accessibility=4

Step 2: Look for Evidence of Animal Feeding

• Bite marks

• Partially eaten fruits

• Scat (droppings) with seeds

• Seeds found away from parent plant

• Coyote scat with prickly pear seeds!

Document with photos!

Arizona wildlife dispersers:

• Coyotes: Eat cactus fruits, mesquite pods

• Donkeys: Eat juniper berries, piñon nuts

• Javelina: Love prickly pear

• Packrats: Hoard seeds and fruits

• Birds: Phainopeplas spread mistletoe, many eat cactus fruits

• Ants: Carry many desert seeds underground

Step 3: Measure Pulp:Seed Ratio

For fleshy fruits:

1. Weigh whole fruit (or estimate volume)

2. Remove seeds

3. Weigh seeds

4. Calculate:

   • Pulp weight = Whole fruit - Seeds

   • Pulp:seed ratio = Pulp weight ÷ Seed weight

High ratio = lots of fleshy reward for animals

Prickly pear fruits have HIGH pulp:seed ratio - lots of sweet flesh as a reward for dispersers!

Step 4: Document Seed Protection

Rate seed protection:

• 1 = No protection (thin coat, easily damaged)

• 3 = Moderate (hard coat)

• 5 = Extreme (very hard shell, impermeable)

Seeds eaten by animals MUST survive digestion!

A seed that travels through an animal's digestive system gets a free ride to a new location, plus natural fertilizer when deposited. But it needs a tough coat to survive the journey!

Examples:

• Mesquite seeds: Rating = 4 (very hard coat, can survive for years)

• Prickly pear seeds: Rating = 5 (extremely hard - need scarification)

• Acacia seeds: Rating = 4

Experiment C: Ballistic/Mechanical Dispersal

For plants with explosive pods:

Some legumes may show this, though it's less common in desert species. Check palo verde or other legume pods in late summer.

If you can find ripe pods:

1. Gently touch pod (or wait for it to pop naturally)

2. Watch seeds eject

3. Measure ejection distance

   • How far do seeds fly?

   • Record maximum distance observed

4. Video if possible - it happens fast!

As legume pods dry in the desert heat, they twist and build up tension. When they finally split, they explosively fling seeds away from the parent plant!

Step 3: Create Dispersal Comparison

Estimate typical dispersal distances for each mechanism:

Desert plants often have multiple dispersal mechanisms as backup. Mesquite pods are eaten by coyotes (animal dispersal) but also fall to ground (gravity) if not eaten.

Step 4: Data Tables

Wind Dispersal Data:

Animal Dispersal Data:

Part 4: Habitat Dominance Survey

Compare gymnosperm vs. angiosperm abundance in different habitat types to see which group dominates where.

By counting plants in different habitats, we'll discover whether gymnosperms (conifers) or angiosperms (flowering plants) dominate. The answer might surprise you and tells us about plant adaptations to different environments!

Step 1: Select Three Different Habitats

Choose locations representing different conditions (modified for Mohave County):

Habitat 1: Desert Scrub / Lower Elevation

• Dominated by shrubs

• Hot, dry

• Full sun

• Examples: Creosote bush flats, low valleys

Habitat 2: Pinyon-Juniper Woodland / Mid-Elevation

• Mixed trees and shrubs

• Cooler than desert floor

• Some shade

• Examples: Foothills, 4,000-6,000 ft elevation

Habitat 3: Riparian Area / Wash

• Near water source

• Higher moisture

• Dense vegetation

• Examples: Along washes, near springs

You may need to travel to different elevations. Lower areas will be heavily angiosperm-dominated. Higher areas show more gymnosperms (especially junipers and pinyon pines).

Step 2: Establish Survey Plot

At each habitat:

1. Mark out 10m × 10m square (100 m²)

   • Use measuring tape or paces

   • One pace ≈ 1 meter for most people

   • Mark corners with rocks or stakes

   • This is your survey plot

Step 3: Survey Adult Woody Plants

Count all woody plants > 1 meter tall:

For each plant:

1. Identify as gymnosperm or angiosperm

   • Gymnosperm: Conifer (needle/scale leaves, cones) - juniper, pinyon, Arizona cypress

   • Angiosperm: Broadleaf or non-conifer - everything else!

2. Measure height

   • Estimate or measure to top

   • Record in meters

   • Use your own height as reference

3. Measure diameter

   • At chest height (1.4 m above ground)

   • Use measuring tape around trunk, divide by 3.14

   • Record in centimeters

   • Arizona note: Many desert shrubs branch at ground - measure largest stem

4. Identify species if possible

   • At least note distinct types

Common Mohave County woody plants:

• Gymnosperms: Juniper, pinyon pine (at elevation)

• Angiosperms: Creosote, mesquite, catclaw, Joshua tree, yucca, cholla, prickly pear

Step 4: Estimate Percent Cover

For each group (gymnosperms and angiosperms):

Look up at canopy:

• What percentage of the sky is blocked by gymnosperm foliage?

• What percentage by angiosperm foliage?

Look at ground layer:

• What percentage of ground is shaded by each group?

Estimate to nearest 10%

Step 5: Count Seedlings

Survey for small plants < 1 meter tall:

Walk systematically through your plot:

• Count gymnosperm seedlings (tiny conifers - look hard!)

• Count angiosperm seedlings (broadleaf sprouts)

This shows recruitment - which group is reproducing successfully!

Adult plants tell us what's there now. Seedlings tell us what the future forest will look like. If you see lots of juniper seedlings but no pinyon seedlings, junipers are winning the recruitment battle!

Desert seedlings are HARD to find - many germinate after rain then die. Focus on areas with recent moisture.

Step 6: Calculate Density

For each habitat:

• Gymnosperm density = (# gymnosperm plants ÷ 100 m²) × 100

  • This gives you "plants per 100 m²"

• Angiosperm density = (# angiosperm plants ÷ 100 m²) × 100

Step 7: Note Canopy Structure

Describe differences:

Gymnosperm-dominated areas:

• Usually: Conical tree shapes (junipers)

• Dense, year-round shade

• Less undergrowth

• Thick needle litter

Angiosperm-dominated areas:

• Usually: Varied shapes - shrubby to spreading

• More variable shade (many desert plants are small)

• More diverse undergrowth

• Variable litter

In pinyon-juniper woodland, gymnosperms provide structure but angiosperms (shrubs, herbs) often dominate ground layer diversity!

Step 8: Data Table

Part 5: Reproductive Investment Comparison

Compare how much energy gymnosperms vs. angiosperms invest in reproduction by analyzing their seeds and protective structures.

Plants have to make choices about how to invest their energy. Do they make MANY small seeds or FEW large seeds? Do they invest in fancy dispersal structures or simple ones? Let's compare strategies!

Reproductive strategies - r-strategy (many small seeds)

Reproductive strategies  - K-strategy (few large seeds)

Step 1: Collect Complete Reproductive Structures

Gymnosperms:

• 1-2 complete seed cones with seeds

• Pinyon cone, juniper "berry"

Angiosperms:

• 3-5 complete fruits with seeds (various types)

• Mesquite pod, prickly pear fruit, acacia pod, desert marigold achene, brittlebush achene

Step 2: Count Total Seeds Produced

For each structure:

• Open carefully

• Count every seed (including empty/aborted ones)

• Note: filled vs. empty

Arizona data:

• Pinyon cone: 10-30 seeds (usually 50% filled)

• Juniper "berry": 1-3 seeds

• Mesquite pod: 10-20 seeds

• Prickly pear fruit: 100-300 seeds!

• Acacia pod: 5-15 seeds

Step 3: Measure Seed Sizes

For each species:

• Measure length and width of 5-10 seeds

• Calculate average

• Note size range (smallest to largest)

Step 4: Estimate Seed Mass

If you have a scale:

• Weigh individual seeds

• Or weigh 10 seeds, divide by 10

If no scale:

• Compare relative masses by feel

• Rate as: Very light / Light / Medium / Heavy / Very heavy

Arizona examples:

• Pinyon seed: Heavy (150-250 mg)

• Mesquite seed: Medium (50-100 mg)

• Prickly pear seed: Light (5-10 mg)

• Brittlebush seed: Very light (<1 mg)

Step 5: Compare Key Features

Create comparison table:

A. Seeds Per Structure

• Gymnosperm cone: _____ seeds (typically 10-100 for conifers)

• Angiosperm fruits: _____ seeds (highly variable: 1 to 300+!)

B. Protection Level

Rate on 1-5 scale:

• 1 = Minimal (seeds exposed)

• 3 = Moderate (thin covering)

• 5 = Maximum (thick, hard protection)

Gymnosperms:

• Protection = Cone scales (woody, open when dry)

• Rating: Usually 2-3

• Arizona: Juniper "berry" = 4 (fleshy coating)

Angiosperms:

• Protection = Fruit wall (thickness varies)

• Nut = 5

• Drupe/Berry = 4

• Prickly pear = 4 (thick fruit flesh)

• Dry pod = 2

• Achene = 1-2

C. Dispersal Investment

What resources go into dispersal?

Gymnosperms:

• Usually: Seed wings (lightweight paper)

• Or: Fleshy coating (juniper)

• Low to moderate investment

Angiosperms:

• High diversity:

  • Wings (low investment)

  • Fleshy tissue (HIGH investment - nutritious pulp for animals)

  • Hooks/barbs (low investment)

  • Parachutes (moderate investment)

  • Tumble mechanism (entire plant investment!)

Making a juicy fruit is EXPENSIVE - the plant has to produce lots of sugars and water. But it works! Animals seek out the fruit and carry seeds far away. Tumbleweeds invest in the entire plant structure as a seed scatterer!

Desert plants show extreme strategies:

• High investment: Prickly pear (huge juicy fruit with hundreds of seeds)

• Low investment: Brittlebush (tiny dry achene, wind-blown)

D. Seed Size Variation

Within species:

• Do seeds vary much in size?

• Measure smallest and largest

• Calculate: (Largest - Smallest) ÷ Average × 100 = % variation

Between strategies:

• Wind-dispersed: Usually small, uniform

• Animal-dispersed: Often larger, more variable

• Arizona: Pinyon seeds vary 20-30% in size (larger ones more likely to germinate)

Step 6: Data Table