Symbiosis & Plant Defense Mechanisms

Lab 12: Symbiosis & Plant Defense Mechanisms

Objective

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

• How plants form beneficial partnerships (mutualisms) with fungi, bacteria, pollinators, and ants

• The amazing diversity of plant defenses - both physical (thorns, spines) and chemical (toxins, latex)

• How to assess the costs and benefits of these interactions

• The ecological chess game between plants and their herbivores

• How plants can be both protectors and victims in nature

Materials

• Ruler with mm scale

• Trowel

• Resealable plastic bags

• Camera

• Paper (white for viewing roots)

• Knife

• Field Notebook

• Heavy leather gloves (ESSENTIAL for desert spiny plants!)

• Tweezers (for removing tiny spines)

ECOLOGICAL BACKGROUND

Plant-Organism Interactions

Plants can't run away, so they've evolved three main strategies for dealing with other organisms:

1. MUTUALISMS (Win-Win)

"Mutualism" means both partners benefit. It's like a business deal where everyone wins. The plant gets something it needs (nutrients, pollination, protection), and the partner gets something it needs (food, shelter).

Main types:

• Mycorrhizae: Fungi help roots absorb nutrients → Plant gives fungi sugars

  • Think of it like: The fungus is an extension of the root system, reaching places the roots can't go

• Nitrogen fixers: Bacteria convert nitrogen → Plant gives bacteria sugars/shelter

  • Think of it like: The bacteria run a fertilizer factory inside root nodules, making nitrogen the plant can actually use

• Pollinators: Animals transfer pollen → Plant gives nectar/pollen rewards

  • Think of it like: The plant runs a restaurant (nectar bar) and the customers (pollinators) accidentally deliver mail (pollen) as they eat

• Ant guards: Ants protect plant → Plant gives ants food/housing

  • Think of it like: The plant hires a security company (ants) and pays them with room and board

2. DEFENSES (Protection)

Physical defenses:

• Thorns, spines, tough leaves, hairs

• Like wearing armor

Chemical defenses:

• Toxins, bad taste, latex, resin

• Like having poison or pepper spray

Goal: Make eating the plant costly for herbivores

Plants invest energy in defenses to make herbivores think twice. A cactus spine might cost the plant energy to build, but it's worth it if it prevents being eaten!

3. PARASITISM (Plant as Victim)

Some plants are parasitic on other plants!

• Mistletoe, dodder, broomrape

• They steal water, nutrients, or sugars

Just like some animals are parasites, some plants are too! Mistletoe taps into tree branches and steals water and nutrients. The tree loses, the mistletoe wins.

The Evolutionary Arms Race

Plants evolve defenses → Herbivores evolve counter-defenses → Plants evolve new defenses → and on and on!

Monarch butterflies evolved the ability to tolerate milkweed toxins. Now they can eat a plant most insects can't touch. Monarchs even store the toxins to make themselves poisonous to birds!

Part 1: Mutualistic Symbioses Survey

Experiment A: Mycorrhizal Associations (Indirect Evidence)

Mycorrhizae = Fungal threads (hyphae) that wrap around or penetrate plant roots.

The fungus acts like an extension of the root system, gathering water and nutrients (especially phosphorus). In return, the plant gives the fungus sugars from photosynthesis.

About 90% of plant species have mycorrhizae! Plants without mycorrhizae must grow extensive fine roots to do the same job.

Imagine your arms were too short to reach the top shelf. You could either grow longer arms OR hire someone tall to help you. Mycorrhizae are the "someone tall" - they extend the plant's reach into the soil.

The fungal threads are microscopic - thinner than a human hair! We need a microscope to see them. But we can see their effects on root structure.

Step 1: Select Study Plants

Choose 6-8 different plant species from diverse habitats (modified for desert):

Include mix of:

• Desert shrubs (creosote, brittlebush, bursage)

• Trees (mesquite, palo verde, desert willow)

• Grasses and forbs

• Riparian plants if accessible

Focus on smaller herbaceous plants and shrubs. Tree roots are massive and hard to excavate!

Plants that typically LACK mycorrhizae (good for comparison):

• Brassicas (mustard family) - some desert mustards

• Chenopods (goosefoot family) - Russian thistle, saltbush

• Sedges

Step 2: Excavate Roots Carefully

For each plant:

1. Choose a small plant (easier to dig up completely)

   • Target plants < 30 cm tall

   • Annuals work best

2. Dig carefully around the base

   • Start 10-15 cm away from stem

   • Dig in a circle, then underneath

   • Arizona soil note: Rocky, compacted soil is difficult! Bring sturdy trowel

3. Excavate root system - get as much as possible intact

   • Work slowly and carefully

   • Desert roots can be surprisingly long!

4. Shake off loose soil - rinse gently if needed

   • Bring water bottle for rinsing

   • Don't scrub - be gentle

5. Examine on white paper for contrast

   • Spread roots out flat

   • Take photo for documentation

A. Fine Root Abundance

Look at the smallest, thinnest roots:

• Many fine roots (looks like hairy mass) = Lower mycorrhizal dependence

  • Plant compensates with lots of roots

  • Doing the work itself

• Few fine roots (thicker, less branched) = Likely high mycorrhizal dependence

  • Fungus does the job of fine roots

  • Plant invests energy elsewhere

Rate: 1 (very few) to 5 (extremely abundant)

If you don't have a helper, you need more arms! Plants without mycorrhizae grow tons of tiny root branches to compensate.

B. Root Branching Intensity

Count branching points in a 5 cm section:

• Highly branched (10+ branches)

• Moderately branched (5-10 branches)

• Lightly branched (<5 branches)

C. Overall Root System Assessment

Color:

• White/cream (healthy, active)

• Light brown (older but healthy)

• Dark brown/black (potentially diseased or dead)

Smell:

• Fresh earthy smell = healthy

• Musty/rotten = potential problems

Thickness:

• Thin and delicate = young, active

• Thick and woody = older, structural

Root hair visibility:

• Can you see tiny hairs on finest roots?

• Desert plants may have fewer visible root hairs due to dry conditions

Step 4: Document Soil Conditions

At each collection site:

Soil type:

• Sandy - loose, gritty, drains fast (common in washes)

• Loamy - mix of sand/silt/clay (rare in desert)

• Clay - sticky when wet, hard when dry

• Rocky - mostly rocks with little soil (very common!)

Organic matter:

• High (dark, rich) - rare in desert, found under bushes

• Low (pale, sparse) - typical desert condition

Moisture:

• Dry - powdery, no clumping (most common)

• Moist - forms ball when squeezed

• Wet - muddy, water visible (riparian areas only)

Nutrient status:

• Rich - dark soil, near washes, under shrubs

• Poor - pale/whitish soil, crusted, open areas

Most desert soils are low in organic matter but can be high in minerals. Mycorrhizae are ESSENTIAL here because the soil is often phosphorus-poor!

Step 5: Compare Patterns

Expected patterns:

Desert prediction: Most native desert plants will show high mycorrhizal dependence - the soil is too poor and dry to succeed without fungal partners!

Step 6: Data Table

Experiment B: Nitrogen-Fixing Symbioses

Nitrogen fixation = Bacteria living in root nodules convert atmospheric nitrogen (N₂) into ammonia (NH₃) that plants can use.

This is HUGE because nitrogen is often the limiting nutrient for plant growth!

The air we breathe is 78% nitrogen, but plants can't use it in that form (N₂). It's like having a locked treasure chest - the treasure is there but inaccessible. Nitrogen-fixing bacteria are the key that unlocks it!

Legumes (pea family) are famous for this. The bacteria are called Rhizobium.

Real-world use: Farmers plant legumes to add nitrogen to soil! Crop rotation with legumes is natural fertilization.

Step 1: Locate Nitrogen-Fixing Plants

Find at least 3-5 individual legume plants.

Trees & Shrubs:

• Mesquite (Prosopis spp.) - most common, screwbean and honey mesquite

• Palo Verde (Parkinsonia spp.) - yellow flowers, green bark

• Catclaw Acacia (Senegalia greggii) - curved thorns

• Smoke Tree (Psorothamnus spinosus) - along washes

Herbaceous:

• Desert Lupine (Lupinus sparsiflorus) - purple flowers, spring

• Desert Locoweed (Astragalus spp.)

• Prairie Clover (Dalea spp.)

Legumes are ABUNDANT in the desert! They thrive because they make their own nitrogen fertilizer.

Step 2: Excavate Roots Carefully

1. Choose small herbaceous legumes (easier than digging tree roots!)

   • Lupines and locoweeds work great

   • For trees, dig around the edge of canopy where feeder roots are

2. Dig around plant carefully

   • Start 15-20 cm from stem

   • Desert legume roots can be deep!

3. Extract root system gently

   • Get at least some of the lateral roots

4. Rinse roots to see nodules clearly

   • Nodules can be hidden under soil

   • Gentle water spray works well

Best time is after rains when soils are softer and plants are actively growing!

Step 3: Search for Nodules

What nodules look like:

Small bumps or swellings on roots

• Size: 1-10 mm (BB-sized to pea-sized)

• Color: Pink, red, white, or brown

  • Pink/red = ACTIVE! This is the key sign

  • White/tan = young or inactive

  • Brown/gray = old or dead

• Shape: Round, elongated, or irregular clusters

• Location: On side roots, not on main taproot

  • Look carefully at branch points

Nodules are like tiny apartments where bacteria live. The plant builds these structures specifically to house its bacterial partners!

Desert legumes may have fewer but larger nodules compared to plants in rich soil - quality over quantity!

Step 4: Measure Nodule Characteristics

For plants WITH nodules:

A. Count Total Nodules

• Count every nodule on the excavated root system

• Record total number

• Note: You probably don't have the entire root system, so actual count is higher

B. Measure Nodule Sizes

• Measure 5-10 nodules with ruler (mm)

• Calculate average

• Note size range (smallest to largest)

C. Assess Nodule Activity

Cut open a nodule with knife:

Pink or red inside = ACTIVE! Bacteria fixing nitrogen

• White, green, or brown inside = Inactive or dead

The pink color is from leghemoglobin, which helps bacteria work. It's similar to hemoglobin in our blood - it carries oxygen but keeps it at the right level so bacteria can function!

The bacteria need oxygen to live but not too much oxygen or they can't fix nitrogen. Leghemoglobin is like a bouncer at a club - it controls how much oxygen gets in!

D. Calculate Root:Nodule Ratio (optional)

If you have a scale:

• Measure total root mass (grams)

• Measure total nodule mass

• Calculate: nodule mass ÷ root mass = ratio

• High nodule ratio = heavy investment in nitrogen fixation

Interpretation:

• High ratio (>0.1) = Plant invests heavily in nitrogen fixation

• Low ratio (<0.05) = Less dependent on nitrogen fixation

Step 5: Compare Nodulated vs. Non-Nodulated Plants

Find legumes both WITH and WITHOUT nodules (look at soil type - poor soils have more nodulation).

Plants in nitrogen-poor soil invest more in nodules than plants in nitrogen-rich soil. It's only worth the energy cost if you need it!

Compare:

A. Plant Size and Vigor

• Measure height (cm)

• Note overall health/robustness

  • Rate vigor: 1 (weak, sparse) to 5 (vigorous, lush)

• Count leaves or branches as indicator of growth

B. Leaf Color

Nitrogen content shows in leaf color!

• Darker green = more nitrogen = better nodulation

• Yellow-green = nitrogen deficiency

Rate: 1 (pale yellow-green) to 5 (dark green)

Chlorophyll (the green in leaves) contains nitrogen. More nitrogen = more chlorophyll = greener leaves. It's a visible nitrogen meter!

C. Growing Location

Record where you found each plant:

• Soil type (sandy, rocky, etc.)

• Location (open desert, wash, under shrub)

• Nearby vegetation

Expected: More nodulation in poor, open soils. Less nodulation in rich soils (under shrubs) or riparian areas.

Step 6: Data Table

Experiment C: Pollination Mutualisms

Pollination = Animals transfer pollen from one flower to another while seeking rewards (nectar, pollen, oils).

This is mutualism - both benefit!

• Plant gets: Its "sperm" (pollen) delivered to another flower for reproduction

• Animal gets: Food (nectar, pollen) or other rewards

It's like paying someone to deliver your mail. You get your mail delivered, they get paid. Everyone wins!

Different pollinators prefer different flower types - creating pollination syndromes.

Pollination syndrome: A set of flower traits that match a specific pollinator type. It's like flowers advertising "BEE FOOD HERE" or "HUMMINGBIRD CAFÉ" with specific colors, shapes, and scents!

Step 1: Document Plant-Pollinator Interactions

Find 10-15+ flowering plant species and observe their visitors.

Early morning and late afternoon are best to avoid heat AND catch peak pollinator activity. Midday heat (100°F+) reduces insect activity dramatically!

Step 2: For Each Plant Species, Record

A. Floral Traits

Color:

• What colors are the petals/sepals?

• Any color patterns (nectar guides - lines or spots pointing to center)?

• UV patterns (invisible to us, visible to bees!)

Common flower colors:

• Yellow (most common desert color - brittlebush, desert marigold)

• Purple/Blue (lupines, phacelia, chia)

• Red (chuparosa, penstemon, ocotillo)

• White (datura, evening primrose, yucca)

Shape:

• Tubular (long narrow tube) - hummingbirds, long-tongued insects

  • Examples: Penstemon, chuparosa, tobacco

• Bowl-shaped (open, flat) - many insects, easy access

  • Examples: Poppies, desert marigold, brittlebush

• Irregular (like snapdragon - upper and lower lips) - bees

  • Examples: Lupines, locoweed

• Tiny clustered (like composite flowers) - small insects

  • Examples: Sunflower family (brittlebush, aster)

Scent:

• Sweet fragrance (insect-pollinated)

• No smell (often bird-pollinated - birds have poor sense of smell!)

• Unpleasant/musky smell (fly or beetle-pollinated)

• Fruity smell (various insects)

• Night fragrance (moth or bat-pollinated)

Size:

• Large (>5 cm across) - visible from distance, larger pollinators

• Medium (2-5 cm) - most common size

• Small (<2 cm) - often in clusters for visibility

B. Observe Pollinator Visitors

Watch flowers for 10-15 minutes per species:

Setup tips:

• Sit quietly at comfortable distance (1-2 meters)

• Don't wear bright colors or strong fragrances

• Morning observations are most productive

• Bring water and shade (umbrella or hat)

Who visits?

Common pollinators:

BEES:

• Large bumblebees (uncommon in hot desert)

• Honey bees (introduced, very common)

• Small native bees (VERY common and diverse!)

  • Carpenter bees (large, black, shiny)

  • Digger bees

  • Sweat bees (tiny, metallic)

FLIES:

• Bee flies (fuzzy, hover)

• Syrphid flies (hover flies, look like small bees)

• Common flies

BUTTERFLIES:

• Painted lady (orange/black)

• Checkerspots

• Swallowtails (larger)

MOTHS:

• White-lined sphinx moth (hummingbird moth - day active!)

• Night-flying moths (need evening observations)

BEETLES:

• Desert blister beetles

• Various small beetles

BIRDS:

• Hummingbirds (Costa's, Black-chinned, Anna's)

• Verdins (tiny, sometimes visit flowers)

BATS:

• Lesser long-nosed bat (endangered, visits agave and saguaro)

• Need night observations (rare for this lab)

WIND:

• No animal visitors

• Look for dangly anthers and feathery stigmas

What do they do?

Document behavior:

1. Land on flower or hover?

2. Probe with tongue/mouthparts - where do they insert?

3. Collect pollen on body - where does pollen stick?

4. Move between flowers - same plant or different plants?

5. Time spent per flower - quick visit or long feeding?

How often?

• Count visits per 10 minutes

• Note if same individual returns

• Note if multiple species visit same flower

C. Assess Pollinator Effectiveness

Does contact occur with reproductive parts?

This is the key question! Not all flower visitors are effective pollinators.

Watch closely:

• Does insect touch anthers (pollen-bearing parts)?

  • Are they positioned to dust pollen on insect?

• Does insect touch stigma (pollen-receiving part)?

  • Is it positioned to receive pollen from insect?

• Or does it "cheat" and steal nectar without touching? (nectar robber!)

Some insects are "nectar thieves" - they drill through the base of the flower or reach in without touching the pollen or stigma. They get the reward without doing the job! It's like sneaking into a movie without paying.

Rate effectiveness:

• 5 = Perfect contact every visit - pollen clearly transferred

• 3 = Sometimes contacts - partial effectiveness

• 1 = No contact (nectar robber!) - ineffective

Carpenter bees sometimes rob flowers by cutting holes in the base. They're smart but lazy pollinators!

Step 3: Match Flowers to Pollination Syndromes

Based on your observations, identify the pollination syndrome:

Step 4: Data Table

Experiment D: Ant-Plant Associations

Ant guards = Some plants house and feed ants, and in return, ants attack herbivores trying to eat the plant.

Win-win mutualism!

The plant hires a security company (ants) and pays them with room (hollow thorns or stems) and board (nectar). The ants patrol the plant and attack anything that tries to eat it!

Two strategies:

1. Domatia: Hollow structures (thorns, stems) where ants live

• Famous example: African bull-thorn acacias

• Less common in North America

2. Extrafloral nectaries (EFNs): Nectar glands OUTSIDE flowers that feed ants

• Found on leaves, stems, or buds

"Extrafloral" means "outside the flower." Regular nectaries are inside flowers to feed pollinators. Extrafloral nectaries are on other parts of the plant specifically to feed ants!

Step 1: Search for Ant-Guarded Plants

Plants with extrafloral nectaries (more common in Mohave County):

Trees & Shrubs:

• Desert willow (Chilopsis linearis) - on flower buds

• Palo verde (Parkinsonia spp.) - on leaf stems

• Mesquite (Prosopis spp.) - check carefully

• Ocotillo (Fouquieria splendens) - sometimes present

Herbaceous plants:

• Sunflower family members - check leaf bases

• Some mallows - on calyx (base of flower)

How to find them:

1. Look for ants crawling on plants (especially on leaves/buds, NOT flowers)

   • Best early morning when ants are active

2. Look for small nectar droplets on leaves or stems

   • Shiny, clear drops that aren't dew

3. Check leaf bases and stipules

   • Common location for EFNs
     
     
     

4. Follow ant trails up plants

   • Ants will lead you to food sources!

Find at least 5 plants with EFNs or ant activity.

Desert ants are EVERYWHERE and very active. You'll find them on many plants, but you need to determine if they're there for EFNs or for aphid honeydew (see below).

Step 2: Document Extrafloral Nectaries

A. Count EFNs Per Leaf or Stem

• Examine 5 leaves/stems

• Count all EFNs on each

• Calculate average

• Note location pattern

B. Measure EFN Size

• Diameter (mm)

• Raised bumps or flush with surface?

• Color (often darker than surrounding tissue)

C. Note Position

Common locations:

• Base of leaf blade (where blade meets petiole)

• On petiole (leaf stem)

• On stipules (small leaf-like structures at leaf base)

• On buds or young stems

• On flower bracts or calyx

Pattern: EFNs are often on YOUNG or VULNERABLE plant parts that need the most protection!

D. Check for Nectar

Best time: Early morning (nectar production highest)

• Is nectar visible (shiny droplet)?

• Touch gently - sticky?

• Does it attract ants?

• Taste test (optional): Very tiny amount on finger - sweet?

In hot, dry conditions, nectar may evaporate quickly. Check early!

Step 3: Observe Ant Activity

Watch plants for 10-15 minutes:

A. Ant Presence

• Present or absent?

• If present, how many ants?

• What species (if identifiable)?

  • Harvester ants (large, red/brown)

  • Carpenter ants (large, black)

  • Argentine ants (small, invasive)

  • Many others!

B. Ant Behavior

Patrolling:

• Walking around systematically

• Following consistent paths

• Investigating leaves and stems

• Like a security guard making rounds

Feeding at EFNs:

• Stopped at nectary

• Antennae moving

• Extended mouthparts

• May stay for minutes

Aggressive:

• Quick movements

• Attacking other insects

• Mandibles open

• May spray formic acid

Tending aphids:

• Gathered around aphids

• "Milking" aphids for honeydew

• This is different from EFNs!

• Ants may defend aphids (but aphids hurt plant)

Simple explanation: Aphid-tending ants are like ranchers farming cows. They protect aphids because aphids produce honeydew (sugary excretion). But this HURTS the plant because aphids suck plant sap! Not a true mutualism - more like corruption.

C. Ant Response to Disturbance

Carefully test defensive behavior:

1. Gently touch a leaf with stick

2. Watch ant response

3. Do ants:

   • Swarm to investigate?

   • Show aggressive defensive behavior?

   • Ignore disturbance?

   • Run away?

Rate response:

• 5 = Immediate aggressive response

• 3 = Investigate but not aggressive

• 1 = No response

Step 4: Compare Herbivory Levels

This is the test! Do ant guards actually reduce herbivory?

Compare plants WITH ants vs. WITHOUT ants:

For each plant:

1. Count damaged leaves out of 20 examined leaves

2. Estimate % leaf area eaten (average across leaves)

3. Note presence of herbivorous insects

   • Caterpillars

   • Beetles

   • Grasshoppers

Expected result: Plants with active ant guards show significantly less herbivory!

Estimate how much herbivory occurred to the leaf.

• 0% = No damage (perfect condition)

• 1-10% = Small holes or minor edge damage

• 10-25% = Moderate damage, several holes or sections missing

• 25-50% = Heavy damage, major portions missing

• 50-75% = Severe damage, more gone than remains

• 75-100% = Critical damage, mostly skeleton

Step 5: Data Table

Part 2: Plant Defense Structure Survey

Physical defenses = Structures that make plants hard, painful, or impossible to eat.

These are costly to build but highly effective!

Simple explanation: Imagine you're a caterpillar trying to eat lunch. Would you rather chew on a soft lettuce leaf or a cactus spine? Physical defenses make the caterpillar choose somewhere else for lunch!

Four main types:

1. THORNS: Modified stems (woody, have buds/leaves)

• Sharp, pointed stems

• Can grow leaves or branches

• Example: Hawthorn, honey locust

2. SPINES: Modified leaves (no buds)

• Sharp, needle-like

• No buds or leaves emerge from them

• Example: Cactus spines, ocotillo, agave

3. PRICKLES: Outgrowths of epidermis (pop off easily)

• Surface structures, not deep

• Break off relatively easily

• Example: Rose prickles, some acacias

4. TRICHOMES: Hairs (can be simple, glandular, barbed, or stinging)

• Can be soft and fuzzy or dangerous

• Range from protection to chemical warfare

• Example: Stinging nettle, fuzzy mullein

Simple explanation for the difference:

• Thorn = If you remove it, there's a woody core and maybe a bud (it's a stem)

• Spine = If you remove it, there's a scar but no bud (it's a leaf)

• Prickle = Pops off easily, shallow attachment (it's skin-deep)

• Trichome = It's a hair, just more dramatic!

Step 1: Survey Defended Plants

Find 8-10+ plant species showing physical defenses.

Where to look:

• Desert shrubs and cacti (MANY spines!)

• Acacias (thorns)

• Riparian trees (some have thorns)

• Agaves (spine-tipped leaves)

EXTREME DEFENSES:

Cacti:

• Cholla (Cylindropuntia spp.) - EXTREMELY dangerous barbed spines

• Prickly pear (Opuntia spp.) - large spines PLUS tiny glochids

• Barrel cactus (Ferocactus spp.) - hooked spines

• Hedgehog cactus (Echinocereus spp.) - dense spine clusters

Agaves:

• Century plant (Agave spp.) - sword-like leaves with terminal spine

• Sharp enough to puncture leather!

Trees & Shrubs:

• Catclaw acacia - curved thorns (like cat claws!)

• Mesquite - straight thorns on branches

• Ocotillo - spines on stems

• Desert thorn (Lycium spp.) - thorns on branches

Herbaceous:

• Thistles - spiny leaves and stems

• Puncture vine (Tribulus terrestris) - spiny fruits (flat tires!)

Step 2: Identify Defense Type

For each structure, determine category:

How to tell:

Step 3: Measure Defense Characteristics

For thorns, spines, and prickles:

A. Length

• Measure from base to tip (mm)

• Measure 5-10 structures, calculate average

• Note range (shortest to longest)

Arizona data examples:

• Cholla spines: 15-25 mm

• Barrel cactus spines: 50-80 mm

• Catclaw thorns: 10-20 mm (but curved!)

• Agave terminal spine: 30-50 mm (SHARP!)

B. Density

Count defenses per 10 cm of stem OR per cm² of leaf surface

• Use ruler to mark 10 cm section

• Count all spines/thorns/prickles

• Record number

Or for leaves:

• Use cm² grid (make from paper)

• Count spines per square

Arizona examples:

• Cholla: 50-100 spines per 10 cm (DENSE!)

• Prickly pear: 1-8 large spines per areole, plus 20+ tiny glochids

• Ocotillo: 3-5 spines per node

C. Sharpness

Rate on 1-5 scale (CAREFULLY with gloved hand or visual inspection):

• 1 = Blunt, not painful (like ocotillo spines when old)

• 2 = Slightly sharp, minor discomfort

• 3 = Sharp, mildly painful (mesquite thorns)

• 4 = Very sharp, definitely painful (barrel cactus)

• 5 = Extremely sharp, dangerous (agave, cholla)

NEVER test sharpness with unprotected hands!

Special feature to note:

• Barbs (like fishhooks) - cholla!

• Hooked tips - barrel cactus

• Smooth - most thorns

• Glochids (tiny, barbed, detach easily) - prickly pear

Glochids are the evil tiny spines on prickly pear. They're barely visible but DOZENS get in your skin with any contact. They're worse than the big spines! Use duct tape to remove them.

For trichomes (hairs):

Density:

Visual assessment:

• None - smooth leaf

• Sparse - can see leaf surface clearly between hairs

• Moderate - fuzzy appearance

• Dense - thick mat of hairs, can't see leaf surface

Type:

• Simple (plain hairs) - soft, fuzzy

  • Function: Reduce water loss, reflect sunlight

  • Example: Brittlebush

• Glandular (sticky to touch) - secretes substances

  • Function: Chemical defense, insect trapping

  • Example: Creosote bush (sticky, aromatic)

  • Touch test: Sticky residue on glove

• Hooked (catches clothing/fur) - barbed tips

  • Function: Deter herbivores, seed dispersal (if on fruits)

  • Less common in Mohave County

• Stinging (causes pain - be VERY careful!) - inject irritants

  • Function: Active chemical defense

  • Example: None common in Mohave County (lucky for us!)

  • If present, causes immediate pain on contact

Step 4: Document Habitat

Does defense intensity correlate with herbivore pressure?

Record habitat characteristics:

• Exposed vs. protected areas

  • Open desert (high herbivore pressure)

  • Under shrubs (lower pressure, shade)

• Elevation

  • Low elevation (hot, dry)

  • High elevation (cooler, more moisture)

• Moisture availability

  • Dry uplands

  • Riparian areas (wetter)

• Herbivore abundance

  • Signs of browsers (deer, bighorn sheep droppings)

  • Insect diversity

Expected: Plants in high-pressure environments invest more heavily in defenses!

Step 8: Data Table

Part 3: Chemical Defense Indicators

Chemical defenses = Plants produce toxins, bitter compounds, digestibility reducers, and irritants to deter herbivores. These are invisible but incredibly effective!

While physical defenses are like armor, chemical defenses are like poison. An insect might get through the armor, but the poison makes it regret the meal!

Common types:

• Alkaloids: Bitter, toxic (nicotine, caffeine, morphine)

  • Affect nervous system

  • Examples: Tobacco, datura

• Tannins: Astringent, reduce digestibility

  • Bind to proteins, making them indigestible

  • Make mouth feel "dry"

  • Examples: Oak, creosote

• Latex: Sticky, gums up mouthparts

  • Flows out when damaged, traps insects

  • Often toxic too

  • Examples: Milkweed, spurges

• Resins: Sticky, aromatic, toxic

  • Especially common in desert plants

  • Examples: Creosote, pine

• Cyanogenic glycosides: Release cyanide when chewed

  • Extremely toxic

  • Examples: Some acacias

We can't identify specific chemicals without a lab, but we can detect their presence!

Step 1: Test for Latex Production

CAUTION: Latex can irritate skin and eyes! Some latex is toxic. Wear gloves and avoid contact with eyes!

Latex = Milky white (or colored) sap that flows when plant is damaged

Simple explanation: Latex is like blood for these plants - it flows out when injured, sealing the wound and gumming up any insect that caused the damage. It's both a band-aid and a weapon!

Plants with latex (possible to find in Mohave County):

Common:

• Desert milkweed (Asclepias spp.) - white latex, toxic

• Leafy spurge (Euphorbia spp.) - white latex, irritating

• Rubber rabbitbrush (Ericameria nauseosa) - yellow latex

• Some wild lettuces (Lactuca spp.) - white latex

Less common:

• Dandelions (if present in irrigated areas)

• Prickly lettuce

How to test:

1. Carefully break a leaf stem or vein

   • Small damage is enough

   • Use gloved hands or knife

   • Avoid getting on skin!

2. Observe immediately:

   • Does milky sap appear?

   • What color?

     • White (most common) - milkweed, spurges

     • Yellow - some daisies, rabbitbrush

     • Clear/watery - not true latex

3. Measure abundance:

   • Low: Just a tiny bead forms

   • Moderate: Small droplet forms, flow stops quickly

   • High: Flows freely for several seconds

4. Time flow duration:

   • Start timer when damage occurs

   • Stop when flow ceases

   • Record seconds

   • High production = flows 10+ seconds

5. Describe consistency:

   • Watery (thin, runs quickly)

   • Moderate (like milk)

   • Thick and gummy (like glue)

   • Very sticky (strings when pulled)

The stickier and more abundant the latex, the better it works at:

• Gumming up insect mouthparts

• Healing wounds (seals cut)

• Deterring future attacks (bad experience!)

Step 2: Test for Resin Production

Resin = Sticky, aromatic substance (especially common in desert plants!)

Resin is like super-concentrated essential oils. It's sticky, smells strong, and tastes terrible. It also doesn't wash off easily - think of how hard it is to clean pine sap off your hands!

Abundant:

• Creosote bush (Larrea tridentata) - FAMOUS for resin, "desert rain smell"

• Brittlebush (Encelia farinosa) - aromatic resin

• Burro bush (Ambrosia spp.) - aromatic

• Junipers - aromatic foliage

• Pinyon pine - sticky resin (if at elevation)

How to test:

1. Crush or scratch leaves/needles between gloved fingers

   • Gentle crushing releases compounds

   • No need to destroy plant

2. Smell: Strong piney, aromatic scent?

   • Terpenes! (volatile aromatic compounds)

   • Rate intensity: 1 (none) to 5 (very strong)

   • Describe: Piney? Medicinal? Turpentine-like? Pleasant or unpleasant?

3. Touch: Sticky residue on gloves?

   • Rub leaves between fingers

   • Feel for tackiness

   • Look for shiny residue

4. Quantity:

   • Slightly sticky (minimal resin)

   • Moderately resinous (noticeable stickiness)

   • Very sticky/gummy (obvious resin coating)

Creosote bush! After crushing leaves, your gloves will be sticky and aromatic. This resin makes creosote nearly invulnerable to herbivores! Creosote resin smell after desert rain is one of the most distinctive desert experiences. That's volatile terpenes being released!

Step 3: Correlate Chemistry with Herbivory

The ultimate test: Do chemical defenses actually work?

For each plant surveyed:

1. Note chemical defenses present:

   • Latex: Yes/No (abundance: L/M/H)

   • Resin: Yes/No (abundance: L/M/H)

   • Strong smell: Yes/No

   • Visual indicators: List any

2. Assess herbivory damage:

   • Examine 20 leaves

   • Count damaged leaves

   • Estimate average % leaf area eaten

3. Look for pattern:

   • Do plants with strong chemical defenses show less herbivory?

   • Which defense type is most effective?

   • Are there specialist herbivores that overcome defenses?

Step 4: Data Tables

Chemical Defense Data:

Part 4: Parasitic Plant Survey

Plant parasites = Plants that steal water, nutrients, or sugars from other plants!

Simple explanation: Most plants make their own food through photosynthesis. Parasitic plants are the cheaters - they tap into other plants and steal what they need. Some are partial cheaters (hemiparasites), others are complete freeloaders (holoparasites).

Step 1: Search for Parasitic Plants

Common parasitic plants:

MISTLETOE (hemiparasite) - VERY COMMON:

Species:

• Desert mistletoe (Phoradendron californicum) - MOST COMMON

• Found on: Mesquite, palo verde, acacia, ironwood, desert willow

Identification:

• Green, leafy clumps in tree branches

• Thick, succulent stems

• Small scale-like leaves or no leaves

• Especially visible in winter

• Orange/red berries (if in season)

Where to look:

• Up in tree branches (use binoculars!)

• Older trees more heavily parasitized

• Washes and riparian areas (more common)

• Look for abnormal green clumps that don't match tree foliage

Mistletoe berries are spread by birds! They eat the berries, and the sticky seeds pass through their digestive system and stick to branches where birds perch.

Step 2: Document Parasite Characteristics

For each parasite found:

A. Identify Host Plant Species

This is critical! Parasites are often host-specific.

• What plant species is being parasitized?

• Are multiple species parasitized or just one?

• Document with photo

B. Count Parasite Abundance

Quantify the infection level:

• How many parasites per host plant?

• Count visible clumps on one tree

Rate:

• Low (1-2 parasites)

• Moderate (3-10 parasites)

• High (>10 parasites)

• Severe (tree heavily loaded)

C. Examine Attachment Points

• How deeply embedded in branch?

  • Flush with surface?

  • Swollen bulge at attachment?

  • Deep penetration visible?

• Any swelling or galling at attachment point?

• Multiple mistletoes on same branch?

D. Assess Host Health

Compare parasitized host to nearby non-parasitized plants of same species:

Vigor:

Rate overall health:

·         1 = (mostly dead branches)

·         2 = (significant die-back)

·         3 = (yellowing, thin canopy)

·         4 = (slight differences)

·         5 = (looks normal)

Leaf loss and growth:

·         Defoliation (losing leaves)?

·         Thin canopy (can see through tree easily)?

·         Dead branches (no leaves at all)?

·         Compare to healthy tree nearby

·         Height/size (smaller than expected)?

Step 3: Data Table