!

Lab$4:$Secondary$growth$and$woody$stems!!

Upon!completion!of!this!lab,!the!student!should!be!able!to:!

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❧ Explain!the!differences!between!primary!and!secondary!growth.!

❧ Explain!how!a!stem!grows!in!both!length!and!diameter.!

❧ Explain!how/why!bud!scale!scars!can!be!used!to!determine!the!age!of!a!

woody!twig.!

❧ Make!a!diagram!showing!external!features!of!a!woody!twig,!labeling!

terminal!bud,!axillary!bud,!bud!scales,!

terminal!bud!scale!scars,!leaf!scars.!

❧ Identify!the!tissues!and!structures!in!a!cross!section!of!a!woody!stem,!as!listed!

in!the!exercise.

❧ Determine!the!age!of!a!stem!by!examining!its!cross!section.!!

❧ Differentiate!between!spring!wood!and!summer!wood.!

❧ Differentiate!between!heartwood!and!sapwood.!

❧ Define!and/or!recognize!in!an!image!the!tissues,!structures,!or!cells!mentioned!in!bold!throughout!the!exercise.!

$Growth!in!herbaceous!(i.e.,!nonDwoody)!plants!is!mainly!primary!growth,!in!which!all!of!the!procambium!differentiates!into!primary!xylem!and!phloem.!!Herbaceous!

plants!typically!do!not!grow!very!large!in!comparison!to!plants!that!undergo!

secondary!growth!and!become!woody.!Banana!plants!(the!genus!Musa)!are!popularly!considered!to!be!the!world’s!largest!herbs.!

! In!plants!that!will!undergo!secondary$growth,!some!cells!in!the!procambium!retain!the!ability!to!divide.!!These!cells!develop!into!a!new!meristematic!tissue!called!

the!vascular$cambium!(VC),!between!the!primary!phloem!and!primary!xylem.!!Cells!of!the!VC!divide!on!a!plane!that!is!parallel!to!the!epidermis!and!to!the!long!axis!

of!the!plant;!this!results!in!an!increase!in!diameter!of!the!stem!(or!root).!Tissues!

produced!by!the!VC!are!called!secondary$tissues.!!The!resulting!increase!in!size!is!called!secondary$growth.!!!! New!cells!formed!toward!the!exterior!of!the!stem!will!differentiate!into!

secondary$phloem,!while!cells!formed!toward!the!interior!will!become!secondary$xylem.!!The!VC!is!most!active!in!woody!plants!–!shrubs!and!trees!–!in!which!it!continues!to!produce!secondary!tissues!for!as!long!as!the!plant!lives.!!Some!

relatively!herbaceous!plants!do!have!a!VC!but!it!is!only!active!for!a!short!period!of!

time,!and!the!secondary!tissues!produced!are!not!very!prominent.!!Monocots!(corn,!

banana,!grasses,!et!al.)!lack!a!VC!entirely.!!

$

I. Secondary$Growth$in$Helianthus.$$We!previously!examined!the!primary!tissues!of!young!Helianthus,stem.!!We!will!now!see!what!happens!as!the!Helianthus!stem!matures,!and!a!little!secondary!growth!begins.!!Obtain!one!of!the!prepared!slides!of!Helianthus!stem!cross!sections,!and!now!observe!the!larger!section!through!the!compound!microscope,!noting!the!following!

tissues!(beginning!at!the!outside!and!moving!inward!toward!the!center).!!You!

should!notice!many!similarities!to!the!young!stem!examined!previously,!but!note!the!

changes!that!have!occurred.!(see!Atlas!p!136,!fig!9.35)!

!

1. Epidermis:!the!single!layer!of!cells!covering!the!surface!of!the!stem.!!Stomata!may!be!present!in!the!stem!epidermis,!but!not!in!large!numbers.!

2. Bundle$Caps:!The!large!clusters!of!redDstained!cells!in!the!outer!part!of!each!vascular!bundle!are!the!bundle!caps.!!These!cells!were!previously!discussed!as!

an!example!of!sclerenchyma!fibers.!!!

3. Cortex:!The!tissue!between!the!epidermis!and!the!vascular!bundles!(not!including!the!bundle!caps)!is!the!cortex.!!What!two!types!of!tissues!can!be!seen!in!the!cortex?!(1)!

4. Primary$phloem!can!be!seen!in!each!vascular!bundle,!immediately!adjacent!to!the!bundle!cap.!!

5. Secondary$phloem:!this!tissue!may!not!be!very!well!developed!in!these!sections.!!Secondary!tissues,!though,!can!generally!be!recognized!by!the!

relatively!orderly!appearance!of!cells!in!rows!and!columns.!

6. Vascular$cambium:!!A!darkDgreenDstaining!band!of!tissue!appears!to!extend!from!some!of!the!vascular!bundles.!!In!some!sections,!this!may!for!a!continuous!

ring!around!the!stem.!!This!is!the!vascular$cambium.!It!develops!first!in!the!vascular!bundles,!from!cells!in!between!the!primary!xylem!and!primary!phloem.!!

What!kind!of!tissue!is!the!VC?!!(2)!What!is!its!function?!!(3)!

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7. Secondary$xylem:!found!immediately!inside!the!vascular!cambium.!!!8. Primary$xylem:!the!tissue!with!the!large,!redDstained!vessel!elements!and!some!

parenchyma.!

9. Pith:!the!tissue!of!the!central!part!of!the!stem.!10. $Pith$rays:!in!some!places,!pith!tissue!appears!to!pass!between!the!vascular!

bundles!and!connect!to!the!cortex.!!These!extension!of!the!pith!are!the!pith!rays.!!!

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Below!is!a!photograph!from!one!of!our!slides.!!How!many!of!the!tissues!mentioned!

above!can!you!identify!and!label?!(4)!

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!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !

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$II. Internal$anatomy$of$a$woody$stem$of$Tilia.$

$The!VC!and!secondary!tissues!are!not!very!well!developed!in!Helianthus.!!For!a!look!at!“real”!secondary!growth,!obtain!a!prepared!slide!of!Tilia,americana!(“American!linden”)!stem!cross!sections.!!Our!slides!include!sections!from!oneD,!twoD,!threeD,!and!

fourDyear!old!stems.!!Look!at!the!slide!under!low!power,!or!even!use!a!dissecting!

scope,!to!survey!the!four!sections!and!identify!the!oneD!and!threeDyearDold!sections!

for!examination.!!!

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A.$OneFyearFold$Tilia,stem$section:$Examine!the!oneDyearDold!section!on!the!slide.!!It!is!the!section!with!ONE!growth!ring,!and!is!probably!the!smallest!section!on!the!

slide.!Beginning!at!the!center!of!the!stem!and!working!your!way!outward,!examine!

each!tissue!as!follows:$!

1. Locate!the!central!pith.!!!2. Primary$xylem$tissue!can!be!recognized!as!masses!of!vessel!elements!that!

extend!into!the!pith.!!These!regions!of!primary!xylem!are!remnants!of!the!

separate!vascular!bundles!that!were!present!in!the!stem!when!it!was!younger!

(they!would!have!looked!similar!to!the!vascular!bundles!in!young!Helianthus).!3. The!ring!of!tissue!next!to!the!primary!xylem!is!the!secondary$xylem.!!Another!

name!for!secondary!xylem!is!wood.!

4. The!vascular$cambium!exists!as!thin!cylinder!of!tissue!surrounding!the!secondary!xylem.!!In!cross!section,!it!appears!as!a!narrow!ring!of!darklyDstained!

cells.!

5. Secondary$phloem!is!the!next!tissue!on!your!journey!to!the!surface!of!the!stem.!!Like!primary!phloem,!this!tissue!contains!sieve$tube$members!and!companion$cells!(c.c.!may!be!difficult!to!identify).!!Additionally,!the!secondary!xylem!of!Tilia!includes!phloem$fibers!and!some!additional!parenchyma!cells.!!!

6. The!primary$phloem!is!at!the!outermost!part!of!the!phloem!tissue.!!It!may!be!difficult!ti!dinstinguish!the!boundary!between!primary!phloem!and!secondary!

phloem.!!!

7. The!cortex!is!just!external!to!the!phloem.!At!this!young!age,!there!may!be!some!epidermal!tissue!still!present!also.!!!

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B.$3FyearFold$stem$of$Tilia:$Once!you!are!familiar!with!the!anatomy!of!the!oneDyearDold!stem,!examine!the!cross!section!of!the!threeDyearDold!stem!on!the!same!

slide.!!!This!section!has!three!growth!rings.!!$!

1. Locate!the!secondary!xylem.!!Notice!the!three!annual!rings.!!Each!annual!ring!contains!both!springwood!and!summerwood.!!How!do!the!cells!(the!vessel!elements)!in!these!two!regions!differ?!(1)!

2. The!secondary!xylem!contains!several!kinds!of!cells:!a. Vessel$Elements:!these!are!the!largest!cells!in!the!xylem,!and!are!the!chief!

waterDconducting!cells.!

b. Tracheids:!smaller!in!diameter!than!vessel!elements,!these!cells!conduct!water!and!provide!structural!support.!

c. Fibers:$thickDwalled!cells!that!primarily!function!in!structural!support.!!!d. Xylem$parenchyma:!rather!than!becoming!conducting!cells!or!fibers,!some!

cells!in!the!xylem!differentiate!into!parenchyma.!!These!xylem!parenchyma!

cells!may!function!as!water!storage!cells.!

3. Vascular$rays:!these!are!lines!of!parenchyma!cells!radiating!outward!from!the!pith!toward!the!stem!surface.!!Rays!conduct!water!and!nutrients!from!the!xylem!

outward!to!supply!the!cells!in!the!outer!stem!tissues.!!They!also!conduct!sugars!

and!other!nutrients!inward.!Xylem$rays!are!those!on!the!xylem!side!of!the!VC;!xylem!rays!are!often!only!a!few!cells!wide.!!Phloem$rays!are!on!the!outer!side!of!the!VC;!some!phloem!rays!become!dilated,!expanding!into!VDshaped!masses!of!parenchyma.!!!

4. Look!at!the!outer!portion!of!the!stem.!!The!epidermis!is!no!longer!present;!it!has!been!replaced!by!layers!of!cork!tissue.!!This!cork!tissue!develops!from!another!

secondary!meristem,!the!cork!cambium.!!!

5. Bark:!all!of!the!tissues!outside!the!vascular!cambium!collectively!comprise!the!bark$of!a!tree.!!!

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C.$4Fyear$old$section$of$Tilia:!Below!is!a!photograph!of!a!cross!section!from!a!fourDyearDold!stem!section!of!Tilia.!!Recognize!and!label!the!following:!pith,!primary$xylem,!secondary$xylem,!secondary$phloem,!vascular$cambium,!xylem$ray,!undilated$phloem$ray,!dilated$phloem$ray,!annual$growth$ring,!springwood,!summerwood,!cork$tissue,!cortex.!!Be!able!to!identify!the!first,!second,!third,!and!fourth!years!of!2°!xylem!growth.

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III. External$features$of$a$woody$(dicot)$stem.$$

Here!you!will!examine!a!woody!branch!from!a!deciduous!tree,!and!locate!the!

following!features!and!structures.!!(See!Atlas!pp!135,!140)!

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1. Terminal$bud.!!The!terminal!bud!is!the!large!bud!at!the!apex!of!the!twig.!!It!is!covered!with!specially!modified!leaves!called!bud$scales.!!They!serve!to!protect!the!dormant!apical$meristem!and!any!leaf!and!bud!primordia!that!have!already!formed.!!!

2. Leaf$scars.!!Look!along!the!stem!for!these!somewhatDtriangular!regions!that!represent!places!where!leaf!petioles!where!attached.!!What’s!a!petiole?!What!

was!the!leaf!arrangement!on!this!twig?!

3. Use!a!dissecting!scope!to!examine!several!leaf!scars.!!You!may!be!able!to!distinguish!small!dots!of!tissue!within!the!scars.!!These!spots!are!remnants!of!

vascular!bundles!that!supplied!the!leaf.!!The!number!of!traces!can!vary!

depending!on!the!species,!and!is!sometimes!useful!in!identification.!

4. !Axillary$buds!are!found!just!above!each!leaf!scar.!!Remember!the!general!relation!of!leaf!primordia!and!bud!primordia!in!Coleus?!!Not!all!twigs!will!have!a!full!complement!of!axillary!buds,!since!these!frequently!get!damaged!and!

knocked!off!the!plant!during!the!growing!season.!

5. Lenticels:$Lenticels!are!openings!in!the!bark!that!allow!for!gas!exchange!into!and!out!of!the!underlying!living!tissues.!!They!are!often!visible!as!small!dots!or!

slits!in!the!otherwise!smooth!bark!surface.!!The!shape!and!size!of!lenticels!can!

also!be!a!useful!characteristic!in!identifying!some!woody!plants.!

6. Terminal$bud$scale$scars.!!Bud!scales!also!leave!scars!when!they!fall!off,!after!the!buds!“break.”!!The!bud!scale!scars!appear!as!thin!rings!of!corky!tissue!

encircling!the!stem.!!The!interval!between!successive!groups!of!bud!scale!scars!

represents!one!year!of!growth.!

7. In!the!space!below,!draw!a!diagram!of!a!woody,twig!of!a!deciduous!tree,!which!had!opposite,leaves,!during!the!winter!after,it’s,third!year!of!growth.!!DO!NOT!draw!a!cross!section,!but!diagram!of!external!features.!Things!to!include:!an!

appropriate!number!of!bud$scale$scars,!at!least!one!node!per!each!year!of!growth,!axillary$buds!and!leaf$scars!in!the!appropriate!places,!and!a$terminal$bud.!!Also!indicate!the!oneDyearDold!and!twoDyearDold!portions!of!the!stem.!!!

8. Check!out!some!of!the!wood!blocks!and!woody!stem!chunks!on!display.!!Note!the!annual$growth$rings,!and!the!boundary!between!sapwood!(active!conducting!tissue)!and!heartwood$(secondary!xylem!that!no!longer!conducts!water).!!