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Investigation:​ ​ ​ ​Leaf​ ​Stomata Background​ ​Information:

Leaf​ ​stomata​ ​are​ ​the​ ​principal​ ​means​ ​of​ ​gas​ ​exchange​ ​in​ ​vascular​ ​plants. Stomata​ ​are​ ​small​ ​pores,​ ​typically​ ​on​ ​the​ ​undersides​ ​of​ ​leaves,​ ​that​ ​are​ ​opened or​ ​closed​ ​under​ ​the​ ​control​ ​of​ ​a​ ​pair​ ​of​ ​banana-shaped​ ​cells​ ​called​ g ​ uard​ ​cells​. When​ ​open,​ ​stomata​ ​allow​ ​CO​2​​ ​to​ ​enter​ ​the​ ​leaf,​ ​and​ ​allow​ ​for​ ​water​ ​and oxygen​ ​to​ ​escape.​ ​In​ ​addition​ ​to​ ​opening​ ​and​ ​closing​ ​the​ ​stomata,​ ​plants​ ​may exert​ ​control​ ​over​ ​their​ ​gas​ ​exchange​ ​rates​ ​by​ ​varying​ ​stomata​ ​density​ ​in​ ​new leaves​ ​when​ ​they​ ​are​ ​produced​ ​(such​ ​as​ ​in​ ​the​ ​spring​ ​or​ ​summer).​ ​The​ ​more stomata​ ​per​ ​unit​ ​area​ ​is​ ​the​ ​stomata​ ​density​. Why​ ​might​ ​it​ ​be​ ​adaptive​ ​for​ ​a​ ​plant​ ​to​ ​control​ ​its​ ​rates​ ​of​ ​water​ ​loss​ ​and​ ​CO​2 uptake?​ ​One​ ​answer​ ​can​ ​be​ ​found​ ​in​ ​the​ ​sun.​ ​ ​Increases​ ​in​ ​heat​ ​could​ ​denature proteins​ ​of​ ​membranes​ ​of​ ​chloroplasts.​ ​ ​When​ ​overheated,​ ​plants​ ​may​ ​open the​ ​stomata​ ​to​ ​evaporate​ ​water​ ​to​ ​lower​ ​the​ ​temperature.​ ​ ​ ​Based​ ​on​ ​this,​ ​you could​ ​hypothesize​ ​that​ ​leaves​ ​in​ ​the​ ​sun​ ​would​ ​have​ ​a​ ​higher​ ​stomata​ ​density than​ ​leaves​ ​in​ ​the​ ​shade. On​ ​the​ ​other​ ​hand,​ ​if​ ​water​ ​is​ ​not​ ​available,​ ​such​ ​as​ ​under​ ​drought​ ​conditions,​ ​excessive​ ​evaporation​ ​might​ ​lead​ ​to desiccation​ ​(drying​ ​out.)​ ​ ​Plants​ ​in​ ​hot​ ​areas​ ​may​ ​benefit​ ​from​ ​fewer​ ​stomata​ ​so​ ​that​ ​they​ ​can​ ​conserve​ ​water. The​ ​discussion​ ​illustrates​ ​an​ ​important​ ​concept​ ​in​ ​experimental​ ​biology,​ ​that​ ​there​ ​are​ ​often​ ​alternative​ ​hypotheses​ ​to explain​ ​variation​ ​in​ ​nature.​ ​ ​ ​In​ ​this​ ​case,​ ​the​ ​stomata​ ​density​ ​may​ ​increase​ ​or​ ​decrease​ ​in​ ​response​ ​to​ ​environmental variation​ ​in​ ​sunlight​ ​and​ ​water​ ​availability. Prelab​ ​Questions 1.​ ​ ​What​ ​role​ ​do​ ​stomata​ ​play​ ​in​ ​homeostasis?

2.​ ​ ​ ​Two​ ​hypotheses​ ​for​ ​stomata​ ​density​ ​are​ ​described.​ ​ ​How​ ​could​ ​you​ ​test​ ​them?

3.​ ​ ​ ​Plant​ ​species​ ​might​ ​have​ ​variation​ ​in​ ​the​ ​number​ ​of​ ​stomata​ ​present​ ​on​ ​their​ ​leaves.​ ​ ​Why?

4.​ ​ ​Plants​ ​move​ ​waters​ ​from​ ​the​ ​roots​ ​of​ ​the​ ​plant​ ​to​ ​the​ ​leaves​ ​through​ ​transpiration.​ ​ ​As​ ​water​ ​evaporates​ ​from​ ​the leaf,​ ​cohesion​ ​pulls​ ​water​ ​up​ ​from​ ​the​ ​roots.​ ​ ​ ​ ​Suggest​ ​a​ ​relationship​ ​between​ ​transpiration​ ​and​ ​stomata​ ​density.

Procedure​ ​for​ ​Counting​ ​Stomata 1.​ ​Obtain​ ​a​ ​leaf.​ ​Trace​ ​the​ ​outline​ ​of​ ​the​ ​leaf​ ​on​ ​a​ ​piece​ ​of​ ​graph​ ​paper.​ ​Estimate​ ​the​ ​surface​ ​area​ ​by​ ​counting how​ ​many​ ​squares​ ​fit​ ​inside​ ​the​ ​outline​ ​of​ ​the​ ​leaf.​ ​If​ ​you​ ​have​ ​partial​ ​squares,​ ​try​ ​to​ ​add​ ​them​ ​up​ ​to​ ​equal whole​ ​squares.​ ​Record​ ​the​ ​surface​ ​area​ ​in​ ​the​ ​table​ ​below. 2.​ ​Paint​ ​a​ ​thin​ ​strip​ ​of​ ​clear​ ​fingernail​ ​polish​ ​a​ ​section​ ​of​ ​the​ ​leaf​ ​(paint​ ​between​ ​parallel​ ​veins).​ ​Allow​ ​the fingernail​ ​polish​ ​to​ ​completely​ ​dry. 3.​ ​Place​ ​a​ ​piece​ ​of​ ​clear​ ​tape​ ​over​ ​the​ ​dried​ ​nail​ ​polish.​ ​ ​Gently​ ​but​ ​firmly​ ​press​ ​the​ ​tape​ ​onto​ ​the​ ​leaf. 4.​ ​Peel​ ​the​ ​tape​ ​from​ ​the​ ​leaf​ ​and​ ​place​ ​the​ ​tape​ ​sticky​ ​side​ ​down​ ​onto​ ​a​ ​microscope​ ​slide.​ ​Examine​ ​the​ ​tape, which​ ​now​ ​contains​ ​an​ ​impression​ ​of​ ​the​ ​leaf​ ​cells.​ ​Try​ ​to​ ​use​ ​both​ ​scanning​ ​and​ ​low​ ​power. 5.​ ​Count​ ​the​ ​number​ ​of​ ​stomata​​ ​in​ ​the​ ​field​ ​of​ ​view.​ ​ ​ ​Move​ ​the​ ​slide​ ​so​ ​you​ ​can​ ​see​ ​other​ ​areas.​ ​It​ ​may​ ​be best​ ​to​ ​take​ ​an​ ​average​ ​of​ ​several​ ​areas​ ​on​ ​the​ ​leaf. 6.​ ​ ​Find​ ​the​ ​density​ ​of​ ​stomata​ ​can​ ​be​ ​measured​ ​by​ ​dividing​ ​the​ ​number​ ​of​ ​stomata​ ​by​ ​the​ ​surface​ ​area​ ​of​ ​the leaf.

Choose​ ​a​ ​question​ ​to​ ​investigate: 1)​ ​How​ ​does​ ​the​ ​density​ ​of​ ​stomata​ ​vary​ ​amongst​ ​different​ ​types​ ​of​ ​plants. 2)​ ​How​ ​does​ ​the​ ​density​ ​of​ ​stomata​ ​vary​ ​amongst​ ​leaf​ ​surfaces​ ​(upper​ ​versus​ ​lower​ ​surface​ ​of​ ​leaf)? 3)​ ​How​ ​does​ ​the​ ​density​ ​of​ ​stomata​ ​vary​ ​among​ ​leaves​ ​found​ ​on​ ​the​ ​same​ ​plant​ ​(leaves​ ​at​ ​the​ ​top​ ​very​ ​leaves at​ ​the​ ​bottom)?

Experimental​ ​Design​ ​ ​-​ ​the​ ​procedure​ ​for​ ​counting​ ​stomata​ ​and​ ​calculating​ ​density​ ​is​ ​described​ ​above. Consider​ ​the​ ​question​ ​you​ ​chose​ ​to​ ​investigate,​ ​collect​ ​data​ ​and​ ​create​ ​a​ ​data​ ​table​ ​to​ ​organize​ ​your observations.​ ​ ​ ​You​ ​may​ ​want​ ​to​ ​take​ ​photos​ ​to​ ​include​ ​with​ ​your​ ​final​ ​report. Make​ ​a​ ​Claim​​ ​that​ ​summarizes​ ​the​ ​answer​ ​to​ ​your​ ​experimental​ ​question. Using​ ​data​ ​from​ ​this​ ​experiment,​ ​provide​ ​Evidence​​ ​that​ ​supports​ ​the​ ​claim. Using​ ​background​ ​knowledge​ ​and​ ​data​ ​from​ ​this​ ​lab,​ ​provide​ ​Reasoning​​ ​that​ ​uses​ ​the​ ​evidence​ ​to​ ​justify​ ​the claim.​ ​ ​ ​ ​For​ ​this​ ​section​ ​you​ ​may​ ​need​ ​to​ ​do​ ​research​ ​outside​ ​the​ ​class​ ​on​ ​the​ ​type​ ​of​ ​plant​ ​you​ ​investigated.

Your​ ​group​ ​will​ ​share​ ​your​ ​data,​ ​claim,​ ​evidence​ ​and​ ​reasoning​ ​with​ ​the​ ​class​ ​as​ ​either​ ​a​ ​poster,​ ​ ​powerpoint slides,​ ​or​ ​a​ ​digital​ ​infographic.