These tiny openings are thestomates,throughwhich a leaf takes incarbon
dioxide from the air. There areabouthalfa millionstomateson an average
sized leaf. Each iscontrolledby two "guard"cellsthatregulate the sizeof
theopening,dependingupon how much carbondioxide the leaf needs. The
guardcells close theopeningto prevent the escapeofwaterwhen the soil is
dry due to lackofrain.
Thestomatesare on the undersurfaceratherthan on the upper surface,
so they willnotbecome clogged by dust or insects. Also, if they were on the
uppersurface, thesun'sdirect light and heat would tend to causegreatloss
ofwater byevaporation,and the leaf would wilt and die.
But some leavesdohavestomateson their uppersurfaces. Forexample,
leavesthatgrow upright have stomates on both upper and lower surfaces.
Waterlily leaves (called"pads")float on the surface of thewater; they have
their stomates on the upper surface. Otherwise the leaves would"drown"
because their air spaces would become filled with water.
SHOWING THAT OXYGEN IS ABY-PRODUCT OF PHOTOSYNTHESIS
Materials:Two smallelodeaplants,twowide-mouthedjars,two test tubes,
two glass funnels, several toothpicks or wooden splints. Ifyou don'thave
elodeain your fishtank,you can buy some at a pet supply store.
Follow this procedure:Place oneplantin eachjar. Inverta glass funnel
over eachplant. Now fill each test tube with water. Holdingyourthumbover
theopeningofthe test tube, invert it. Lower itunderthe waterin thejar;
take yourthumbaway and place the test tube over theinvertedfunnel. Do
the same with the secondjar.
Place onejarin thesunlightfor a day. Place theotherin a darkcloset.
(The secondjaris called the"control." Ithelps you be surethatthe results
ofyourexperimentare valid.)
Atthe endofa day carefully remove the test tube from thejarwhich was
in the sunlight. Keep your thumbover the test tube opening. Now, light a