You willobserve:The leaf has becomeeithergreyish or completelywithout
pigment. But after it wastreatedwith dilute iodine, itturnedblue-black. The
alcoholin which the leaf was boiled hasturnedgreen.
By boiling the leaf in alcohol, you removed itschlorophyll,orfood-making
substance. Andlater, when youtreatedit with dilute iodine, you tested it for
the presence ofstarch. If iodine causes a blue-blackshade to appear, you
knowthatthesubstanceyou testedcontainsstarch.
You rememberthata green leaf is afood-makingfactory for the entire
plant,andthatchlorophyll,which makes the leaf green, is the basicmachinery
ofthis factory. Itputstogetherrawmaterialsto make a finishedproduct,in
this case starch. In the plant,the rawmaterialsare water (which the plant
gets from the soilthroughits roots) and the gascarbondioxide. A leafactually
breathesthroughmicroscopicopenings calledstomateslocatedon its under-
surface;this is how a leaf takes incarbondioxide from the air.
Every factory needs power, or energy, to run its machinery. Sunlightis
the source of power for every leaf factory. Withoutsunlight, leaves wouldnot
be able tomanufacturefood for plants, nor,indirectly,for animals andhumans.
The first food productthata leafmanufacturesis a formofsugar, butthe
plantchanges this sugar to a type ofstarchfor easierstorage.
This miraculous food-making process-on which all life depends-is
calledphotosynthesis. "Photo" refers to light (in this case, sunlight) and
"synthesis" refers to the "manufactureof." The process of photosynthesis
is the manufactureofsimple sugars by the green plant in the presence of
sunlight.