What the potassium hydroxide did

Two leaves on one plant, in the same sunshine, for the same five hours. One made starch and the other made none.

Figure to be added

Figure 2.8 — the two leaves after the starch test. Leaf D is still its own pale green, labelled as showing no colour change. Leaf C is a deep blue shading towards dark purple across the whole blade.

The observation

When iodine solution was added to leaf D, no colour change was seen; when it was added to leaf C, the leaf turned dark purple or blue.

Reading the result

The potassium hydroxide contained in the polythene bag holding leaf D absorbed the carbon dioxide inside that bag. Leaf D therefore did not receive carbon dioxide, so photosynthesis did not take place in it, and no colour change occurred with the iodine solution.

Leaf C did receive carbon dioxide, so photosynthesis took place, and a colour change occurred with the iodine solution. Starch has not been produced in leaf D, and starch has been produced in leaf C.

The conclusion

It can be concluded that carbon dioxide is necessary for photosynthesis.

Notice what was not done

Nobody added extra carbon dioxide to leaf C to make it photosynthesise harder. The experiment works by subtraction only: everything is left normal except that one gas is taken away from one leaf. That is the cleanest form an experiment can take, and all four factor tests in this chapter are built that way.

Take the carbon dioxide away and nothing is made

A destarched leaf sealed with KOH, which absorbs carbon dioxide, shows no colour change with iodine; an identical leaf sealed with water does. Carbon dioxide is necessary for photosynthesis.