Why starch is the clue
You cannot watch photosynthesis happen. Every experiment in the rest of this chapter works around that by looking for something it leaves behind.
The reasoning
Because the glucose produced in photosynthesis is stored temporarily within the leaves themselves, as starch, the starch test is carried out in order to find out whether or not photosynthesis has taken place.
The chain of reasoning, in three links
- If photosynthesis happened in this leaf, glucose was made.
- If glucose was made, some of it is sitting in the leaf right now as starch.
- So if the leaf contains starch, photosynthesis happened — and if it contains none, it did not.
It works because the store is in the leaf itself
Suppose the plant sent every molecule straight out to the roots as it was made. Then a leaf would tell you nothing, and the whole method would collapse. The temporary storage is an inconvenience for the plant and a gift to the experimenter: it leaves the evidence at the scene.
Six experiments, one test
Every one of the experiments that follows ends the same way — take the leaf, do the starch test, see whether the colour changes. What differs between them is only what was taken away from the plant beforehand. Learn the test once and the experiments become a single idea repeated.