If you take a leaf growing in the sunlight, and cover a portion of it, leaving the rest exposed, you will be able to see the effect of light and darkness on the starch-building powers of this particular leaf. To do this use two flat pieces of cork or thick cardboard, covered with silver paper or tin foil about 1 in. to 1-1/2 in. big, and of the same size and shape. Place a part of a healthy leaf between them and bind them tightly together, as in fig. 14. If the weight of the cork makes the leaf bend down out of the full sunlight, then support it so that it lies in a position where it is well lighted. Leave it untouched for three days, and then in the middle of a bright day cut the leaf from the tree, remove the cork when you get into the house, and immediately treat it as described above for the iodine test. You will find that the part of the leaf which was exposed shows a good violet colour, proving that starch is present there, while the part which was covered is only yellowish, showing that starch has not been developed in this portion (see fig. 15). This proves that ~the covered part of the leaf could not build starch~, so that exposure to the light and air seems to be necessary, as we expected. This further suggests that it is only in the daytime that the plant can build starch. You can see that this is actually the case by testing leaves from the same tree at different times of the day and comparing the starch in them. For example, test a leaf from a certain plant in the early afternoon, when it has been exposed all day to good sunlight, and compare it with one which is gathered just before sunrise, if you can get up so soon (this is, of course, easier in the spring or autumn, when the sun does not rise so early as in midsummer). You will find that the leaves picked in the early afternoon are packed with starch, while those picked before the day begins show very little or none. [Illustration: Fig. 15. Same leaf as in Fig. 14 treated with iodine. It shows that the covered part had formed no starch.] What then becomes of the starch during the night? You will remember that we found much starch in potatoes, which you know grow right underground, and therefore, according to the experiments we have just done, should not contain starch. But it is found that the starch is ~made in the leaves~ through the day, and is slowly carried down the stems in solution, and then ~stored (not made) in the underground parts~, such as bulbs, potatoes, thick roots, and many others. It is like the shopkeeper, who collects some money each day and sends it every evening to the bank to be stored for him. The leaves of the plant are then fresh next day to begin the work of building up more starch. [Illustration: Fig. 16. Striped leaves; the white stripes show no starch when stained with iodine.]