Now, ~what use could the cotyledons be if they only shrivel away~?
Take a freshly soaked seed and cut a thin slice of the nurse-leaves and drop it into a little solution of iodine;[1] the tissue will go a violet blue colour. Then drop iodine on a piece of bread, a piece of potato, and some boiled rice, and you will find that they also go blue, or almost black. The food in the nurse-leaves is in some ways the same as that in bread, potato, and rice, and in many other things we eat.
[1] Get a chemist to make a solution of iodine and potassium iodide, which should be a bright, clear, orange colour.
The part of the food which goes blue with iodine is ~starch~, and this blue colouring of starch with iodine is an easy and safe test for it. You will see the same colour if you take some ordinary laundry starch and stir it up with hot water and a little iodine. Look now at the corn seed; the white solid mass in the seed contains starch, as you can prove with iodine, and although it is not in the cotyledon, yet it is quite near the young plant, which can get at it easily.
We have found, therefore, that ~young plants have a store of food in their nurse-leaves, or near them in the seed~, and that this food is the same as very much of our own food, that is, it is starch. There are other food substances present, too, but they are more difficult to find. The seed, therefore, contains not only the young living plant itself, but also a storehouse of food for its use, ~and as the plant grows we see this store getting less and less~ in the shrivelling cotyledons. This shows that the young plant uses up this food in the course of its growth.
But you must not forget that, although we find the young plants provided with food in this way, we have not yet settled the question of the food supply for all plants. As we see, the cotyledons shrivel up and are emptied of their store long before the plant is full grown. Remember that baby calves have milk for food, while old cows have grass. And when the store of food supplied in the seed is finished the older plants must find new supplies for themselves.
In growing seedlings you must always keep them well supplied with water, the soil or sawdust in which they grow ~must~ be kept moist. If you take one out of the sawdust and try to grow it only in the air, you will find that it soon dies. Even for the seedling the storehouse of food is not enough; it requires to have water too.