We must always use the pollen from the same kind of a plant, however. It would be of no use to put nasturtium pollen on a morning-glory stigma, for instance, for it could not affect the ovule in the least. The protoplasm knows in some way its own plant and will not fertilize any other. This is a very good thing, otherwise we might have a funny mixture of all sorts of plants. Many delicious fruits have been produced by fertilizing one plant with pollen from another. New varieties of grapes and berries are constantly obtained in this way. If you live on a farm or have a garden, you might try to develop some new kinds of berries or fruits. You might not succeed, but it would do no harm to try. [Illustration: [Ovule]] CHLOROPHYLL. [Illustration: [Leaves]] Chlorophyll is plant green. That is what the word means. We are so used to seeing green leaves that we think very little about it. It probably never has occurred to most of us that the green coloring matter of plants can be of much importance. Yet it is one of the most important things in the world. Like many other things, it is not what it seems. It is not merely a dye as one might suppose, but much more than that. We cannot really see what it is without a microscope, and when we look at a piece of green leaf through the microscope we are surprised to find the leaf is not green at all. It is colorless like glass, but in the cells just behind the skin cells we see little roundish green bodies packed away. These are the chlorophyll grains, and when there are a great many of them close together they show through the skin and make the whole plant green. The skin protects them, you see, and yet it is transparent and allows the light to get to them, which is a matter of great importance to the chlorophyll grains, for they are hard workers, but cannot do a single thing without sunlight. [Illustration: [Cells]] Chlorophyll grains lie just behind the skin cells in all parts of the plant that look green. The cells they lie in are often long with their short ends towards the skin. Leaves contain several layers of chlorophyll cells. The inner ones are not long like the outer ones, and do not contain so many chlorophyll grains. In the illustration, _a_, _a_ represent the upper and lower skin and _b_ the cells containing chlorophyll. The under side of a leaf usually has fewer chlorophyll grains in its cells, for the light is not so bright there, and chlorophyll needs plenty of light. Sometimes the cells in the middle of a leaf, that is, halfway between the upper and lower surfaces, have no chlorophyll at all. Now what _do_ you suppose is the work the chlorophyll grains have to do?