We _do_ know about sugar and how the little green chlorophyll people run their starch factories in all the green parts of the plant,—under the skin of stems sometimes as well as of leaves, for wherever a stem is green, we may be sure chlorophyll is at work making starch in it. And we know how the protoplasm in the different cells changes the starch into sugar. We know, too, how wood and other tough substances are made of starch. But there is something else in plants as important as starch and very different,—the protoplasm. Protoplasm itself is not made entirely of starch; it requires materials not found in starch. These materials are nitrogen, sulphur, and phosphorus. Nitrogen is the most important, and this the plant gets chiefly through the roots. Nitrogen is found in the earth combined with hydrogen and other substances. The protoplasm tears to pieces these nitrogenous substances which the roots suck up, and so enables the plant to take the nitrogen. The other two substances which the protoplasm needs, sulphur and phosphorus, the plant gets partly from the air and partly from the earth. Sulphuric acid exists in _very_ small quantities in the air and goes in through the stomata, attracted, no doubt, by the protoplasm inside. But other sulphurous and phosphorous compounds are taken up by the roots. So we see protoplasm is complicated. It contains carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus united in a very complicated way. Although protoplasm itself is made only of carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus, it can make use of a great many other things. When the protoplasm of certain cells wants to build hard, tough walls, it uses potash and soda or even silica, which you know glass is made of. Just draw a blade of sedge grass through your fingers if you want to feel the silica in it. You will probably cut your fingers, but that will help make you remember about silica. Then the protoplasm uses iron to color the petals and other parts of the plant. It uses magnesia, too, and salt and lime and a number of other materials for building walls or making dyes or something else. Every material in our own bodies is found in plants, and sometimes the plants have materials that we do not have. Of course materials are put together differently in plants from what they are in us. When Mother Nature combines her carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, magnesia, iron, and all the other things to make a plant, she does not go to work as she would if she were going to make an animal. Just what the difference is it would be difficult to tell, but there _is_ a difference. Plants contain a good deal of sugar as a rule, and if you remember cloves you will admit that at least _some_ flowers are made of spice, for cloves are the dried flower buds of the clove tree.