Skin covers over and protects what is underneath. It is thin compared with what it covers, but it is important, as we discover when we lose a piece of our own skin. A fluid substance or even blood oozes out, and the spot where the skin is off is very painful. Plants have a skin too, and it does for them what our skin does for us. It is tough and protects the soft inner parts and keeps the sap from oozing out. Skin, of course, is built up of cells. These cells generally lie close together, touching each other, except at certain spots, where there is an opening. Skin cells are usually long and wide, and their outer walls, as you would expect, are thicker than the inside walls. The protoplasm builds up hard material on the outside to protect the rest of the leaf or stem. Leaves and young stems and roots and flower parts all have skin. The skin is alike in all in a general way, just as all houses are alike in a general way. They all have a roof, walls, partitions, doors, and windows, though these are of different sizes and arranged differently in different houses to suit the needs of the people who live in them. So with plants. The skin cells are different in size and shape and thickness in different plants to suit the needs of the plants, though in all there is a general resemblance. [Illustration: [Skin cells]] Here is a row of skin cells (_a_) with other cells (_b_) back of them. See how thick the skin cells are on the outside (_c_). They are very tough there too. _d_ is an opening between two cells, and all is magnified several hundred times. [Illustration: _a_] Sometimes there are several layers of skin cells where the plant needs a particularly thick skin; _a_ in the illustration is an example of such a skin. But it would not do to have an air-tight skin, even for a plant. Our own skins are full of holes, or pores, as you know, to let out the extra water and other waste materials in what we call perspiration. The plants need such an arrangement as much as we do. So in their skin we find pores. You see the plant needs a great deal of water. The water is used in making the substance of the plant. It is also used in the sap to carry food about from place to place. Sap contains a great deal of water in order that it may flow easily. This water cannot all be used by the plant, and when it comes up from the roots in the sap a large part of it has to be got rid of by the leaves. If the skin were solid, the water could not escape. But you know what protoplasm can do. If the skin needs pores, it will make them. And this is how it does it. If you peel off a bit of skin from the under side of a leaf and put it under the microscope, you will see something like this. [Illustration: [Pores]] The round forms are the pores. The crooked lines between are the edges of the cell walls, and you are looking at them right through the outer wall of the skin, which is transparent like glass, otherwise you could not see the edges of the partitions.