Of course there are no stomata in bark. We find them only in the living skin. Bark does not need stomata, as it does not regulate the water supply. The young green parts of the plant do that by means of their covering of living skin. Living skin is usually transparent like glass. It is tough and yet transparent. You see, the light must get through it to the cells which lie behind it. There is usually no green color in skin. Sometimes there are other coloring materials, though not as a rule. The living skin covers the leaf or stem or other part of the plant like a window of tough glass. Even where the skin is several cells thick, the light can pass through, just as it can through thick glass. [Illustration: [Flower]] TUBE CELLS. [Illustration: [Tube Cells]] The top of a tree is a long way from the roots. Yet the leaves must have food from the roots, and the roots must have food from the leaves. [Illustration: [Tube Cells]] It is not an easy matter to move all this food material up and down, you may be sure. I wonder how _you_ would manage it? Why, you say, if I had to raise sap from under the ground to the top of the tree, I should certainly build some pipes and have a pump at the top. That is the way the plant has decided. So pipes there are, plenty of them,—pipes or tubes of many sizes and shapes. You know how cells grow, lying next each other. Well, tube cells are long and contain protoplasm in the beginning. They lie end to end. But, you see, it would not be very easy for the sap to pass through _millions_ of cell walls on its way up. So when the protoplasm has built a row of cells with good thick walls, it passes out through thin places or openings it has left in the walls. The end partitions between the tube cells are thin and break away, and lo and behold! we have a long, strong tube with nothing in it but air. Up this tube the sap creeps or down it the sap runs. A great many of these tubes, which are as fine as hairs or much finer in some cases, are needed in a plant. They run all through the stems and out into the leaves. They are collected into bundles, and form part of the veins and the framework of leaves. I do not know what the plant would do without them. But what makes the sap run _up_ the tubes? Now you are asking questions! It took a long time for people to find that out, for there is more than one reason why the sap runs up. For one thing, the root cells keep drawing in water and other things, and the fluid already in is pushed up by that behind; so there is a sort of pump at the bottom of the plant, you see,—a force pump. The sun shining on the leaves and stems evaporates the water above, and the water below then easily takes its place; so there is a sort of suction pump at the top.