Just as in animals the whole body is covered over with a ~skin~, so in plants we find a special outside sheet of cells, which protect the inner tissues and form a thin skin. You can get this off very well if you break across an iris leaf, and pull along the thin, colourless layer on the outside. If you examine it with your lens, you may perhaps see something of the mosaic-like pattern of the cells which build it up. You should certainly see that it is colourless, although the tissue of the leaf beneath it is quite green. On the large branches of trees and the bigger plants, we do not find this delicate protecting layer, but instead there is a thick brown ~cork~. When the cork layer gets very thick it splits irregularly as the tree grows too big for it, and so forms a rugged bark. The cork layers have much the same duty as the fine skin, only they are thicker and stronger, and more suited to hold out through the winter. You know already from daily life the practical use of cork, for you put it into bottles to keep the liquid in the bottle and the damp and dust in the air from entering. Just what the cork does for the bottle, the sheets of cork wrapping round the branches do for the plant. They prevent it from being dried up by cold winds, and they keep out the heavy rains of winter which would injure it. Roots have a cork coating also when they get old. As you may remember, it is only the tip of the root which can absorb water for the plant, so that in the young part of the root a cork layer would be very much out of place, and you will never find it there. You will find instead the little delicate root-hairs, which absorb water and pass it on to the older parts; these old parts do no more absorbing, they are only the water carriers and food storers, and so have no hairs and are protected by a layer of cork. As we found before, plants ~breathe in~ air like animals, and you may ask how they can do this when they are covered with their thick air-tight layers of cork. Examine a fairly old elder twig, and you will see all over its brown skin numbers of darker brown spots. If you look at these with your magnifying glass, you will see that they are quite spongy and soft. They are the special entrances for air, and are the breathing spots or ~lenticels~ (~see~ fig. 96). They are to be found in all corky stems, although they are not always so easy to see as in the elder. [Illustration: Fig. 96. Piece of Elder twig, showing the breathing pores in the bark.]