SKIN CELLS.
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.
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.
