STRENGTHENING CELLS.
Plants need something more than cells of working protoplasm and something more than tubes, just as we need more than flesh and blood vessels.
We would be in a sad plight if we had no bones to keep us in place, and plants would be in a sad plight if they had no—well, not exactly bones, but something to serve the same purpose.
Think of the weight a tree has to bear. You could not begin to lift the crown of a large tree, yet the tree trunk has to hold it up in the air. Not only that,—it has to hold on to it when the wind blows, which is a much harder task. Even small bushes and tender garden plants have quite a weight to bear and quite a task to keep their leaves and stems from being blown away. They could never hold on to them if it were not for the wood and other tough cells they have,—never in the world.
These wood cells and other tough cells are made by protoplasm, of course.
The protoplasm builds them very much as it does the tube cells, long and slender, as you see in the picture at the beginning of the chapter, and then when the hard, tough walls are all done, the protoplasm slips out and leaves the strong framework of tough fibres to do its duty. This framework is not only strong, it is elastic, so it can bend easily. If it were not, the first strong wind or the first thing that happened to bend the plant would snap it off short.
You cannot break wood easily, and, if you do succeed, it always bends more or less first. Some wood bends more easily than others, as you know. A willow twig can be tied into a knot, it bends so easily.
Nearly all land plants have these stiffening cells. They run out of the stems down into the leaves and help make their framework of “veins.” The tubes and the strengthening fibres run along in bundles side by side. You see this saves space. If the tubes and strengthening fibres each took a different road, that would not leave much space for the chlorophyll and other working cells. But all the tubes and fibres are closely packed together and run lengthwise, through the stem. All around these long fibres are placed the other cells which are not long and do not form tubes or fibres. Most of those other cells in the leaf contain chlorophyll. They contain protoplasm, and do the work of transforming food materials into plant material.
