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.
WE AND THE PLANT PEOPLE.
We live and the plants live. Probably neither we nor the plants spend much time thinking about what we owe to each other.
The plants are excusable for this, for they are not great thinkers, at least so far as we know.
But we owe so much to them, we ought to stop and think about it once in a while. We are indebted to them not only for the food we eat, but for the air we breathe.