Plants need something more than cells of working protoplasm and
something more than tubes, just as we need more than flesh and blood
vessels.
[Illustration: [Strengthening Cells]]
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.
[Illustration: [Strengthening Cells]]