Why, you say, if I had to raise sap from under the ground to the top of
the tree, I should certainly build some pipes and have a pump at the
top.
That is the way the plant has decided. So pipes there are, plenty of
them,—pipes or tubes of many sizes and shapes.
You know how cells grow, lying next each other. Well, tube cells are
long and contain protoplasm in the beginning. They lie end to end. But,
you see, it would not be very easy for the sap to pass through
_millions_ of cell walls on its way up.
So when the protoplasm has built a row of cells with good thick walls,
it passes out through thin places or openings it has left in the walls.
The end partitions between the tube cells are thin and break away, and
lo and behold! we have a long, strong tube with nothing in it but air.
Up this tube the sap creeps or down it the sap runs. A great many of
these tubes, which are as fine as hairs or much finer in some cases, are
needed in a plant. They run all through the stems and out into the
leaves. They are collected into bundles, and form part of the veins and
the framework of leaves. I do not know what the plant would do without
them.
But what makes the sap run _up_ the tubes?
Now you are asking questions! It took a long time for people to find
that out, for there is more than one reason why the sap runs up.
For one thing, the root cells keep drawing in water and other things,
and the fluid already in is pushed up by that behind; so there is a sort
of pump at the bottom of the plant, you see,—a force pump. The sun
shining on the leaves and stems evaporates the water above, and the
water below then easily takes its place; so there is a sort of suction
pump at the top.
Then the tubes are so _very_ fine that the fluid in them tends to move
up, just as water will soak up into a towel if the fringe happens to get
into the water; for you know that if you hang a towel so that the fringe
dips into a basin of water, after awhile the whole towel will be wet, as
a result of what we call capillary attraction. For all these reasons the
sap creeps up the stems through the tubes the cells have made.
Every plant has these tubes, from the tiniest weed in the garden to the
tallest forest tree. Although so small, they are often very prettily
marked by lines and dots.