If you have been reading the earlier offerings in this series, which
is meant to be an introduction to what is going on inside the plant,
you will have had this idea thrown at you more than once already. Each
part has its separate function that contributes to the plant as a whole,
and the function of the leaves is to use the energy of light to power
chemical reactions that form simple sugars from carbon dioxide and water.
The process gives off oxygen as a waste product, which is a good thing
for us in the animal kingdom.
The chemical reactions are
quite complicated, and take place in two stages, only one of which requires
light. The actual sites at which they go on are green-colored structures
called chloroplasts. These are located in cells of the leaf,
typically in a few tightly packed layers close to the upper surface
of the leaf. At this stage we need a diagram to which to point.
There may not be a leaf exactly like this in nature, but it will illustrate
what I need to explain.
| Let's stay
with the closely packed cells for a moment. If the leaf is growing
in very strong light, there may be more layers of cells than you
would have in a shade-grown leaf (remember how the leaves that grew
back on that ficus when it got used to the light in your living
rooms, were larger and thinner than the ones that it lost when you
brought it in from the nice bright nursery).
There are veins running all
through these layers, carrying liquids in and out. In fact, no cell
is more than a couple of cells away from the tiny end divisions of the
veins. It has to be that way, doesn't it, for the transport system to
On the top and bottom surface
of most leaves is a waterproof layer that keeps the leaf from drying
up. But the cells need to be in touch with the outside air so that they
can get the gases that they need and get rid of waste gases. And something
else happens in the leaf which could not go on if there was a complete
seal over it. That something else is the controlled loss of water, which
sets up a water tension in the cells of the leaf that acts like a suction
pump to pull the water up from the roots. This is the way in which the
water and all the substances dissolved in it are able to travel all
over the plant.
So, on the one hand, the leaves
have to be protected from drying out, but on the other hand they have
to allowed to dry in a very controlled manner so that the water can
move around the plant. The drying happens in the loosely-packed cells
in the underside of the leaf. The air spaces between these cells are
in contact with the outside air through openings which can be opened
or closed. When they are open, the drier outside air picks up water
from the moist surfaces of these loosely packed cells and carries it
away. The drier cells then draw water from wetter cells which they touch,
and these in turn become under water tension. This transfer of tension
eventually reaches a cell in the fine branches of the veinlets, and
is passed on to other cells, and the effect is to suck water up from
the roots. At that end of the plant, the tension draws water in from
the soil, and the pump is in operation.
The leaves are thus at the
heart of two very important activities of the plant: making the simple
sugars using the energy from light, and causing the suction to develop
in the conducting tissue that brings in liquid from the soil and transports
it around the plant.
to some other things about leaves
the Table of Contents