Plant Cells and Transpiration
Cells are the basic unit of plant life. Plant cells generally combine
into larger subgroups called tissues. There are two
major types of plant tissues: xylem and phloem. Xylem consists mainly of
dead cells and serves to transport water. Phloem consists of live cells
and transport food substances. These two types of tissues combine into
organs. There are three main organs in the plant: stem, root and leaves.
The focus of this article is on the xylem tissue. Xylem cells run
continuously from the leaves to the roots. Their role is to conduct
water from roots to the leaves. In this article, we shall review the
mechanism by which plants take up water from the soil through its roots.
Contrary to what may be expected, the mechanism of water uptake in
plants begins not at the roots, but at the leaves. In contact with the
dry atmosphere, moisture from the leaves evaporate via the tiny pores
located on the leaf surface. The loss of water creates an osmotic
gradient. Osmosis is the process by which water moves from an area where
it is in abundance to an area where it is scarce. Thus, since an osmotic
gradient is created from the leaf tissues to the atmosphere, water vapor
will leave the cells of the leaf surface for the atmosphere. Since the
surface cells experience a water shortage, an osmotic gradient is again
established between the water-poor surface cells and the water-rich
deeper tissues. A chain of osmotic gradients is thus created from cell
to cell causing water to move from the deeper layers to the surface.
This osmotic pull is ultimately felt by the xylem cells, causing water
to move up the xylem tissue into the leaves.
The osmotic gradient finally reaches the root cells and the procedure
is repeated until the surface cells of the root are drained of water as
well. By osmosis therefore, water surrounding the roots moves into the
root cells. Through this mechanism water is taken up from the roots into
the entire plant organism. This entire process is called transpiration.
It must be noted that transpiration is entirely passive. Energy is
not used at all. Furthermore, through this process, the plant absorbs
only the quantity of water that its cells need. If its cells are turgid
(i.e. contain enough water), there will be no osmotic gradient and no
water uptake.
If the atmosphere is saturated with water (100% humidity levels),
then there will be no osmotic gradient created as well. Transpiration
will not occur in this case. The higher the humidity level, the lower is
the rate of transpiration. Conversely, a low humidity level increases
the water uptake as the osmotic gradient is very steep. Under such
conditions, if the soil contains insufficient water, transpiration will
result in an overall loss of water. This results in wilting of the plant
and may lead to death if such conditions persist long enough. Some
plants, especially desert plants such as the cactus, have adapted
themselves to prevent such unbridled transpiration. Their leaf pores
close down under intense heat. |
