ez019165 at rocky.ucdavis.edu
Sun Feb 26 17:26:00 EST 1995
NoNeatName (noneatname at aol.com) wrote:
: I recently did a lab in my High School advanced placement bio. class.
: Basically, I did a survey on the structure of a rhododendron leaf. I
: noticed under the compound scope that there was a higher concentration of
: stomata on the underside of the leaf than on the top. Why would this be?
: Any help would be greatly appreciated. Thanks in advance.
: noneatname at aol.com
A survey of angiosperms shows that leaves have a higher
concentration of stomata on the lower (abaxial) surface of the leaf than
on any other portion of the plant. The reason for this has to do with
the need for the plant to maintain a balance between carbon dioxide
uptake and water loss.
As a plant photosynthesizes, carbon dioxide is required to form
photosynthate. The greatest source of CO2 is through the uptake of
gaseous CO2 from the atmosphere. The primary function of the stomata is
to serve as an interface between the plant and the atmosphere since the
waxy plant cuticle, important in preventing desication of the plant,
prevents any CO2 exchange as well.
The plant pays a price for obtaining this CO2 in that the plant
loses water due to a steadily more negative water potential from the
mesophyll cells in the leaf to the immediate atmosphere around the leaf.
The plant may try to use water loss to its advantage through evaporative
cooling to lower the temperature of the leaf.
Since the atmosphere around the leaf under most conditions pulls
water from the leaf, a plant must maintain a system that minimizes the
water potential gradient around the leaf, known as the boundry layer.
The characteristics of the boundry layer are determined by temperature, air
speed, and relative humidity. The top (adaxial) surface of the leaf,
being the surface usually (but not always) being the surface facing away
from the plant and towards the environment, is subject to higher
temperatures, light intensity, air speed, and many other factors than the
abaxial surface. (Note: The differences in the factors just mentioned
are very slight. To our everyday perception both leaf surfaces look to be
affected equally. However, using sensitive equipment, differences can be
detected and in the physiology and physics of a plant these differences
become quite significant.)
Since the abaxial surface of the plant is usually more conducive
to more efficient CO2-water vapor exchange, many plants have adapted to
possess leaves with a higher stomata concentration on the abaxial rather
than adaxial surface. This is a long explanation, but the question leads
to a lot of physics, physiology, and anatomy in order to say why. You
should see the textbook and lecture explanations! But don't let that
deter you; asking why and trying to understand is always rewarding. Hope
University of California, Davis
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