Stomata question

[Jason Pfafman]

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.

: Justin
: 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 
this helps,

					Jason Pfafman
					Botany
					University of California, Davis