Re: Do leaves absorb water throught the top or bottom of the leaf?

Complete Question:

My 10 year old son is doing a science project. He wants to show if leaves 
absorb water from the top or bottom. I am trying to help his present this. Can 
you help on how to pose the question? our thought was to take two plants, coat 
one with petroleum jelly on the bottom of each leaf, the other the top of each 
leaf. Each day for two weeks he will brush either the top or bottom of the 
leaves with water, then chart the results. Or do you have any other ideas that 
he might try? Directions would be helpful.....thank you


Reply:

What I would strongly recommend would be to change one word in your project 
title from "absorb" to "lose" because leaves rarely absorb water, but they lose 
large amounts.

Probably the best way to determine if leaves lose more water from the top or 
bottom surface would be to count the number of stomata on top and bottom 
surfaces. My book explains how to count stomata using a cast of the leaf 
surface made using finger nail polish and a microscope. The dried nail polish 
cast is removed by sticking it to a piece of scotch tape and placing it on a 
microscope slide. The nail polish method works best on nonhairy leaves. I have 
also seen SuperGlue recommended in place of nail polish but have not tried it 
myself. For some leaves, such as wandering jew, you can sometimes peel off the 
epidermis after snapping the leaf in half and examine the epidermis directly 
under the microscope.

The idea of coating the top or bottom of a leaf with petroleum jelly might work 
if you coat all the leaves on a potted plant and measure the weight loss of the 
potted plant over a day or more. You would also want to cover the pot with a 
plastic bag and tie it around the plant stem to minimize water loss from the 
soil surface and pot walls, for a clay pot. 

It might not work as well with individual leaves because when leaves are 
detached from a plant, the stomata will generally close when their water supply 
is cut off. You could try it that way and weigh the leaves to determine the 
water loss. If that method doesn't work, maybe place the petiole of each leaf 
in a small container of water and measure the weight loss of the container and 
leaf. Cover the container opening with some plastic wrap to minimize 
evaporation from the water surface and insert the petiole thru the plastic 
wrap. You will need a more sensitive balance for an individual leaf than for a 
potted plant.
 
Reference

Hershey, D.R. 1995. Plant Biology Science Projects. New York: Wiley.