Though leaves vary considerably, most share some basic
features. Lets look at a typical leaf up close:Leaf
Cross-Section
The upper and lower epidermis cells form a protective layer for the
delicate interior cellssimilar to the way your skin protects inner tissues. The
upper epidermis is located on the side that normally faces the sun; the lower epidermis is
on the underside of the leaf. The cuticle, (La, cutis=skin)
made from a waxy substance called cutin (CUE-tin)
covers the entire leaf surface to help prevent moisture loss.
Sandwiched between the epidermis cells is the mesophyll (MEZ-oh-fill; Gr. meso=middle, phyll=leaf).
The mesophyll contains two layers. The upper layer consists of tightly-packed palisade cells; most photosynthesis takes place
in these cells. (Well talk more about photosynthesis next week.) Below that is a
layer of loosely-packed spongy cells that allow gases and water
vapor to move about. And woven throughout the mesophyll are the veins, or vascular tissues, responsible for
transporting liquids to and from cells.
Concentrated on the lower leaf surfaces are tiny openings
called stomata* (stow-MAH-tuh; Gr. stoma=mouth)
that allow gases to pass in and out of the leaves. The stomata are surrounded by guard cells that collapse and swell to close and
open the stomata, depending on the plants needs.
*plural stomata or stomates; singular stomate
or stoma
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What's
Stomata With You?
To give you an idea of the size of stomata, here is
a list showing the number of stomata per square centimeter on the lower leaf surfaces of
some different plants.
begonia
4,000
corn 6,800
sunflower 15,000
lilac 33,000
scarlet oak 103,800
Note: One square inch = 6.25
square centimeters, so multiply these numbers by 6.25 to calculate the number of stomata
per square inch. |
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