the right you see what I saw when I nonchalantly walked by a Black Oak tree near my home
one January morning, held the leaf up against the sun, and looked at it with my handlens
. The area you see is smaller than a US 10¢ piece. Pretty
The big blackish lines as well as the smaller silvery lines are all veins in
the leaf. The largest black line is the leaf's single "midvein," running from
the petiole to the leaf tip, the smaller blackish lines branching off the midvein are
"secondary veins," and all the slender, silvery lines are smaller veins. All the
veins, from midvein to the smallest silvery one, serve the same functions.
- They lend support to the leaf tissue, helping it keep its shape,
like the ribs of an umbrella help it keep its umbrella shape
- They conduct substances within the leaf, rather like the veins in
our own bodies conduct blood.
You can see that the network is very extensive. If
the veins in a single elm leaf were placed end to end they would be more than 700 feet
long (210 m)!
The word "vein" is a general one. If we were in a botany lab studying leaf
anatomy, we might refer to leaf veins as vascular bundles. The
"bundle" part of that term arises from the fact that each vein is actually a
bundle of these two different kinds of conducting tissue:
- Xylem consists of dead, woody cells that conduct water from the plant's
roots to where water is needed in the leaf's photosynthesizing cells.
- Phloem transports the tree's photosynthesized food from the leaves to
other parts of the plant where it's needed, or to where it's stored.
We also meet xylem and phloem on our twig page.