Re: 1. Why do trees have rings? Why don't palm trees have them?

Date: Mon Nov 2 16:52:47 1998
Posted By: Evelyn Tsang, Staff, Plant Science, McGill University (Mac Campus)
Area of science: Botany
ID: 889537742.Bt

Message:

Interesting questions!

To start, let's consider trees.

The growing part of a tree is found at the outer edges, just under the bark (which is why we shouldn't carve our initials on the trunk!). This area contains all the channels that bring water up from the roots (xylem) and food down from the leaves (phloem). These channels together form the "cambrium", and it grows quickly in the springtime, more slowly in the summer and not much at all in the autumn and winter. The fast growth produces big cells, called the early (spring) wood, and the slower growth produces small cells, called the late (summer) wood. When you look at a cross-section of a trunk, you can see a pattern of the alternating thick and thin circles of early wood and late wood, and these are the trees' growth rings. However, in areas of the world where the seasons are not as distinct as in Canada, the trees may continue to grow throughout the year, and the growth rings will not be seen in the cross-section of a trunk.

Next, stomata.

Leaves are typically covered with a waxy layer called a cuticle to prevent water from evaporating from their cells. Air-exchange occurs at "breathing holes" on the underside of the leaf, called "stomata" (singular: stomate). Two specialized cells line the stomate, they are the "guard cells". Up-close, they resemble a pair of lips, with the side around the hole being thicker than the side attached to the leaf. These cells open and close using osmotic pressure. Ionic changes in the cell causes water to rush in, and the thick side of the guard cell cannot expand, so it arches away from the hole as the thin side of the guard cell stretches from the water pressure. This opens the stomate. To close, water is drawn away, and the guard cells deflate back towards each other.

Water lilies!

Most plant leaves have stomata on the underside of the leaf, so that they are in the shade. This way, the leaf will lose less water from transpiration. Water lilies, however, have no problem with losing water; instead, they need air! So, their stomata are found on the UPPER-side of the leaf, which float on the surface of the water. And that's how water lilies exchange gas!

Hope this answers your questions!
Cheers,
Evelyn

-----------
Admin note:
David Hershey adds the following:

Trees with rings are dicots or gymnosperms, which both have a vascular cambium. A vascular cambium is responsible for the increase in the trunk diameter and the rings. Palm trees are monocots, which have no vascular cambium. Even palm trees in cold winter climates will not have rings.

Reference

Stern, K.L. 1991. Introductory Plant Biology. Dubuque, Iowa: Wm. C. Brown.

Current Queue | Current Queue for Botany | Botany archives

Try the links in the MadSci Library for more information on Botany.