Plant Propagation
Fall 2008
Lecture 7 - Roots and stems


Remember that your required reading includes this page at the Ohio State University website on Herbaceous Roots and Stems. It is a great summary of what I want you to know, so give it a read either after or before you complete the remainder of this lecture.

So why a palm tree in the header above? I'll answer that at the end of the lecture.

Here is the mp3 audio companion for lecture 7.


Roots

The three main functions of the roots are to:

Flowering plants have two types of roots systems based on whether the roots of the mature plants arise from hypocotyl or epicotyl tissue:

Take the time to pull up a plant and look at the roots, then at the stem. The stem has nodes that divide the stem into segments, with internodes between the nodes. Branches, leaves and flowers all originate at nodes on the stem. Now look back at the roots. Notice that the roots don't have nodes. As I noted above, the primary root that originates from the hypocotyl spawns secondary roots, but these secondary roots don't grow out of nodes in the same way that branches originate at nodes on the stem.

Remember from last lecture that a root has the following features:


If we cut a cross section of the root just above the root hairs in the younger part of the mature root, you would find that the cells have differentiated to form the following tissues:

Stems

The main stem of the plant develops from the epicotyl of the embryo. The tip of the growing stem has an apical meristem which is the site of new cell division. Stem branching occurs at nodes where there are auxillary buds containing apical meristems of their own, unlike roots where lateral roots are initiated in the pericycle.

Herbaceous vs Woody Plants
Herbaceous plants (that is, plants whose above-ground plant parts die back to the soil surface at the end of the growing season) grow upwards due to cell divisions their apical meristems positioned at the tip of the plant. Stems of herbaceous plants typically do not thicken very much and rely on branching to grow laterally. Continued growth in girth, like you find in a tree, requires an active lateral meristem or cambium (a lateral meristem lying between the xylem and phloem) that continues to produce new xylem and phloem cells. This secondary growth is typical of woody perennial plants. We will study plants with secondary growth in a subsequent lecture. Some herbaceous plants are annuals, like tomatoes and pansies, where the whole plant dies overwinter. Other herbaceous plants, like Kentucky bluegrass and chrysanthemums, are herbaceous perennials where only the above-ground growth dies overwinter (unless it is a really nasty winter). New stems grow the following year from nodes on stem tissue that survived the winter by being positioned very close to the surface and covered by mulch and snow, or even being positioned slightly undergound .

Dicotyledonous Stems

The herbaceous dicot stem to the left shows four basic parts (from outside to inside): epidermis, cortex, vascular bundle and pith. Notice how the vascular bundles of dicots are arranged in a ring around the plant with cortex to the outside and pith to the inside.

To the right you will see a sunflower stem cross section. Note the collenchyma cells in the cortex just under the epidermis. As you likely know, the sunflower stem is quite tough, and this toughness is in part due to the layer of collenchyma. Toward the outside of each vascular bundle you will find fibers of sclerencyma which also contribute to the sunflower stem's toughness. Then, moving from the outside to the inside, you will find phloem, a layer of cambium (not labeled on this cartoon) and then xylem. The pith is in the center.

Now you might recall that just above I said that herbaceous annuals (like sunflower) don't have cambium to increase the girth of the stem, so what's with the cambium in this picture? Well, you will see a layer of cambium between the xylem and phloem of dicot stems, and it is active for a while and produces a modest amount of xylem and phloem, but it is not consistently active as in woody stems, and the plant does not survive through the winter.

Monocotyledonous Stems

The tissues of the monocot and dicot stems are essentially the same as what we saw above in dicots. As you see on the left, the main difference is that in monocots the vascular bundles are scattered throughout the stem instead of being oriented in a ring like dicots. Since there is no ring of vascular bundles, there is no "inside" pith and "outside" cortex. All the ground tissue is considered to be cortex.

On the right is a monocot vascular bundle. The phloem is always oriented toward the outside of the plant and the xylem toward the inside. There is no cambium and no secondary growth. Around the outside of the vascular bundle is a layer of parenchyma cells called the bundle sheath. I'll refer to this layer of cells later when we review photosynthesis. For now we will consider it a protective covering and supportive sheath around the vascular bundle.

So why the palm tree at the top of the page? Because it is an exception.

A palm tree is a monocot, but unlike other monocots, their primary stems do increase in girth from year to year even though they do not have secondary cambium. They have a special layer of meristematic cells (called the primary thickening meristem) oriented toward the outside of the stem that each year can initiate new vascular bundles and new parenchyma cells. Each year the stem expands in girth as a result of the palm's production of new parenchyma and vascular bundles.


That will do for now. Thanks for sticking with all this detail.

- Tom