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.
The three main functions of the roots are to:
- Anchor the plant in the soil.
- Absorb water and dissolved minerals from the soil.
- Store nutrients like starch for subsequent use by the plant
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.
- Tap root system - plants of this type have primary roots that arise from hypocotyl tissue and persist into maturity. Secondary roots arise from the primary root, and tertiary roots arise from the secondary. This primary -> secondary -> tertiary root formation is the usual rooting system for dicots. Next time you are walking past an empty lot or a weedy patch of ground and you see a sunflower or buttercup or other weed that doesn't look like a grass, yank it up and look at the root system. That's probably a tap root. To the right is an image of a young soybean taproot system with many secondary roots branching off of the primary root.
- Adventitious or fibrous root system - in plants of this type, the primary root, which originates from hypocotyl tissue, declines prior to maturity and new, adventitious roots arise from epicotyl tissue. This root system is typical of monocots like grasses. Just dig up a clump of turfgrass and look at those roots. That is a fibrous root system. To the right is a young cereal plant from the Poaceae family (grasses) showing adventitious roots coming from the base of the epicotyl.
Remember from last lecture that a root has the following features:
- Root Cap - shaped like a thimble this structure covers the tip of the root and provides protection as the root drives into the soil. These cells are produced by the root apical meristem. The outer cells of the root cap are continuously being worn away and new cells are added to the inner portion. The cells may be covered with a lubricating slime.
- Root apical meristem - the cells here divide rapidly to form new cells. These new cells later mature into more specialized root tissues.
- Region or Zone of Elongation - in this region the cells previously produced by the meristem undergo rapid elongation. Elongation (along with production of new cells) results in root growth which pushes the root further into the soil. Within the region of elongation just behind the meristem you will find the following undifferentiated tissues:
- Protoderm - undifferentiated tissue on the exterior of the root which will mature to become the epidermis
- Procambium - undifferentiated tissue in the central part of the root which will mature to become the the vascular cylinder
- Ground meristem - undifferentiated tissue lying between the protoderm and procambium that will mature to become the cortex
- Region of differentiation - here the root becomes thicker and secondary or lateral roots are developed. In this region the protoderm, procambium and ground meristem cells undergo differentiation into specialized cells.
- Root-hairs begin to form in the region of differntiation. Root hairs are fine outgrowths of epidermis. They increase the area of absorption of the root.
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:
- Epidermis - a one-cell thick layer of cells surrounding the root. These cells do not have the cutin or waxy covering that you will find in stem epidermis cells because roots don't need to keep moisture inside the root. Quite the opposite, the roots need to be permeable to moisture. Root hairs arise from the epidermal cells.
- Cortex - The cells inside the epidermis make up the cortex. Cortex cells in the root usually consist of parenchyma cells with numerous intercellular spaces through which water can move. However, the innermost cells of the cortex, called the endodermis, are rectangular in shape with thicker walls, are stacked close together, and have a bandlike deposit of waterproof suberin that wraps around each cell filling in all of the gaps (intercellular spaces) between adjacent endodermis cells . This thick band of suberin is called the casparian strip.
The casparian strip forces the water, which has been moving freely through intercellular spaces, to move through a layer of cells before it gets into the xylem. That way, the cells can exert some regulation on water passage into the root vascular system. Here's a summary of cortex function:
- allows for the diffusion of water, mineral salts and oxygen from the root hairs inwards.
- stores food reserves, especially starch.
- the endodermis, with the aid of the Casparian strips, facilitates the movement of water from cortex to xylem.
- Vascular Cylinder or Stele - The vascular cylinder begins just inside the endodermis.
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 .
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.
- Epidermis - this tough covering is a single layer of living cells. These cells are closely packed and function to protect the internal parts of the plant. The walls are thickened and covered with a thin waterproof layer that retains water called the cuticle. Stomata with guard cells are found in the epidermis for gas exchange. In some stems either unicellular or multicellular hair-like outgrowths, called trichomes, appear from the epidermis.
- Cortex - containing collenchyma and parenchyma cells.
- Vascular bundles - this region contains sclerenchyma fibers that strengthen the stem and provide protection for the vascular bundle. In dicots, the vascular bundles form a distinct ring. A mature vascular bundle consists of three main tissues - xylem, phloem and cambium. The phloem is always located towards the outside of the bundle and the xylem towards the center. The cambium separates the xylem and phloem, and, in those plants where secondary growth takes place, the cambium produces new xylem and phloem cells: xylem toward the center, phloem toward the outside.
- The pith occupies the central part of the stem and is composed of thin-walled parenchyma cells often with larger intercellular spaces than you would find in the cortex.
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.
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.