Root Structure and Function

Objectives: By the end of this lab you should...

1. Recognize taproots, fibrous roots and adventitious roots.

2. Know the four regions of a root tip, how they develop, and their functions.

3. Recognize the tissues and cell types in a mature dicot root, and know their functions.

4. Know how a monocot root differs from a dicot root.

Activity 1: Root Types Most plant roots can be described as either a taproot or a fibrous root system. When there is one main root with a few much smaller branches, it is called a taproot. Taproots always develop from the radicle of the embryo in a seed. If the plant has many equally sized roots, it has a fibrous root system. A fibrous root system develops from adventitious roots which usually arise from stem tissue.

Look at the examples of whole roots shown in lab. Compare with figure 25-2 in your textbook. Write the name of the plant and its root type in the space provided. Also note whether the plants are monocots or dicots.

If we can generalize from these examples, what type of root system do dicots usually have? What type of root system do monocots usually have?

Which type of root system would be better at stopping soil erosion?

Note the oak seedling again. As most tree seedlings grow to sapling and then tree size, the main root stops growing and the lateral roots increase in size and number, eventually forming a large, woody, fibrous root system.

In what way would this be adaptive?

When you are outside on campus or at home, look for the visible adventitious roots at the base of a palm tree. What type of root system do you think palms have?

Are palms monocots or dicots?

Try pulling up various weeds and observing their root systems as well.

Activity 2: Regions of the Root Tip

Obtain a prepared slide of a longitudinal section through a root tip. We may use either Zea mays or Allium, both monocots. From your text or notes, review the 4 regions of a typical root tip. List their major characteristics or functions. Look at the prepared slide provided. You may want to draw your specimen in the space at right. Use your review sheet on tissues and cell types to determine what types of cells you are seeing.

Root Cap: Most of the cells in this region are what type?

Apical Meristem: These meristematic cells are small,

and lack a large central vacuole.

The outermost cells are called protoderm

(young epidermis). Look for two or more dark

columns of cells on either side of the center of the

root. These cells are called the procambium;

they will develop into the vascular cylinder.

Other cells here are the ground meristem;

they will become the parenchyma cells of the root

cortex and/or pith.

Region of Elongation: Observe how the cells in this

region are elongated, due to the filling of the vacuole

and stretching of the primary cell wall.

Region of Maturation: Look carefully to see root hairs on

the epidermal cells. They are quite fragile and may have

broken off during preparation of the specimen. Do you see

any development of xylem or phloem cells in the vascular

cylinder? Once a cell has specialized (differentiated), it

usually can no longer divide or become meristematic.

Root hairs are very fine outgrowths of single epidermal cells that may extend several millimeters into the soil. How are they adaptive?

Observe a petri plate with germinating radish seedlings.

Look at the root hairs under the dissecting microscope. Are they present all along the root tip, or just in one region?

We will look at a cross-section of a mature, non-woody dicot root. Obtain a slide with a dicot root cross-section (usually Ranunculus ). On low power, you can distinguish two distinct zones or regions- a) an outer cylinder composed of the epidermis, cortex, and endodermis, and b) the vascular cylinder in the center of the root. See figure ______ in your text. Water and dissolved minerals move laterally across the outer cylinder from the epidermis, through the cortex, to the endodermis. After passing into the vascular cylinder, the water and minerals move longitudinally through the xylem, towards the root tip or the stem.

On figure 1, label the

At higher magnification, look at the details of the epidermis and cortical cells. The epidermal cells are small, tightly packed, and in older roots, may have a waxy cuticle, meaning they are no longer able to absorb water from the soil. The cells of the cortex are large parenchyma cells with many intercellular spaces. Water can move easily through the cortical cells, along their walls, and through the intercellular spaces. The cortex is also a major site of food storage for many plants. Look for blue or purple stained bodies inside the cortical cells. These are starch granules, formed from sugar produced in the leaves and translocated here by the phloem tissue.

Locate the endodermis. Look for thickened cell walls and a band of suberin , which usually stains pink or red, embedded in the top and side walls, but not tangential walls, of the cells. These bands, called Casparian Strips, are most likely to be visible where two endodermal cells meet. What is the function of the endodermal cells with their Casparian Strips?

Just to the inside of the endodermis is a band of thin-walled parenchyma cells called the pericycle. This cells remain undifferentiated (unspecialized), and are therefore meristematic- able to divide and form new tissues. Division of several pericycle cells will give rise to a branch or lateral root, which pushes through the cortex and the epidermis as it grows.

The remainder of the vascular cylinder consists of primary xylem and phloem. The conducting cells of the xylem- vessel elements- have thick walls that usually stain red on our slides. They form a 4 or 5 armed "star" in the center of the vascular cylinder. (On some slides, not all the vessel elements in the center are mature.) Tucked in between the arms or ridges of the xylem is the phloem, consisting mainly of sieve tube members and their companion cells. (You will not be able to distinguish between these on our slides.) These cells usually stain blue or green on our slides. In those dicots which develop woody roots, a layer of parenchyma cells between the primary xylem and phloem can divide and form secondary xylem and secondary phloem, causing the root to grow in diameter. These cells are called the vascular cambium. They may not be readily identifiable on our slides.

Label on fig. 2

pericycle,

Activity 5: Branch Root Formation

Look at a slide which shows the development of branch roots. Note where the new root tip originates. What tissue layer divides to form the branch roots?

Obtain a slide of a mature monocot root cross-section. On low power, compare its gross structure to that of the dicot root. Does it consist of two cylinders?

Are the cylinders similar in proportion to the dicot root?

At higher magnification, look for and label on figure 3 the epidermis (may be more than one layer of cells), the cortex, and the endodermis. The vascular cylinder has many arms or ridges of primary xylem, with clusters of primary phloem between the tips of the ridges. In the very center of the root are parenchyma cells, which make up the pith of the root.

Most tree, and many herbs and shrubs develop a mutualistic association with a particular type of fungus. resulting in the formation of mycorrhizae, or "fungus roots". The fungi act as fine roots and root hairs, absorbing water and minerals from the soil more efficiently than roots can. In return, the fungi obtain energy in the form of sugar from the host plant. Mycorrhizal associations are particularly important where water and /or nutrient concentrations are limited. They are extremely common in the tropical rainforest, conifer and deciduous forest, and desert communities. Most wetland species and most grasses do not have mycorrhizal associations.

Observe the two demo slides provided. If the fungal partner forms a mantle around the root, and penetrates the spaces between the cells of the cortex, they are called ectomycorrhizae. If the fungus actually enters the cortical cells, they are called endomycorrhizae. Compare the demo slides with the figures in your text.