Roots

Materials

1. Radish or grass seedlings germinated on damp filter paper in petri dishes

2. Prepared slides of cross sections of young buttercup

(Ranunculus) and greenbrier (Smilax) roots

3. Prepared slides of willow (Salix) roots showing lateral

roots

Some Sugggested Learning Goals

1. Understand the differences between root hairs and lateral roots.

2. Know the locations and functions of root tissues such as epidermis, cortex, endodermis, pericycle, phloem, and xylem.

3. Know the location and composition of Casparian strips.

Introduction

Roots function primarily in anchoring plants and in absorbing water and dissolved substances vital to growth and maintenance of living tissues. The typical regions of a young root tip (root cap, meristematic region, region of elongation) were briefly examined in Exercise 3. In this exercise we want to concentrate on the region behind the region of elongation-the region of maturation (also referred to as the region of differentiation or root hair zone). This region is where the cells originally produced in the meristematic region become differentiated into several different types, each with a specific function.

Flowering plants, primarily on the basis of differences in flower parts, are grouped into two large classes commonly referred to as dicots and monocots. However, dicots and monocots also differ in the structure of their roots, stems, and leaves. In this exercise, we will briefly examine both dicot and monocot roots.

A. Root Hairs

Root hairs develop in a zone a short distance behind the root cap. As new root hairs are produced near the root cap, the older root hairs farther back die. Root hairs greatly increase the absorptive surface of a root.

Mount about half a centimeter of a living radish or grass seedling root, including the tip, in a drop of water on a slide, add a coverslip, and examine under low power. (Be sure your root tip is intact; if the tip has already been removed, discard the seedling and take another one.) Note the numerous root hairs of various sizes. Each root hair is a part of the epidermal cell from which it protrudes; it is not a separate cell itself. Can you distinguish the root cap that functions primarily in protection of the delicate membranes behind it as the root pushes through the soil?

B. Dicot Roots

Examine a slide showing a cross section of a buttercup root (Ranunculus xs). Note that the outermost layer of cells, the epidermis, is only one cell thick. Are there any root hairs present on your slide? Next note the extensive tissue with numerous starch grains (often stained purple) interior to the epidermis. This tissue functions primarily in food storage, and is known as the cortex. In carrots and similar roots, it comprises the bulk of the root.

The distinctive tissues in the center of the root are surrounded by a single layer of conspicuous cells, most of which appear to have relatively thick walls. This layer, the endodermis, forms the inner boundary of the cortex, and separates the tissues in the center of the root (known collectively as the stele) from the other root tissues. The endodermis was believed to play a role in regulating the movement of water and dissolved substances entering or leaving the stele, but this is now in question. Endodermal cells have bands of fatty suberin (Casparian strips) around the inner faces of the walls. Casparian strips are generally difficult to discern because the fatty substances are dissolved when the slides are being prepared, but in buttercup roots a conspicuous wall layer (that usually stains red) is deposited inside the Casparian strips. Although suberin itself is impervious to water, endodermal cell walls have many paired pits (thin areas where there is no suberin) that allow water to pass through.

The tissue in the center with relatively thick-walled cells (usually stained red) is primary xylem, which functions in conducting water. Between the arms of the xylem are patches of primary phloem, a food-conducting tissue. In older dicot roots, a vascular cambium usually develops between the primary xylem and phloem, and produces secondary xylem and phloem. The addition of secondary tissues by the vascular cambium will increase the girth of the root. Note the pericycle, a single layer of thin-walled cells located adjacent to and inside the endodermis. The cells of the pericycle usually do not appear different in form from a the young primary phloem cells. Lateral roots originate in the pericycle. Unlike dicot stems, dicot roots have no pith.

C. Monocot Roots

Examine a slide showing a cross section of a root of greenbrier (Smilax xs), a monocot. Note the pith in the center, and the phloem within the patches of xylem. Locate the endodermis, pericycle, cortex, and epidermis. What differences and similarities are there between dicot and monocot roots?

D. Lateral Roots

Examine a slide of a cross section of a root of willow (Salix, branching xs). This slide shows at least one specially stained lateral root beginning to grow out from the stele. In which specific tissue is the base of the branch root located? Which tissues does it push through as it grows?

Drawings to Be Submitted

I . Label the provided illustration of a young root through the region of maturation. Indicate the CORTEX, EPIDERMIS, and ROOT HAIRS. Label an EPIDERMAL CELL and a ROOT HAIR on the longitudinal section through a young root, and add a ROOT CAP to the bottom.

2. On the illustration of the cross section of a buttercup root, label EPIDERMIS, CORTEX, ENDODERMIS, and STELE. On the illustration of the enlargement of the stele, label PRIMARY XYLEM, PRIMARY PHLOEM, PERICYCLE, and ENDODERMIS.

3. Label the following on the illustration of the cross section of a greenbrier (SMILAX) root: EPIDERMIS, CORTEX, ENDODERMIS, PERICYCLE, PRIMARY PHLOEM, PRIMARY XYLEM, and PITH.

4. Label a cross section of a willow root, showing a developing lateral root. Label EPIDERMIS, CORTEX, ENDODERMIS, PERICYCLE and LATERAL ROOT.

Review Questions

1. With which specific region of roots is this exercise concerned?

2. In which tissues do the following originate?

Root hairs____________________________ Lateral roots_______________________________________________

3. What evidence of the food-storage function of cortex is present in buttercup roots?________________________________

4. Which tissue surrounds and borders the stele of a dicot root?

Which tissues comprise the stele?

5. What is the function of the vascular cambium?

6. Of what substance are Casparian strips composed?

7. Is a pith present in all roots?______________________________________________________________________ If not, in which roots is it present?___________________________________________________________________

8. As lateral roots develop inside a primary root, through which tissues must they grow to reach the surface?________________

Laboratory Preparation

Quiz 4

Roots

1. From which tissue do lateral roots arise?

2. Between which tissues is the vascular cambium located?

3. Which tissue of stems is not present in dicot roots?

4. In which tissues are root hairs to be found?________________________________________________

5. Which tissue is immediately adjacent to the endodermis on the side toward the center?

6. In which region of the root does differentiation of cells into various cell types take place?

7. What is present in cells of the cortex that gives evidence of its function as a food-storage tissue?_______

8. Of what fatty substance are Casparian strips composed?

9. What tissue produces cells that add to the girth (diameter) of the root?

10. What water-conducting tissue is present in the center of a dicot root?