Lab 6 Roots - Napa Valley College 6 Roots.pdf ·...
Transcript of Lab 6 Roots - Napa Valley College 6 Roots.pdf ·...
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Lab Exercise 6: Roots (Atlas pp. 130-‐133, Text Ch. 24)
The primary root system is the plant’s interface with its physical substrate (typically the soil). A root is the first organ to emerge when a seed germinates; the water absorbed by that first root (the radicle) is vital to the survival of the seedling. As underground structures, roots function to anchor the plant, absorb water and inorganic nutrients, and conduct those substances to the aboveground shoots. Many plants have roots that are adapted for additional specialized functions, such as food storage, structural support, gas exchange, photosynthesis, parasitism, and interaction with mutualistic microorganisms. Upon completion of this lab, the student should be able to: • Differentiate between taproot systems and fibrous root systems, and between primary roots and lateral roots.
• Explain how adventitious roots differ developmentally from primary and lateral roots; locate/recognize adventitious roots on plants.
•Identify the four growth regions of the root tip, and explain how the activity in each region contributes to the growth of the root.
• Identify, and list the functions of, the three major regions of dicot and monocot root cross sections, and their component tissues and specialized structures.
•Discuss the differences between dicot and monocot roots , and recognize each type based on their cross section.
•Compare and contrast the structures of roots and stems as seen in cross section.
•Answer the questions in the exercise and understand the terms presented in bold.
I. External Morphology of Roots A) Root Systems
1. Distinguishing taproot from fibrous root systems: Examine the plant material available in lab and familiarize yourself with the different types of root systems they possess (taproot or fibrous).
a) How do these two systems differ in appearance?
b) How do they differ in development?
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2. Identifying adventitious roots: Examine the specimens of Chlorophytum comosum (“spider plant”) and Echeveria sp. (“hen-‐and-‐chicks”). Both of these species produce adventitious roots: roots that arise from locations outside of an existing root system. Can you find them?
a) From what organ(s) do the adventitious roots of Chlorophytum seem to originate? From what organ(s) are adventitious roots of Echeveria derived (if any are present)?
b) What do adventitious roots “do” for the plants that the primary root systems can’t? How are they advantageous?
c) In monocots, the embryonic root, or radicle, is short lived, and serves to absorb water and nutrients only during the initial stages of germination and seedling growth. The mature fbrous root systems develop from adventitious roots that arise from the base of the stem.
3. Diagrams: Diagram a taproot system and a fibrous root system. Where
appropriate, label the primary root, lateral roots, and adventitious roots.
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B) The Root Tip
Examine a prepared slide of a longitudinal section of a root tip from Zea mays (corn), Raphanus sativa (“radish”), and identify these four growth regions:
1. Root Cap. The root cap is a thimble-‐shaped cell mass that covers the apex of the
root. Its main function is to protect the root apical meristem as the root tip is pushed through the soil.
2. Meristematic region. This is the root apical meristem, a region of active cell
division. The cells in this region should appear smaller, and the density of cells and cellular contents should appear greater, in comparison to the other regions of the root.
3. Region of Elongation. Here, the cells produced by in the meristematic region
are actively enlarging. Most of the size increase is along the main axis of the root (thus, elongation). The cells in this region should appear somewhat more rectangular. The elongation taking place in this region pushes the root tip through the soil, and results in an increase in the length of the root.
4. Region of Maturation. Cells in this region are beginning to differentiate into
mature cell types. The more darkly stained cells near the center of the root will become the vascular tissues. You may be able to distinguish maturing vessel elements that have begun to develop their secondary wall structure. The outer layer of cells differentiates into the epidermis. In some sections, slender, tapered structures can be seen projecting from the epidermis. These are the root hairs. Root hairs are delicate and difficult to preserve in prepared slides; only small portions of the hairs remain attached to the epidermis. The remaining cells between the epidermis and vascular tissues will become the cortex of the root.
5. Complete the diagram of the root tip below by adding root hairs to show their general distribution/location. Label the root hairs and the four growth regions of the root tip.
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C) Root Hairs Examine the intact root hairs of a recently germinated radish seedling. Before making your slide, notice the general appearance of the young root. 1. Make a wet mount of the root from a radish seedling: Using forceps, transfer the
root to a slide. Do not let the root hairs dry out! Remove the seed portion before adding the cover slip. After putting the cover slip in place, check that the root is completely surrounded by water. If not, add a drop or two to fill the space under the cover slip.
2. Examine at low power, paying attention to the youngest (shortest) root hairs,
found closest to the root cap. Are the root hairs unicellular or multicellular structures? (Q1)
3. Add a drop of methylene blue at the edge of the cover slip and draw it under by touching the corner of a paper towel to the water on the opposite edge of the cover slip. The dye should stain the nuclei of the cells. Nuclei of the epidermal cells typically migrate into the developing root hairs. If you look carefully, you may be able to find a few root hairs with nuclei in them.
4. Draw a single epidermal cell with a root hair extending from it, and with the
nucleus present in the root hair. II. Internal Structure of Roots A) Herbaceous dicot root cross section. Examine a prepared slide of Ranunculus (“butter-‐cup”) root cross-‐sections. All of the tissues in these roots are primary tissues. What does this mean? 1. Using low power, observe how the root appears to be divided into three major
regions in cross section: the central vascular cylinder (or stele), the epidermis, and the cortex (the region between the epidermis and the stele). Then switch to higher power for detailed examination of each tissue region.
2. The epidermis: a single layer of cells surrounding the entire root.
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3. The cortex of the Ranunculus root consists of several layers of cells. Except for the noticeably concentric layers adjacent to the epidermis and the stele, the cortical cells lack an orderly pattern of arrangement. The darkly stained bodies within the cortical cells are starch grains. What does this tell you about the function of the root cortex? (Q2)
a. What type of tissue/cell makes up the bulk of the cortex?
b. How does this compare to the cortex of the stem, as observed in
Sunflower (Helianthus) for instance?
c. Can you think of any possible reasons for this difference between stems and roots?
4. The figure below shows Ranunculus root in cross section. Label the epidermis,
cortex, and stele. 5. The endodermis is the innermost layer of the cortex, and often appears as a
red-‐stained ring in our slides. The radial and transverse walls of these cells are impregnated with suberin, a hydrophobic waxy material that stains red, like lignin. Initially suberin is deposited in a thin band around each cell, called the Casparian Strip. Suberin solidifies the wall and renders it impermeable to water. Thus even a narrow Casparian strip will completely block the apoplastic pathways through the endodermal cell walls, and prevent nonselective diffusion of solutes into the stele.
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In Ranunculus, most of the endodermal cells have developed a uniformly thickened wall impregnated with suberin throughout, so the Casparian strip is not visible. Some endodermal cells, called passage cells in the text, retain a relatively thin wall with an evident Casparian strip. Look for passage cells in the sections on your slide.
6. The stele consists of all of the tissues enclosed by the endodermis (the
endodermis being the innermost layer of the cortex, not part of the stele). 7. The primary xylem occupies the central portion of the stele. It is generally
star-‐ or cross-‐shaped, with arms extending toward the endodermis. Differentiation of primary xylem begins at the tips of the arms (protoxylem), near the endodermis, and proceeds inward. Protoxylem are usually the smallest xylem cells. The larger cells of the primary xylem differentiated last, and are the youngest. These are the metaxylem elements. (Q3)
8. The primary phloem develops between the arms of the xylem in dicot roots. The phloem cells are much smaller than those of the xylem, and lack secondary wall thickenings. Between the xylem and phloem lies a band of parenchyma cells that are difficult to discern. These cells develop into a vascular cambium in roots as they begin to undergo secondary growth.
9. The pericycle is the single layer of cells just inside the endodermis; it is most easily distinguished where it runs between the endodermis and the tips of the xylem arms. The pericycle gives rise to lateral roots.
10. In this image of the dicot stele cross section, label the cortex, starch grains, endodermis, passage cell, primary xylem, primary phloem, and pericycle.
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B) Monocot root. Examine a prepared slide of root cross sections from the monocot species provided (Smilax or Zea). The tissues of the monocot root are organized into the same three regions seen in Ranunculus (a dicot). 1. Compare the relative size of the steles in dicot and monocot roots.
Ranunculus stele = __________________ of the total root diameter. (estimate % or fraction)
Monocot stele = _________________ of the total root diameter. (estimate % or fraction)
2. Notice the layer of cells with thickened walls that lies beneath the epidermis. This is the exodermis.. This tissue begins to develop in the maturing portion of the root, a few centimeters behind the root tip, in an area of the root that is becoming less active in absorption of water and minerals. What is the function of the exodermis? (Q4)
3. The endodermis should appear as a prominent ring of cells with “U-‐shaped” wall thickenings, surrounding the stele. Where are the cell walls the least thickened? Why might that be important? (Q5)
4. The pericycle in monocots is found in the same relative location as in dicot roots. However, some monocots, such as Smilax, have a multiseriate pericycle (several cell layers thick).
5. Primary xylem and primary phloem alternate with each other, just inside the pericycle. The primary xylem is more conspicuous. What tissue occupies the central portion of this stele? How does this differ from Ranunculus? (Q6)
6. In the photo of the Smilax root section, label: epidermis, exodermis, cortex, endodermis, pericycle, primary phloem, primary xylem, and pith.
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C) Lateral root origin in Salix. Obtain a prepared slide of a cross section of Salix (willow) roots showing the development of lateral branch roots. The same primary root tissues are present as were observed in the previous sections, with the addition of a lateral root primordium pushing through the cortex and epidermis to emerge into the soil. 1) From what tissue do the lateral roots develop? Is this evident in the cross section? 2) Do the lateral roots appear to develop from cells adjacent to the primary phloem or the primary xylem? 3) Can you identify the root cap covering the tip of the newly forming root? 4) What is the advantage of lateral root emergence from within the stele of the parent root, rather than from the cortex or epidermis? 5) Is Salix a monocot or a dicot plant? How can you tell from the root cross section?
(This lab exercise was adapted in part from Exercises for the Botany Laboratory, Kazmiersky 1999).
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Biology 241 General Botany Name:__________________________________
Take Home Quiz for Lab 6: Roots Due Wednesday March 11
1) It is hypothesized that the endodermis is part of the cortex (ground tissue) in roots. Which primary meristematic tissue does the endodermis develop from? (2) 2) What is the function of the pericycle? (1) 3) What isthe function of the root hairs? (1) 4) How do root hairs make roots more efficient in absorbing water and mineral nutrients from the soil?(1) 5) What would happen to root hairs if they formed immediately behind the apical meristem instead of in the zone of maturation? (2) 6) List four identifying/differentiating characters of monocot and dicot roots visible in the root cross sections. (4) Monocot root (Smilax) Dicot root (Ranunculus) a. a. b. b. c. c. d. d.
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7) List three characteristics that differ between dicot roots and dicot stems seen in cross section. (4) Dicot Root(Ranunculus) Dicot Stem (Helianthus) a. a. b. b. c. c. 8) List two characteristics that differ between the roots and stems of monocots when seen in cross section. (2)
Monocot Root (Smilax or Zea) Monocot Stem (Zea) a. a. b. b. 9) How do branches form in aboveground stems? How does this compare to branch (lateral root) formation in the root system? (3) Lateral root: Lateral branch: ________/20