EXERSCISE I: MICROSCOPIC OBSERVATIONS OF PLANT TISSUES
Using Figures 1 and 2, compare Slide #13, a cross-section through the stem of Zea mays (corn - a monocot) with Slide #11, a cross-section through the stem of Medicago sp. (alfalfa - a dicot). Identify the following underlined structures in both specimens.
1) Epidermis -- a single layer of cells which covers the stem. Observe the cuticle, which is present on the outer extremity of each epidermal cell.
What is the specific function(s) of the epidermis of the Zea mays stem?
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How do you think the epidermis of the stem would differ from that of the root considering the location and function of the two organs?
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2 a) Cortex of a Monocot (Slide #13)
In Zea mays (corn), the cortex is a large, inner area comprised primarily of parenchyma with several layers of sclerenchyma just inside the epidermis. There is also a layer of sclerenchyma around each of the vascular bundles (Figure 1). Sclerenchyma cells function in structural support of the stem and protection of the vascular bundles. Sclerenchyma cells are small and posses tough, thick cell walls. These walls are often impregnated with lignin, a highly branched polymer that makes cell walls more rigid. Mature sclerenchyma cells can not elongate and are found in regions of the plant that have stopped growing in length. At maturity sclerenchyma cells may actually be dead.
Parenchyma cells are the most common type of plant cell. They are quite large and posses thin walls, which remain thin and flexible even when the cells are mature. These cells contain leucoplasts which function in the storage of starch produced in photosynthesis. Leucoplasts appear as purple stained granules on your slide. Parenchyma cells of the stems of some plants contain chloroplasts and have photosynthetic functions.
You will also notice vascular bundles, which are randomly scattered throughout the cortex (Figure 1). These bundles represent vascular tissue. The function of vascular tissue is discussed in step 3.
b) Cortex and Pith of a Dicot (Slide #11)
The main storage part of the stem of Medicago sp. (alfalfa) is divided into two distinct regions by the ring of vascular tissue (Figure 2). Note that there is a distinct difference in the arrangement of the vascular tissue between the Monocotyledonae and Dicotyledonae. In a monocot stem, the vascular bundles are scattered throughout the cortex (Slide #13, Figure 1), whereas the vascular bundles in a dicot stem are organized into a ring near the periphery of the stem (Slide #11, Figure 2).
The region between the epidermis and the vascular tissue is called the cortex. It is composed of several layers of parenchyma cells, some of which may contain chloroplasts, and thick-walled collenchyma cells. Collenchyma cells are specialized for structural support and are located as cylinders of tissue beneath the epidermis. They support elongating (still growing) parts of the plant. Collenchyma cells are relatively flexible and allow the plant to bend without breaking. Their cell walls are thinner than those of sclerenchyma and slightly thicker than those of parenchyma. The walls do not contain any lignin.
The region internal to the vascular tissue is called the pith and is composed of parenchyma.
What is the function of sclerenchyma tissue?
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What is the function of collenchyma tissue?
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How do the cells of sclerenchyma differ from those of collenchyma in terms of function and structure?
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What is the function of parenchyma tissue?
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3. Vascular Tissue (Bundle)-- is a conductive tissue and consists of two types of tissues: xylem and phloem. The xylem tissue is always located more internally than the phloem tissue and the cells of this tissue are much larger in comparison to the cells of the phloem tissue. Observe large xylem cells which are also known as xylem vessels. These cells are dead when mature, lose their end walls, and form lengthy continuous tubes. Their function is to conduct water and dissolved minerals upward in the plant. The function of the phloem tissue is to conduct water and carbohydrates, (used by plants for food) upward and downward in the plant.
How does the function of xylem tissue differ from that of phloem tissue?
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How does the arrangement of vascular bundles differ in stems of monocots as compared to dicots?
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Figure 1A: Schematic cross section of the mature stem of a monocot plant, Zea mays (corn).
Figure 1B: Microscopic view of cross sections of the mature stem of a monocot plant, Zea mays (corn).
More pictures of the Vascular Bundle.
Figure 2A: Schematic cross section of the stem of a dicot plant, Medicago sp. (alfalfa).
Figure 2B: Microscopic view of cross sections of the mature stem of a dicot plant, Medicago sp. (alfalfa).
Figure 2C: Close up view of the vascular bundle of the stem of a dicot plant, Medicago sp. (alfalfa).
