Mitosis and cytokinesis in plants BOTANY LAB #1 MITOSIS AND PLANT TISSUES In plants the formation of new cells takes place in specialized regions of meristematic tissue. Meristematic tissues contain immature, undifferentiated cells that continuously undergo mitotic division. Mitosis is nuclear division involving exact replication of separation of the genetic material (DNA) so that each of the daughter nuclei carries a chromosome complement identical to that of the parent nucleus. Cytokinesis is division of the cytoplasm following nuclear division. Plants grow through the production of new cells by mitosis. Primary growth takes place at the tip of the stem or root in a specialized primary tissue areas called the apical meristem (an area of continuous mitosis). Secondary growth takes place in specialized secondary tissue areas called the vascular cambium (also an area of continuous mitosis). In the first part of this lab we will examine the meristematic tissue on a prepared slide of a longitudinal section of an onion root tip. Examine the root tip at low magnification. The extreme area of the root tip is called the root cap. The meristematic tissue you want to examine for stages of mitosis is found just above this area (see the photo below). Onion (Allium) cells each contain 16 chromosomes. Not all of them will be visible in each cell, but they should be easy to observe. Look for the following phases of mitosis on your microscope with a magnification of 100X. When you find a particular phase examine it under a magnification of 400X.
The Cell Cycle Interphase is not a mitotic stage, but it does represent the greatest portion of the cell cycle (see the diagram above). The following characteristics define Interphase: (1) Most cells seen will be in this phase. (2) Relatively large and intact nucleus. (3) Nuclear membrane is intact and can be seen. (4) Nucleoli can be seen inside the nucleus. (5) The chromosomes are not visible as distinct structures but appear inside the nucleus as a dark grainy structure.
Prophase Prophase is the first phase of mitosis. The following characteristics define prophase: (1) The chromosomes become visible. (2) The nucleoli and nuclear membrane begin to disappear in the early stages. (3) In late prophase the nucleoli and nuclear membrane will have completely disappeared. The photo below illustrates late prophase. Notice that the nuclear membrane and nucleolus have disappeared. Metaphase Metaphase is the second phase of mitosis. The following characteristics define metaphase: (1) The chromatids line up along the equatorial plate. (2) Spindle fibers form and extend from one pole of the cell to another.
Anaphase Anaphase is the third phase of mitosis. The following characteristics define Anaphase: (1) The chromosomes separate and move toward opposite poles. Telophase Telophase is the fourth stage of mitosis. The following characteristics define Telophase: (1) The nuclear membrane reforms and the nucleoli reappear. (2) Cytokinesis starts and a plate appears between the two groups of chromosomes.
Daughter Cells The two cells that result from mitosis are called daughter cells. Genetically they are identical to the mother cell. PLANT TISSUES As we saw on the slides of mitosis above; new cells are formed in the meristematic tissues. Some of the new daughter cells stay in the meristem to help the plant grow, and other tissues are incorporated into other tissues such as those discussed below. The tissues of higher plants can be grouped as follows: (1) Meristems of which there are two types: a. Apical meristems function in primary growth to increase the length of the plant. b. Lateral meristems function in secondary growth to increase the diameter of the plant. (2) Mature permanent tissues which are of three general types: {1} Dermal or surface tissues - of which there are two specific types: a. Epidermis. b. Cork. {2} Simple fundamental ground tissue - of which are of four specific types: a. Parenchyma. b. Collenchyma. c. Sclerenchyma. d. Endodermis. {3} Conducting tissue (vascular tissue) of which there are two specific types: a. Xylem. b. Phloem.
Meristematic Tissue will be discussed in the next lab handout. Surface Tissues Epidermis The outermost layer of the plant is the epidermis. This layer lacks chloroplasts but functions in preventing water loss from the plant. For this portion of the lab we will examine a thin section of the bottom surface of a leaf from a Zebrina plant (Wandering Jew). Tear the leaf in two sections as show below: Select a small portion of the lower epidermis from the torn margin (see small square in the photo above). Prepare a wet mount using distilled water. Look at the under portion of the leaf epidermis under high magnification. We are looking for Guard Cells. They regulate the stoma size to control the passage of water and gases. The guard cells should look something like this: Notice that chloroplasts can be seen only in the guard cells. The diagram below is from your text.
PERIDERM or CORK Cork makes up the most abundant part of the periderm tissue in woody plants. In this exercise we will examine cork tissue. (1) Make a very thin section of cork tissue. Do not add water or use a cover slip. (2) Examine the tissue at 100X and 400X magnifications. (3) Notice the fairly uniform size and general appearance of these cells. These are dead cells with no internal contents, just a thick, waxy cell wall. Your slide should look like the photo below: Cork cells 100X Cork cells 400X
SIMPLE FUNDAMENTAL TISSUES Collenchyma - is a simple tissue composed of elongated cells with tapered ends. In cross section you should see small cells with irregularly thickened cell walls. In this exercise we will examine Collenchyma cells in celery. (1) Make a wet mount of a very thin cross-sectional piece of celery. take the tissue from the celery as shown below: (2) On low power (40X) locate the small clusters of light grey cells found in "little islands" of tissue in a ring around the outside of the celery. It should look like the picture below at 40X magnification. These are strands of collenchyma tissue, making up the "celery strings". They function as support tissue in the subdermal area of the celery petiole (leaf stalk).
(3) Add a drop of neutral red stain to the slide. Place a drop on one side of the cover slip and draw it through to the other side using a paper towel as a wick. Is should look the picture below a 100X magnification. Collenchyma cells stained with neutral red 100X Notice the characteristic irregular thickness of the cell walls. Parenchyma is the most common kind of plant tissue. It is found in all parts of the plant (roots, stems, leaves, flowers and fruit). It makes up the bulk of the body of a herbaceous plant. (1) On the same slide of cross section of celery stalk look for the larger cells in the center of the petiole. It should look like the photograph below. Celery Parenchyma cells stained with neutral red 100X Notice the thin cell walls and the intercellular spaces (air spaces between the cells where three or four cells are in contact. Sclerenchyma cells have thick secondary walls and are dead at functional maturity. The cells walls are thick and hard and contain lignin. In contrast to collenchyma,
sclerenchyma tissue has cell walls the are evenly thickened. There are two types of sclerenchyma cells: elongated fibers and sclereids (come in a variety of shapes). Sclereids (1) Prepare a wet mount of a very small amount of the fleshy part of a pear. (2) On low magnification you should notice clusters of sclereids that should look like the photograph below: A perfect example would look like this Cluster of thick-walled sclereids from the pulp of a pear. Such clusters give pears their gritty texture.
Fibers are elongated sclerenchyma cells with thick cell walls and small lumens. They are found in association with vascular bundles, forming a bundle cap of cells just outside the phloem. (1) Look at a prepared slide of Helianthus (Sunflower) on low power (40X) The clusters of cells that form a ring around the periphery of the stem are vascular bundles. They should look something like the photograph below: Phloem Sclerenchyma Xylem Toward the outside of each vascular bundle is a cluster of sclerenchyma fibers. Examine them on high magnification. They should look something like the photograph below on 100X magnification: Note the thick walls and small central lumen On the photograph below the thick walls and small lumens are even more obvious.