CELL DIVISION

 

Objectives:  

            1.  Know where cell division takes place in the plant.

2.  Know the events that take place in Interphase, the 4 phases of Mitosis, and

     Cytokinesis.

            3.  Recognize cells and nuclei in all of these stages.

 

            Cell division in plants is largely restricted to localized areas called meristems.  The most active meristems are those at the tips (apices) of roots and shoots. Roots, in particular, often exhibit extraordinary growth rates, making them excellent places to observe mitosis or nuclear division. Terminal buds are useful for looking at the development of the three primary meristems. Cell division can also take place in a) the bark and vascular bundles in order to provide lateral growth, b) in the vascular cylinder of a root to form a branch root, and c) in stem tissue to heal wounds.  We will see some of these regions in other exercises.

 

Activity 1:  Apical Meristem of the Dicot Shoot Tip

Obtain a slide of a longitudinal section through a Coleus shoot tip.  Note that you are looking at a vertical slice through the terminal bud of a dicot plant. The apical meristem is a population of small, blocky looking cells that are densely stained and appear darker than non-dividing cells. The apical meristem consists of three types of

cells, named for the tissues they will eventually form: a) the protoderm – which

gives rise to the epidermis, b) the procambium (provascular meristem) – which gives rise to the vascular tissues, c) ground meristem – which forms the cortex

and pith of the stem, plus mesophyll of the leaves.  Locate the three primary meristems on Figure 1. The protoderm will be the outermost layer of cells. The procambium will be dark traces extending from the center of the two emerging leaf primordia down the stem. The ground meristem is mostly lighter colored cells in between the other two. Leaf primordia will become leaves. There may be two or four on your slide. Two bumps of meristematic cells on the sides of the stem are bud primordia, which will become the axillary buds. Label the parts in bold type on Figure 1.

                       

 

      Figure 1:  Coleus Stem Tip

 

 

 

Activity 2:  Mitosis in the Onion root tip

            Obtain an onion ( Allium ) root tip slide.  Orient the slide so that the specimens appear to be pointing at you as you look through the microscope.  There may be two or three sections of a single root tip on the slide.  They are stained to show the chromosomes (mitotic figures) more prominently.  Scan one of the sections on low power.  Look for the loose, clear cells of the root cap, at the very tip of the root.  Now move to the region just above the root cap, where you should find small, blocky cells that are densely stained.  This is the apical meristem of the root.  See Figure 2, below.

 

 

 

 

 

 

At medium and high power, look at selected cells of the apical meristem.  On Figure 3, Locate and identify all of the following stages of the cell cycle, and write down the major characteristics of each stage.

 

1. INTERPHASE:

 

 

2. PROPHASE

 

 

 

 

3. METAPHASE

 

 

 

 

4. ANAPHASE                                                                      

 

 

 

 

5. TELOPHASE

 

 

 

                                                                                          

 

 

6.CYTOKINESIS                                      

 

Figure 3: Mitosis in Onion Root Tip