A student prepared a stained squash of cells from the root tips of a garlic bulb (Allium sativum) to calculate a mitotic index. She:
- Cut the terminal 5 mm5\text{ mm}5 mm from 6 garlic6\text{ garlic}6 garlic roots.
- Placed the root tips into a watch glass containing 4 cm34\text{ cm}^34 cm3 of 2 M2\text{ M}2 M hydrochloric acid for 8 minutes8\text{ minutes}8 minutes in a water bath set to 55∘C55^\circ\text{C}55∘C.
- Rinsed the root tips thoroughly in ice-cold distilled water.
- Placed one of the root tips on a microscope slide and added a few drops of toluidine blue stain.
- Placed a coverslip onto the slide, covered it with a paper towel, and pressed gently but firmly downwards on the root tip without twisting.
- Observed the prepared slide using an optical microscope.
Figure 1
Suggest why the student soaked the root tips in hydrochloric acid in step 2.
Pressing the coverslip downwards enabled the student to observe the stages of mitosis clearly. Explain why.
Name the stage of mitosis shown in Cell K in Figure 1. Explain the appearance of this cell.
Use Figure 1 to calculate a mitotic index for the cells in this field of view, given that there are exactly 505050 cells in total, with 888 of these cells undergoing active stages of mitosis.
Other students in the class followed the exact same protocol, but calculated different mitotic indices. Apart from student errors, suggest two explanations why.