A student prepared a stained squash of cells from the root tips of a broad bean plant (Vicia faba) to calculate a mitotic index. The protocol was as follows:
- Cut the terminal 5 mm from 6 young Vicia faba root tips.
- Placed the root tips into a watch glass containing 4 cm3 of 1 M1\text{ M}1 M hydrochloric acid at 60∘C60^\circ\text{C}60∘C for 8 minutes8\text{ minutes}8 minutes.
- Rinsed the root tips thoroughly in distilled water.
- Placed one of the root tips on a clean 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.
- Observed the prepared slide using an optical microscope at high power.
Figure 1
1.[2]
Suggest why the student soaked the root tips in hydrochloric acid in step 2.
2.[2]
Pressing the coverslip downwards enabled the student to observe the stages of mitosis clearly. Explain why.
3.[2]
Name the stage of mitosis shown in Cell K in Figure 1. Explain the appearance of this cell.
4.[1]
Use Figure 1 to calculate a mitotic index for the cells in this field of view. Show your working.
5.[2]
Other students in the class followed the same protocol, but calculated different mitotic indices. Apart from student errors, suggest two explanations why.