There are two things to get from these slides:

• The organization of the root apical meristem
• The stages of cell division or mitosis

The root apical meristem is much simpler than the shoot meristem, and if you can figure out the arrangement of tissues here it will help in looking at the shoot. The root is one of the best places in the plant to see mitosis because the dividing cells are in a more compact zone than elsewhere.

Make an outline diagram of the arrangement of primary meristems in the transverse and longitudinal sections of the onion root tip.

Use a low power objective to find the root tip. Focus on the dark stained region, using the X40 objective. (These slides have been stained with Feulgen stain for DNA)

Look for cells undergoing mitosis on this section. Shade their approximate distribution on your drawing. (You can even use the scale on the microscope stage to get some idea of the size of this region).

1. The root apical meristem produces something else apart from the primary meristems - what and where is it?

   
   

2. What is the commonest stage of mitosis (other than interphase)?

   
   

3. What is the next commonest stage?

   
   

4. Which stages do you not often see?

   
   

5. What does this tell us about the time for different stages of mitosis?

   
   

If you have time you can try to find mitoses in your own preparation from Allium root tips. Yellow onions have mostly been treated with maleic hydrazide to inhibit mitosis, so use green onions. Why would an onion grower want to inhibt mitosis?

Chromosome squash

1. Collect two onion root tips and trim so that you have about 2 mm of the very tip.
2. Soak the tips in a few drops of 1 M HCl in a watch glass for about 10 min.
3. Transfer the tips to a few drops of acetocarmine stain for about 15 min.
4. Transfer one tip to a few drops of water on a slide.
5. Lower a cover slip onto the tip and press it down gently so that the tip squashes out to about 7 mm (1/3 inch) around.
6. Look for mitoses under high power (X40). Let me know if you strike it lucky!

(If this fails for any reason try your other root tip)

WARNING: Acetocarmine is corrosive and stains skin and clothes. It does not come out. Take care in handling it.