1. The Onion Root Tip As a Model for Growth

2. Click on the red spots to find out what is happening in each zone.
ZONE D
ZONE C
ZONE B
ZONE A
l.s. root tip
Root Cap

3. Zone of Cell Division Information: Note New cells are
CUBOIDAL in shape
Have thin cellulose cell walls
Have dense cytoplasm with no vacuole
Information:
New cells are produced by MITOSIS

4. Zone of Expansion
Information:
Cells in this region have
Thin cellulose cell walls (primary)
Small vacuoles
cytoplasm with low  (water potential)
Information:
 Of surrounding solution is HIGH so water moves into vacuole of cell
As a consequence of water movement the volume of the cells increases and pushes out the thin cell walls

5. Zone of elongation
Information:
Cells in this region are typified by
Large vacuoles
Column shape
Thin (primary)cell walls
Information:
The arrangement of cellulose fibres in the cell wall results in the expansion of cells only in a lengthways direction
Water continues to enter the cells down a water potential gradient – as a consequence the cells elongate

6. Zone of Differentiation
As the cells mature, it lays down more cellulose inside to form a secondary cell wall. This prevents further growth but allows for DIFFERENTIATION e.g.
The end cell walls may become perforated as the cells develop into PHLOEM SIEVE TUBE ELEMENTS
If the secondary cell wall contains LIGNIN, then the cells will develop into XYLEM or SCLERENCHYMA

7. A simple experiment – A bean root can be marked at 1mm intervals
A simple experiment – A bean root can be marked at 1mm intervals. The bean can be left to grow and the root observed every 12 hours.
Note – the region of greatest elongation is just behind the tip