Variation: Instead of using one cylinder with water and one with food dye, use two
different colour food dyes (e.g. blue and red). At first the flower will showtwo separate colours, but as time goes by the whole flower will show both dyes.
This is because water can move sideways between xylem vessels through openingsalong their length. The ability of water to move sideways between vessels is
useful for when air becomes trapped in a vessel, causing a blockage. If you cutthe stem right up to the base of the flower, this will limit movement between
the xylem vessels.
Variation: Try using celery stalks with leaves. Cut open the celery stalk (cross-section)
and you will see that the little holes inside are coloured – these arethe vessels.
An example of this experiment with photographs can be found at:
http://www.practicalbiology.org/areas/intermediate/cells-to-systems/transport-
in-plants/investigating-transport-systems-in-a-flowering-plant,70,EXP.html
Secondary growth
Meristematic tissue
- Meristematic tissue consists of small cells that are unspecialized. These cells
divide by mitosis to form new cells that can differentiate (undergo changes intheir structure) and can become specialized tissue (e.g. xylem, phloem,
epidermal cells)
- Primary meristematic tissue is found in the tips of roots, stems and buds. When
it divides new cells are produced which causes the plant to grow longer. Thisis referred to as primary growth.
- Secondary meristematic tissue originates from permanent tissue, usually
parenchyma tissue which divides by mitosis. Cambium is secondary meristematictissue that is found in roots and stems. When these cells divide by mitosis it
results in the plant becoming wider. This is called secondary growth.
- Every growing season the stem of a plant increases in width – this is
known as secondary thickening.
- Towards the end of the first year of growth, the parenchyma cells between the
vascular bundles become meristematic and link up with the cambium tissue toform a cambium ring.
- The cells in the cambium ring start dividing to form secondary phloem (on the
outside) and secondary xylem (on the inside).
- Each year another ring of secondary phloem and secondary xylem is formed, making
the stem grow wider.
- It is not possible to see the layers of secondary phloem but the secondary xylem
are visible. These form rings called annular rings which can be used to workout the age of a plant.
- As new rings are formed each year, the older rings are pushed inward and the
xylem vessels collapse due to the pressure. The wood in the centre becomesdenser and harder than the wood at the surface and is called heartwood.
- The youngest annual rings found on the outside serve its function of
transporting water. This wood is not as dense and is called sapwood.
- The light-coloured rings are called spring wood. They are formed during spring
and summer when the growing conditions are favourable. The rings are thereforerelatively thick and light in colour as the xylem cell walls are thin.
- The dark-coloured rings are called autumn wood. They are formed during autumn
and winter when the growing conditions are unfavourable. The rings aretherefore relatively thin and dark in colour as the xylem cell walls are thick.
