Everything Life Sciences Grade 10

primary phloem

secondary phloem (1st year)

secondary xylem (2nd year)

primary xylem

cork cambium

cork

Figure 6.13: Process of secondary thickening in stems.

It is not possible to see the layers of secondary phloem but the secondary xylem layers

are visible. These form rings calledannualorannular ringswhich can be used to work

out 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 becomes

denser and harder than the wood at the surface and is calledheartwood. The youngest

annual rings found on the outside transport 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. These rings are therefore relatively

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 are therefore relatively thin and dark in colour as the xylem cell

walls are thick. Rings may be thicker in some years than in others, depending on the amount

of rain and nutrients received.

Figure 6.14: The annular rings in a tree trunk give a measure of the tree’s age as well as the environ-

mental conditions of the tree’s growth.

168 6.2. Anatomy of dicotyledonous plants