Wood: Rings, Sap, Heart, and Tissues

Wood is secondary xylem produced by the activity of vascular cambium in dicot stem, consisting largely of dead cells involved in the transport of water and minerals as well as support.

Annual rings (A secret to know the age of tree)

The early wood gradually merges with the late wood of the season. But there exists a sharp line of demarcation between the late wood of one season and the early wood of the next season. That explains the formation of annual rings, each of which consists of two parts – inner early wood and outer late wood.

The wood formed during spring is called early wood (or spring wood). In summer, cambium is less active and forms narrow vessels; this wood is called late wood (or summer wood).

The spring wood is lighter in colour and exhibits low density whereas the summer wood is darker and has higher density.

One light and one dark coloured zone comprise one year’s growth and this is known as the annual ring or growth ring. The age of a plant can be approximately determined by counting the number of annual rings.

Science dealing with predicting the age of a tree by counting the annual rings is called as Dendrochronology.

Sap Wood and Heart Wood

Xylem elements are sap wood or alburnum. This is of light colour and contains some living cells also in the association of vessels and fibres. This part of the stem performs the physiological activities such as conduction of water and nutrients, storage and food etc. Sap wood which is functional and consists of recently formed secondary xylem.

The central region of the old trees, which was formed earlier, is filled up with tannins, resins, gums and other substances which make it hard and durable is called heart wood or duramen. It looks black due to the presence of various substances in it. The function of heart wood is gives only mechanical support to the stem. As the plant ages in the central part of stem, the inner cells of sap wood becomes non-functional and dark in colour.

Differences between wood and heart wood

Table showing difference between sapwood and heartwood.

Sap wood

Heart wood

It is the outer wood of an old stem

It is the central wood of an old stem

Light coloured

Dark coloured due to presence of gums, resins, oils, tannin etc.

It is also called as alburnum

It is also called as duramen

It is soft and not durable

It is hard and durable

Contains living cells

Living cells are absent

Wood is lighter in weight

Heavier in weight

Less durable because of susceptibility to attack of pathogens

More durable, resistant to attack of the pathogens

Commercially less valuable

Commercially more valuable

Vessels are not blocked by tyloses

Vessels are blocked by tyloses with various deposits

The function of this region is conduction of water and nutrients and storage of wood

The function of this region is mechanical support

The amount of sapwood remains almost constant

Amount of heartwood increases as the tree grows older

Mechanical tissues in stem

The stem of a tall tree needs to resist against pulling forces of wind and to stand erect against gravity. Stem gets this strength from Sclerenchyma in hypodermis and its patches in the pericycle and secondary phloem, abundant lignified vessels, tracheids and fibres in secondary xylem i.e. wood and sclereids in pith.

Explore NIOS Notes