Biology Today — December 2017

• On the contrary, during unfavourable seasons or inactive period, there is less demand for sap transport as cambium is less
  active and consequently narrower elements are formed. In tropical areas where only one kind of season prevails i.e., long
  dry summers, distinct rings are not formed.


• The number of annual rings as well as the width of rings formed in successive years vary greatly in plants.

Fig.: Detailed structure of annual rings

Autumn wood

• Dark in colour


• Higher density


• Smaller and
  narrower vessels


• Abundant
  tracheids and
  fibres.

Spring wood

• Lighter in colour


• Lower in density


• Larger and wider
  vessels


• The wood formed during spring is called spring wood or early wood while that formed in winter is called winter or late wood.


• These two types of wood appear together as concentric ring known as annual ring in the transverse section of stem. A
  sharp contrast between late autumn wood and early spring wood makes the successive rings distinct even to naked eye.


• The transition from spring to autumn wood is gradual but the transition from autumn wood to spring wood of next year
  is sudden.
  Dendrochronology is the science of dating and analysing annual growth rings of trees.

Heartwood and Sapwood

Sapwood

• Also called alburnum.


• Outer light coloured part with living
  cells.


• Used for conduction of water and
  mineral salts from root to leaf.


• Less durable and susceptible to
  microbial (fungal) or insect attack.

Heartwood

• Also called duramen.


• Inner, dark coloured with dead cells.


• Non-functional as vessels become plugged with tyloses, the parenchymatous ingrowths. Hence, the
  function is mechanical support only.


• Strong and durable as dead cells are lignified and filled with antiseptic extractives as oils, resins, gums,
  tannins, aromatic substances, etc. Therefore, resistant to microbial attack.

Stem Wood

In older trees, the wood of stem gets differentiated into two zones : heartwood and sapwood.

Tyloses

Fig.: Vessels with tyloses

Tangential

surface

Transverse

surface

Radial

Sapwood surface

Heartwood

Growth rings

Inner bark

Outer bark

Pith

Fig.: T.S. of a tree trunk