A transverse section of the trunk of a tree shows concentric rings which are known as growth rings . How are these rings formed ? What are the significance of these rings ?

Explanation

(i) The activity of vascular cambium is under the control of many physiological and environmental factors.
(ii) In the spring season, cambium is very active and produces a large number of xylary elements having vessels/tracheids with wide lumen.
(iii) The wood formed during this season is called spring wood or early wood. The tracheary elements are faily thin walled.
(iv) In winter, the cambium is less active and forms fewer xylary elements that have narrow vessels/tracheids and this wood is called autumn wood or late wood.
(v) The tracheary elements are with narrow lumen, very thick walled.
(vi) the spring wood is lighter in colour and has a lower density whereas the autumn wood is darker and has a higher density.
(vii) The annual ring denotes the combination of early wood and late wood and the ring becomes evident ot out eye due to the high density of late wood.
(viii) Sometimes annual rings are called growth rings but it should be remembered all the growth rings are not annual .
(ix) Additional growth rings are developed within a year due to adverse natural calamities like drought, frost, defoliation, flood, mechanical injury and biotic factors during the middle of a growing season , which result in the formation of more than one annual ring. such rings are called pseudo-or false-annual rings.
(x) Each annual ring corresponds to one year's growth and on the basis of these rings, the age of a particular plant can easily be calculated.
(xi) The determination of the age of a tree by counting the annual rings is called dendrochronology.
Significance of growth ring :
(i) Age of wood can be calculated.
(ii) The quality of timber can be ascertained.
(iii) Radio-Carbon dating can be verified.
(iv) Past climate and archaeological dating can be made.
(v) Provides evidence in forensic investigation.