6 LINKS WITH THE PAST [CH. 



cylinder of wood composed of relatively wide con- 

 ducting tubes. After the first rush of life is succeeded 

 by a phase of more uniform and gentler activity, the 

 demand for water becomes less exacting and the 

 wood which is formed during the rest of the growing 

 season consists of narrower water-pipes. A period of 

 rest ensues, until in the following spring new layers 

 of larger tubes are laid down in juxtaposition to the 

 narrower elements of the latest phase of the preceding 

 summer. This alternation of larger and smaller 

 tubes produces the appearance of concentric rings on 

 a cross-section of a tree. It is not the pause in the 

 active life of the plant which is responsible for the 

 effect of rings, but the fact that the wood produced 

 immediately before and immediately after the pause 

 is not structurally identical. In trees grown under 

 the more uniform conditions of certain tropical 

 regions, the annual rings are either feebly developed or 

 absent ; for example, in some Indian oaks the wood 

 shows no concentric rings of growth. 



Stated in general terms, rings of growth in the 

 wood of a tree are the expression of a power possessed 

 by the plant of regulating the structure of its com- 

 ponent elements in response to the varying nature of 

 the external stimuli. In certain circumstances, for 

 example after the destruction of the young buds by 

 caterpillars, the tree makes a special effort to repair 

 the loss by producing a new set of shoots. This may 



