FOREST SOCIETIES. 



533 



or is so slight as to be practically imperceptible. Here the tree 

 may stand for a long time, but it finally dies. Some have sug- 

 gested that certain natural forces inhibit or set a limit to growth 

 in height, as wind for example. But trees in protected valleys 

 do not go on increasing in height beyond the normal height 

 for the species. Certain trees like the mangrove or banyan, 



Fig. 4966. 



Banyan tree moved in one direction by trade wind. The older portion of the 

 tree is at the right. 



which spread by branches growing down into the ground, may 

 live indefinitely, but the older trunks finally die. The banyan 

 tree in windy exposures develops to the leeward, and in time 

 the old trunk may thus travel considerable distances. Trees 

 finally die of "old age." In nearly all organisms growth is at 

 first slow, then is accelerated, reaches a maximum, then the rate 

 of growth declines, and finally ceases, when life may be main- 

 tained for years with no perceptible increase in size, but a gradual 

 waste and lessening in energy and vitality, until finally death 

 ensues. This is in the later stages often accelerated by para- 

 sitic organisms which take advantage of the weakened constitu- 

 tion, if the tree does not in the mean time fall before the force 

 of the wind. Many trees live for several centuries. A few 



