64 



LETTERS ON TREES. 



length acquire, causes the wind to act on them to their 

 destruction, the older they become, at an advantage 

 infinitely greater than in their earher years. 



15. Further, all dead organic matter sooner or later 

 undergoes certain purely chemical changes, which lead 

 to its decay or decomposition, and end in its disap- 

 pearance ; and this process once begun goes on all the 

 more rapidly that the conditions favourable to it obtain. 

 I need scarcely remind you that, according to the 

 theory, the greater part of all trees consists of vege- 

 table tissue that has lost its vitality ; and it must be 

 obvious to you, that exposed as they are to the full 

 action of the weather, trees are naturally placed in 

 circumstances highly conducive to the decay of this 

 tissue. Endogens, indeed, are for the most part swept 

 away or otherwise destroyed before there is time for 

 the occurrence of such decay. But in Exogens, the 

 process is matter of daily observation, while its results 

 are familiar to every one. From this cause, the heart- 

 wood of all of them after a time disappears, leaving 

 the trunk hollow within, often reducing it to a mere 

 shell, and thus necessarily weakening the mechanical 

 support given by it to the growing parts above, as 

 well as its power of resistance. And it is obvious to 

 remark, that the older an Exogen becomes, its liability 

 to be uprooted or dismembered by any passing storm 

 of wind, increases in a double ratio, and from two dis- 

 tinct but concurring causes, — on the one hand (in sum- 



