LETTER VII. 



63 



trees, and some kinds more than others, are subject to 

 certain influences from without, and to certain changes 

 from within, which unfaihngly entail, not the natural 

 decline and death, but the accidental destruction of 

 the far greater number of them, — and that before the 

 lapse of any very lengthened period. In Endo- 

 genous trees, for example, the trunk, incapable of 

 yielding sideways, has, after a time, its interior so 

 thoroughly filled up below with the roots of the plants 

 above, that the sap can no longer ascend ; whereupon 

 either the last-growing plant dies, or the bud of the 

 next in succession is not developed. A change some- 

 what similar, and attended with the like result, appears 

 to occur in old trees of the Exogenous kind. The 

 internal and more recently formed layers of bark are 

 prevented from yielding by the drying and hardening 

 of the older layers of bark without ; while the inner 

 and older wood loses its porosity, partly by the pres- 

 sure of the younger wood without, and partly from 

 deposits {crystalline?) of organic matter in its sub- 

 stance. And thus it happens that neither can the 

 roots of the growing plants readily find room to grow, 

 nor can the sap rise freely upwards. Again, in Endo- 

 gens, the trunk becomes at length so disproportionate 

 in height to the naturally narrow basis of sustentation 

 under ground, as to be easily blown down. And even 

 in Exogens, proportionally broad as this basis is, the 

 vast height and breadth of surface which they at 



