262 OX THE CONSERVATION OF 



rapidly decompose them. " They afford soil and mechanical support 

 to the new cells. The new cells in their turn give hirth to other 

 cells, and in their turn die, and their offspring encloses them again 

 in their protective mantle. Thus the growing tree goes on and 

 stops, grows old and becomes young again, ends and begins, until it 

 has reached its highest ideal of form, and its longest term of exist- 

 ence." The difference between plant life and animal life is, that 

 the former exists and is promoted by the growth of additional cells, 

 while the latter is maintained by the means of substituted cells. 

 Thus a tree may be said to have within its trunk only one generation 

 of active living cells at one time ; year by year this one series of 

 active cells becomes dormant, and is replaced or rather is added to 

 by another series or generation, while there are as many generations 

 of dead or inert and inactive cells built up in the tissues of the wood 

 of the tree as it is old. Only the present year's growth may be 

 held to be active ; the rest is entirely composed of past generations 

 of heart- ivood, not literally dead, but inactive and passive, and 

 which would decay were it not for the protection from the weather 

 and alternations of climate which the living layers of tissue outside 

 afford. Year by year the sap adds new tissues to the structure, just 

 as the blood circulation in animal economy is requisite to repair and 

 replace the decayed waste tissues; and as in the animal world, in 

 the period of youth, the restorative process is more powerful, and 

 new tissues are then added to the old ; and in maturity, the relative 

 processes of reproduction and decay are equally balanced ; while in 

 old age the destructive or declining process outruns the restorative, — 

 so, in the vegetable kingdom, during the younger years of plant or 

 tree life, corresponding processes are at work, which tend, during 

 the various stages of growth, maturity, and decay, to similar results. 

 In attempting, therefore, to conserve old timber trees, and bear- 

 ing these vital principles in view, attention must be directed, in the 

 case of large trees still vigorous, though evincing incipient signs 

 of decay, to the prolongation of the growing period of the tree, 

 and efforts must be directed to stimulating the formation of addi- 

 tional cell tissue, and thus keeping the health of the tree in a pro- 

 Lvely vigorous condition. On the other hand, should the 

 subject of treatment be in a declining state, whether from acci- 

 dental or natural causes, efforts must be used to arrest the decline, 

 by stimulating the growth of young wood, branchlets, and leaves, 

 so as to aid in the elaboration of sap throughout the head of the 

 tree. 



