Care of the Aged I2i 



covered with pavement, it permits free use of the sidewalk 

 up to within a foot of the tree trunk. Grills are regularly 

 used with the iron guards in Paris, Berlin, and London in 

 places where they are needed. Their use in many other 

 cities would greatly improve the general condition of street 

 trees." 



In later years the rapid multiplication of electric wire 

 lines has introduced new dangers. These, and the pro- 

 tective measures, have already been discussed in a preceding 

 chapter (page 71). 



Care of the Aged. As we have seen, there is no inherent 

 reason in tree nature why a tree need ever die of old age, 

 for all its living parts are annually renewed, and, indeed, 

 there are specimens of various species extant which count 

 their age by thousands of years. Some Dragon Trees on 

 the island of Teneriffe are estimated at over 5,000 years of 

 age, and some of our Big Trees in California are, more 

 certainly, over half that age; many historical trees have been 

 known for 500 to 1,000 years. 



Yet, as we hsive also seen, old trees experience difficulties 

 in carrying on their functions and in resisting the insidious 

 attacks of parasites. When their vigor is impaired by unfav- 

 orable changes in their environment, especially as regards 

 water-supply, the capacity of renewing lost parts and repair- 

 ing damage is more or less lost, with some species sooner than 

 others, so that we can speak of short-lived and long-lived 

 species. Some also respond more readily than others to 

 the pruning knife, especially when cutting in old wood; 

 those known as good sprouters (see page 26 and Chapter 

 VIII; belong to the first, the poor sprouters to the latter 

 class. This knowledge indicates that greater care in the 

 use of the pruning knife must be exercised in old age with 

 trees of this class. \\ hile an old oak may be cut back 



