Care of the Aged 121 
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 I,000 years. 
Yet, as we have 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. While an old oak may be cut back 
