154 



from the present methods of street paving which fill the spaces between the 

 stones with a binding material, and even at times the asphalt covers the soil 

 entirely. The injury to the trees is two-fold; on the one hand, the air is 

 cut ofif, on the other hand watering is insuflicient. This affects the older 

 trees principally. For young trees, the circle of sod around the tree is 

 sufficient, especially if an iron grating laid over it prevents passers-by from 

 tramping the soil. We find that old trees die much more quickly when a 

 regulation of sidewalks and a lowering of the ground w^ater level is com- 

 bined with street paving. In large cities another factor must be added, i. e., 

 laying pipes for gas, electricity and sewers. In all this work, the choi)ping 

 off of the larger root branches is unavoidable. 



Therefore, the root space is not only limited by the pipes, and the soil 

 dried, but also the trees' organs for the absorption of water are decreased. 

 To this cause may be ascribed the gradual break up of old trees as shown 

 by the dying branch tips. 



Different varieties of trees suiTer in \arying degrees and the linden, 

 a favorite and most frequently planted species, is among the most sensitive 

 of varieties. In it the dryness of the soil, with which is associated also 

 dryness of the air, is expressed by a premature defoliation. The large 

 leaved linden suffers more quickly than does the smaller leaved variety, and 

 it is a well known phenomenon, that, in the summer months, when the in- 

 habitants of the city want shade most, the linden and chestnuts often for 

 some time have leaves only on the outermost tips of the branches. The 

 older leaves, covered with red spider, have dried up and fallen. The city 

 adminstration endeavors to overcome this condition by abundantly watering 

 the ground about the tree thereby favoring a second leafing out in the late 

 summer, which is produced even without artificial watering when the trees 

 have lost their leaves prematurely. In this buds are forced to unfold 

 which should develop in the following year ; under such conditions a second 

 time of blossoming is also often produced (Aesculus, Robinia). 



Many of the shoots artificially produced by this watering do not 

 mature sufficiently and are injured by frost. Thus in different years, in the 

 middle of the favorable early summer, the twigs die ofif accompanied by 

 fungous infection. The winter, therefore, did not kill these less mature twigs, 

 but made them susceptible to fungous attack, thus giving the primary cause 

 for later death. This theory also explains the death of the cherry trees along 

 the Rhine, which has occupied the attention of investigators during the last 

 few years^ A Valsa (V. leucostoma) plays a part here as in the case of 

 the alders. We will return to this case in the chapter on injuries due to frost. 



Such bad conditions in street planting may be avoided by a choice of 

 less sensitive varieties. First of all, the elm should be recommended as 

 such ; this has the added advantage of being very resistent to the acid gases 

 of smoke. Also oaks and plane trees are used with advantage according 

 to the kind of soil present. In broad, airy streets Acer platanoides also 



Cf. Deutsche Landwirtschaftl. Presse 1899, Nos. 83, 86, and 1900, No. 18. 



