322 



Bulletin 256. 





(8.) Smoke and gas from factories. 



The presence of smoke and atmospheric gases often causes the 

 death or the stunted condition of trees in the streets and parks 



of our cities. The functions of 

 the leaves are retarded in two 

 ways: first, the breathing-pores 

 or stomata become choked with 

 the soot ; and second, many- 

 gases in themselves may be 

 poisonous, even when diluted 

 with the atmosphere. 



The list of trees that are more 

 or less immune to the effects of 



Fig. 242. — A grating generally covers the 

 "well," and also prevents the area around 

 the base of city street trees from becoming 

 impenetrable to surface water. 



smoke and gas is very small, and therefore the range of selection is 



narrow. Trees should be selected on the basis of results secured with 



similar species in other cities and towns under similar conditions. The 



presence of injurious elements often depends on the 



direction of the prevailing winds. Trees in and around 



Liverpool show the damaging influences of gases 



which are carried a distance of fifteen to twenty 



miles by the prevailing north-west winds. On the 



other hand, in the vicinity of London these injuries 



are not so marked. Corporations having factory 



interests should be urged to control the production of 



gases, and the municipal authorities in such places 



should permit no trees to be planted that are not 



known to be capable of withstanding a certain amount 



of poisonous vapors and choking of the stomata. 



(p.) Overcrowding and improper placing. 



Two kinds of injury result from overcrowding and 

 improper placing of street trees, — the presence of large 

 quantities of dead wood, and the presence of de- 

 formed and sickly specimens. The evil begins with 

 the improper spacing of the young trees when first 

 planted, and it continues to increase in proportion to 

 the length of time during which such trees are per- 

 mitted to remain. The greatest evil attending the 

 close planting of young trees in avenues is the neces- 

 sity of their removal at a subsequent date, when only a person possessing 

 the courage of his convictions will take out the trees that should be 



Fig. 243. — Bridge- 

 grafting, for the 

 pr e s er vation 

 of girdled trees. 



