116 SHADE TREES. 



becomes abundant there is no successful method of control. 

 Diseased trees should be burned, but it is not safe to plant 

 young trees in the same soil. 



GAS. 

 Illuminating gas escaping from defective 



Gas injury. . ., ' . 



pipes impregnates the soil, poisons the roots 

 and causes the death of trees. The remedy for this is evi- 

 dent, but in replanting it is frequently necessary to remove 

 the soil from a considerable area and refill with a fresh 

 supply from outside sources. (See page 28.) 



4. DISEASES DUE TO ENVIRONMENT. 



Plants respond readily to their surroundings, and (in a 

 state of nature, undisturbed by man) the best growths will 

 always be found where the surroundings are most favorable. 

 The most important natural factors which influence the 

 growth of plants are soil, water and temperature. The soil 

 may be unfavorable to plant growth owing to the lack, or 

 improper proportions, of food substances ; or it may be too 

 shallow or may not hold the proper amount of water. The 

 water content of the soil is an important factor, varying 

 with the requirements of the various species of plants. The 

 amount of water that may be unfavorable for a tree will fre- 

 quently be favorable for the organisms of disease. Tem- 

 perature is also an important factor, both as causing direct 

 injuries which may retard the growth of trees or cause their 

 death, and by making it possible for fungi and other destruc- 

 tive organisms to gain entrance. 



Smolce, gases, etc., are also the causes of many injuries 

 and heavy losses. When trees stand close together, the ef- 

 fects of smoke and free gases are first noticed in the tops, 

 but in single trees the injuries may be distributed through- 

 out the crown. As in cases of poisoning by illuminating gas 

 (page 29), the first symptoms are discoloration of the young 



