ROOT DISEASES AND INJURIES 77 



Symptoms. 



The symptoms are general in nature, being about the same 

 as those produced when any agency causes a gradual death of 

 the root system. Partial freezing-to-death of the roots and 

 cumulative gas-injury cause similar symptoms. The turning 

 yellow or brown of the foliage is a common symptom, probably 

 due entirely to the interference with the conduction of the 

 necessary supply of water. Later, the living tissues in the root 

 and trunk will be found dry and turning brown, showing that 

 the lack of water and the poisonous properties of the gas com- 

 bined have killed the tissues. At this stage various branches 

 die and saprophytic fungi attack the dead bark. The length 

 of time which it takes for a tree to die from gas-poisoning de- 

 pends entirely on the amount of gas in the soil. Even a 

 small quantity of gas present continuously will produce serious 

 injury within two or three years. Conifers are much more 

 resistant to gas than deciduous trees. At times, the former 

 recover after the injury becomes apparent, while deciduous 

 trees which begin to develop the symptoms of poisoning rarely 

 recover, even if the leak is repaired. 



Cause. 



The injury to the roots caused by illuminating gas is probably 

 of two kinds : first, true asphyxiation, since the air necessary to 

 the roots is replaced by the gas ; and second, the living tissues 

 are poisoned. INIany toxic substances are contained in the dif- 

 ferent kinds of illuminating gases. These substances when 

 dissolved in the soil-water are absorbed into the root and the 

 cumulative effect is shown in the death of the cells. 



Control. 



The remedy for gas-injury is to stop the leakage and stir the 

 soil until all the gas has escaped. If only a portion of the root 

 system and trunk is killed, the tree may be saved by the use of 

 surgical methods to remove the dead tissue (see page 345). 



