224 MASS. EXPERIMENT STATION BULLETIN 170. 



also taken exhaustive notes on every symptom shown by trees dying 

 from various causes, and from these notes may be had many interesting 

 data on the relative importance of various symptoms. In diagnosing 

 gas injury one must learn to detect either by chemical means or from direct 

 observations and experience the presence of the poisonous constituents 

 of illuminating gas which are absorbed by the roots and circulate to a 

 certain extent through the tissues of the wood. 



As already intimated, no two species of trees suffering from gas poison- 

 ing present precisely the same symptoms, and there is much difference 

 existing in the same species, the location, season of the year and other 

 factors having an important modifying effect. Trees, for example, when 

 examined in the fall, will show slightly different symptoms from those 

 examined in the spring. This is also true of trees poisoned by gas from 

 different manufacturing plants, which varies considerably. The varia- 

 tion in the chemical constituents of the soil here and there may to a certain 

 extent account for the variations in the reaction of gas on the tissues, 

 but this factor is probably not very important, since these variations in 

 the soil are likely to be found even in a single town supplied with gas 

 from one source, and as a rule the symptoms of trees injured by gas from 

 a single manufacturing plant are alike. Tables giving the results of gas 

 analysis from various corporations, however, show that there is consider- 

 able difference in the composition of gas, and that gas from a single manu- 

 factory is likely to vary slightly from day to day, not only in the percent- 

 ages of carbon monoxide and hydrogen, but in the other products. 



The odor and color of the tissue should first be examined carefully when 

 diagnosing a gas-injured tree, although it is possible by the use of chemi- 

 cals to obtain reactions and to detect certain products in the tissue. 

 There are different odors associated with the wood of trees which die 

 from various causes, and it is therefore necessary to become familiar with 

 these before relying too closely on this factor. For instance, molds and 

 other micro-organisms found in the sap of trees dying from various causes 

 often cause decomposition with resultant odors. But there will always 

 be found in trees killed by gas peculiar characteristic odors difficult to 

 describe, and more easily recognized, at least above the ground, after a 

 tree has been dead for a few weeks or months. The odor is more prominent 

 in moist than in dry trees, and can be detected in the tissues of the bark 

 as well as of the wood. Sometimes this odoriferous wood is found deep in 

 the sapwood, and can be recognized in the stumps of trees freshly cut, 

 but in old stumps, where decay has set in, it is not always discernible. 

 In such cases some part of the root system, if dug up, is likely to give a 

 characteristic odor, except when the wood has become too dry and a more 

 or less advanced stage of decay has set in. Even the rate of disintegration 

 and the nature of the decay are often characteristic, and are of some 

 value in diagnosis. 



It should be remembered that the odors of different species of trees, 

 even when in normal condition, differ greatly; i.e., the natural odor of the 



