222 MASS. EXPERIMENT STATION BULLETIN 170. 



gas principles has passed up through the roots and stems, the sapwood, 

 cambium and bark become abnormal. The first symptoms take the 

 form of a characteristic dryness of the cambium and other tissues outside 



the wood, this being the first indication of 

 the approaching deatli of the tissues. Later, 

 these tissues — ■ cambium, phloem and cortex 

 — turn brown, and disintegration follows. 

 The roots, which first absorb the poison, 

 are naturally the first to become abnormal, 

 but later, as absorption and translocation 

 proceed, the poisonous constituents may be 

 detected in the wood at the base of the tree. 

 It not infrequently happens that the tissue 

 here is dead, while that in the trunk a few 

 feet above is alive, but this condition does 

 not endure, the whole tree sooner or later 

 becoming affected. When the underlying 

 tissues die the tissue tensions are destroyed 

 and the bark changes color, gradually grow- 

 ing darker, and its physical properties become 

 greatly changed. Soon various species of 

 fungi, such as Polystictus, Schizophyllum and 

 others, find a foothold on the bark, and 

 borers and other insects attack the dead 

 tissue. Even bacteria and molds, like Peni- 

 cilliurn, become active and hasten the proc- 

 ess of disintegration. The smaller twigs 

 become dry and brittle, and the ends often 

 break off; the upper limbs usually lose their 

 bark first, but eventually the larger limbs 

 present the same appearance. Disintegration 

 may take place so rapidly that in one and a 

 lialf to three years the bark disappears and 

 most of the larger branches break off, and 

 soon nothing but a portion of the trunk and 

 a few stubs remains. 



It must be understood that many of the 

 symptoms mentioned above vaay also be 

 found in trees dying from other causes and 

 do not necessarily constitute reliable guides 

 FIG. 94. -Effects of illuminating ^^ ^j^g detection of gas injury. The tissue 



gas on elm tree one and one" ... ,ii, , r ^• 



i.aif years after leakage oc furnishes the best symptoms for diagnosis, 



curred. and the writer, who has for the past twenty 



years been examining hundreds of trees killed 



by gas, from the first found it necessary to make a thorough examination 



of the tissue to warrant any degree of accuracy in the diagnosis. He has 



