80 EXPERIMF.NT STATION. [Jan. 



detected by removing small portions of the outer bark on the 

 sunny side of the tree, the sun's heat causing a slight volatiliza- 

 tion and perceptible odor. 



Gas oil, a heavy oil used in the manufacture of water gas, 

 is very injurious to trees when used as a spray. A few years 

 ago several hundred shade; trees were severely injured in one of 

 our eastern cities by spraying the trunks with this oil to kill 

 clusters of gypsy moth eggs, it being used without any knowl- 

 edge of its adaptability to this purpose (Fig. 9). The oil 

 quickly soaked into the bark, cortical tissue and cambium, and 

 in some cases extended into the sapwood for one-half to three- 

 fourths of an inch. This injury occurred even on trees with 

 fairly thick bark, killing all the living tissue wherever the oil 

 was applied. While in some instances the trees did not show 

 extensive injury, in others the trunks w^ere 50 to 90 per cent, 

 girdled, and many of the trees died from complete girdling. 

 The most striking feature of this case was the ability of the 

 trees to produce perfect foliage even after serious injury had 

 been incurred. One tree w^as examined whose trunk was girdled 

 for a height of 15 to 20 feet, but this tree persisted in produc- 

 ing foliage for two years after the bark had fallen off. An 

 explanation of this remarkable case consists in the fact that the 

 heavy oil soaking into the sapwood prevented it from checking 

 or cracking, therefore the supply of water from the roots was 

 uninterrupted. The trees treated were elms, different species 

 of maples, etc. The presence of oil in the sapwood in the cases 

 cited above Avas of the greatest aid in preventing cracking and 

 in helping to maintain the transpiration current and a full crop 

 of foliage, and this bears out the recommendation that tree 

 wounds should be painted or treated in some way very soon 

 after they are formed to prevent cracking. It is sometimes 

 necessary to scrape the wound before applying the paint. 



Ordinary house paint, although a crud(> enough treatment, 

 has sometimes been used by ignorant persons on smooth-bark 

 trees, with of course resultant injury. 



Occasionally commercial oils used for sju-aying fruit trees 

 for the San Jose scale cause local injury, and some shade trees 

 have been known to be affected by their use. 



Oils and other materials to keep down the dust in roadbeds 



