212 MASS. EXPERIMENT STATION BULLETIN 170. 



this trouble, as shown by the fact that the white cedar, arbor Aatse, etc., 

 accustomed to wet situations, are quite susceptible to sun scorch when 

 grown out of their natural habitat. There is also reason to believe that 

 in some cases a peculiar chemical condition of the soil or some variation 

 in the root absorptive capacitj^, which limits absorption, is at the root of 

 the trouble. 



We have been observing for a long time maple trees which sun scorch 

 badlj'' each year. During especially severe droughts every leaf is affected, 

 wliile trees located near by are not in the least injured. The leaves of 

 trees suffering from sun scorch do not usually fall off, but remain alive, 

 although discolored. It is impossible for them to perform their full func- 

 tions, but little injury results to the tree. 



Rhododendrons, arbor vitses and other conifers often burn in the spring 

 before the frost is out of the ground, when strong, warm, dry winds occur. 

 When they are mulched the frost remains in the ground longer than it 

 otherwise would, and the winds cause more transpiration of water than 

 the roots can suppl3\ Manj^ rhododendrons meet their fate in tliis way, 

 their death usually being attributed to winterkilhng. Tliis can be pre- 

 vented by removing the mulching early in the spring and allowing the 

 sun to thaw out the frost. 



"Bronzing" of foliage is merely another form of sun scorch common in 

 very dry, hot periods. It is not caused by wind, and there is no laceration 

 of the foliage. Examination shows that the cells near the veins and vein- 

 lets of the leaves are alive, but those farthest away are dead. This bronz- 

 ing is caused by a lack of water supply to the cells of the leaves located 

 most remotely from the veins or source of water supply. Like sun scorch, 

 it is associated with excessive transpiration and diminished root absorp- 

 tion. The leaves become a reddish brown or bronze color, the dead tissue 

 giving them this peculiar hue. It is most commonly met with on the rock 

 maple, though other trees sometimes show the same trouble. 



Mechanical Injuries. 



Although trees possess quite a remarkable power of growth, by means 

 of which they are able under certain conditions to overcome apparently 

 insurmountable obstacles, they do not always make use of this power. 

 When roots and other organs are restricted in some way in their growth, 

 they often lift objects weighing many tons, but when there is opportunity 

 for active tissues to flow around the object, as it were, this more practical 

 and easier method is used. Every type of injury to a tree acts as a stim- 

 ulus, hence there usually follows an accelerated growth of the tissues 

 around the wounds, which often produces disfigurement. 



Under the heading "mechanical injuries" may be described many 

 injuries arising from various causes. The injuries due to wires have 

 been treated in Bulletin No. 156. In cities and towns perhaps one of 

 the most common injuries to be seen on roadside trees is that caused by 

 horses' teeth. Trees located between the sidewalk and the road are 



