204 MASS. EXPERIMENT STATION BULLETIN 170. 



are not alike in all cases, they are easily distinguished from those 

 of troubles caused by other agents, such as gas poisoning, etc. In the 

 majority of cases trees showing this staghead effect, whether from drought 

 or winterkilling, die gradually, and even when their death is more or less 

 rapid there are few of the symptoms characteristic of gas poisoning. Trees 

 poisoned by gas usually die and disintegrate rapidly; also the diagnostic 

 features to be found in the tissues of trees killed by gas are entirely 

 different. 



Winter Injuries above Ground. 



There are numerous cases of injury occurring above ground from the 

 effects of winter, such as the dying back of California privet, various 

 _^ fruit trees and vines, our native alders, 



white birches, the terminal twigs of trees 

 like the horse-chestnut, Norway maple, 



%^ sycamore, Japanese maple, and a con- 



^^^ siderable variety of exotic trees and shrubs. 



^fc^ Some of the specific types of winter in- 



^^^H jury to trees will be best treated under 



the different names by which they are 



known. 



Winter injuries, like other types of in- 

 jury responsible for the production of dead 

 tissue, are usually followed by various 

 species of fungi, a common form being 

 Nectria cinnabarina, characterized by the 

 appearance on the bark of numerous cin- 

 namon-colored pustules, fruiting bodies 

 of the fungi. 



FIG. 76. Same as Fig. 75, en 

 larged, showing pustules. 



Frost Cracks. 



Frost cracks are often seen on many of our shade and fruit trees in 

 winter, and are particularly common to the elm and linden, although 

 occasionally seen on maples. They extend down the trunk for some dis- 

 tance on the sunny side of the tree, and are caused by severe changes in 

 temperature during the winter. Some of our forest trees are also subject 

 to frost cracks; e.g., the striped maple when planted in the open, but 

 never in the dense forest, its native habitat, showing that the trunks of 

 certain trees need to be shaded. Frost cracks open in winter and close 

 more or less in summer, although quite often they never succeed in entirely 

 healing over. In the spring they usually bleed profusely, giving forth a 

 sour, dingy-colored sap called "slime-flux," which shows under the micro- 

 scope various species of fungi, algse and yeast. 



The opening and closing of frost cracks vary with the temperature, 

 barometer and relative humidity, and so closely is this variation allied 

 with meteorological factors that the weather conditions can be deter- 



