1913.] PUBLIC DOCUMENT — No. 31. 75 



iiution the wood of the slowly dying trees is very brittle, and the 

 limbs and branches fall oif easily. Old trees reaching the end 

 of their grand period of growth usually show the same charac- 

 teristics, and for this reason they, too, become dangerous. For 

 many years we have observed the falling of limbs and trees, and 

 have noticed that outside, of course, of tempests, ice storms, etc., 

 a large percentage fall when there is no wind and when the 

 atmosphere is unusually quiet, especially during October and 

 l^ovember after the leaves have fallen. The exact cause of this 

 we do not know, although it may be associated with some 

 phj'sical or chemical change in the tissues occurring after defo- 

 liation. It is known, however, that water, oils and various 

 chemical substances modify the physical properties of the tissue, 

 and the water contents of a tree at ditlerent seasons of the year 

 vary, as shown by Clark,^ from 25 to 70 per cent., and in 

 severe periods of drought the percentage of water may be very 

 low. That this variation may have some influence here is sug- 

 gested by the behavior of chemically treated paper containing 

 wood fiber. When wet its tensile strength is increased. 



Injury to Cork Cambium. 



A great deal of injury to the roots and other parts of trees 

 occurred from the severe cold of the winter of 1903-04. The 

 death of tissue from winter killing is induced by a variety of 

 causes and conditions, and we shall not attempt to discuss them 

 here except to state that any plant, whether indigenous or intro- 

 duced, is susceptible to winter injury if the conditions are 

 favorable. But it is not always the case that all the living 

 tissues are injured, since winter killing is often local, and even 

 a single tissue may be aifected. Following this unusually cold 

 winter many trees died outright, and we found many elms and 

 some sycamores which had lost their outer bark, apparently 

 because of injury to the cork cambium. Sometimes the bark, 

 cortex and phloem were all injured, but in most cases noticed 

 only the outer bark was involved (Fig, 7). This type of in- 

 jury would not in most cases be noticeable at first, but gradu- 

 ally the dead outer bark which had remained on the tree for 



> Twenty-second Ann. Rept. Sec'y Mass. State Bd. of Agr,, 1874, p. 288. 



