KNUDSON: CAMBIAL ACTIVITY 537 
seventy years of age and were growing in an old orchard. In both 
cases examination of sections from one-year-old, three-year-old; 
and five-year-old branches revealed the fact that by July 17 the 
last few layers of xylem cells were being formed, and sometimes 
between July 23 and July 30 the xylem and phloem formation was 
completed and the cells were completely differentiated. The 
cambium was then in its resting condition. Material was also 
taken from three-year-old and five-year-old branches of the pear. 
The tree was about twenty years of age and was growing in a yard. 
In each case xylem formation was completed by July 17. In- 
cidental observations made on the plum, peach, and cherry on three- 
and four-year-old branches indicated that xylem formation was 
complete by July 23 and the cambium was in a resting condition. 
These trees were bearing trees about twenty years old and were 
growing in a farmyard. 
In concluding, mention should be made of the fact that no 
evidence was obtained to indicate that phloem formation continued 
later than xylem formation. 
