1915I BROWN— PJNUS STROBUS ' 237 



while conservative structures in many respects, exhibit much more 

 irregularity in annual ring formation than do stems. 



Summary 



1. The winter condition of the secondary cortex and cambium 

 of white pine is sinular to that of Pinus rigida. The marked differ- 

 ences which occur between the mature bark of white pine and 

 pitch pine are occasioned by changes which take place in the outer 

 cortex (periderm). 



2. The cambium varies both in number of cell layers (2-10) and 

 thickness in different parts of a tree. It is smallest in both these 

 respects in the twigs and young branches, and increases gradually 

 in dimensions from the apex downward, until that point is reached 

 in the bole where the last annual ring is the thickest. Thereafter, 

 the decrease in the diameter is not proportional to the falling off 

 in the diameter of the last formed ring. 



3. Phloem development continues until late in the autumn, 

 much longer than xylem development. Sieve tubes in all stages 

 of formation occur between cambium and fuUy formed phloem. 

 The seasonal growth of phloem exhibits little or no compression 

 as late as October first. Subsequently contraction occurs, due 

 to the extreme cold temperatures of winter. All the seasonal 

 growth of phloem is crushed with the exception of the last 6 or 

 8 transitional tracheids. Compression is greater in the crown 

 than below. 



4. The processes of primary thickening and secondary thicken- 

 ing overlap, and both may be going on in closely neighboring spots 

 in the tree at the same time. 



5. Growth in white pine is divisible into (a) growth without 

 cell division and (&) growth with cell division. The first begins as 

 early as March and the elements concerned (phloem) increase in 

 radial diameter from 50 to over 100 per cent. The awakening of 

 growth is due apparently to the rise of soil water with an accom- 

 panying increase in temperature. 



6. Growth by cell division begins during the last half of April. 

 At the start it is very rapid, and more elements are formed at the 

 inside of the cambium than at the outside. The formation of 



