270 SUMMARY OF CURRENT RESEARCHES RELATING TO 



between the mature bark of the two species are occasioned by changes 

 which take place in the outer cortex (periderm). The cambium varies 

 bothin number of cell-layers (two to ten) 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 dimensious from the apex 

 downward, until that point is reached in the hole where the las! 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. 

 Phloem development continues until late in the autumn, much longer 

 than xylem development. Sieve-tubes in all stages of formation occur 

 between cambium and fully formed phloem. The seasonal growth of 

 phloem exhibits little or no compression as late as October 1. Subse- 

 quently contraction occurs, due to the extreme cold temperatures of 

 winter. All the seasonal growth of phloem is crushed with the exception 

 of the last six or eight transitional tracheids. Compression is greater 

 in the crown than below. The processes of primar thickening and 

 secondary thickening overlap, and both may be going on in closely 

 neighbouring spots in the tree at the same time. 



Growth in white pine is divisible into (a) growth without cell- 

 division, and (b) 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 p.c. The awakening of growth is due 

 apparently to the rise of soil-water with an accompanying increase of 

 temperature. 



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 new xylem elements 

 follows the same order as in P. rig ids, that is, it begins first in the bole 

 at some distance below the apical shoot, and spreads upward and down- 

 ward. As a result growth at the base of a tree may begin several weeks 

 later than in the crown. The awakening and rapidity of growth is 

 dependent on three factors : — moisture, available reserve food, and 

 temperature. The first two are at an optimum in the spring ; the 

 amount of growth therefore is directly proportional to prevailing tem- 

 peratures. The intensity of growth is a variable factor which changes 

 from day to day, and even within a single day. Two periodic optimums 

 of growth intensity occur, one during May and early June, the second 

 in July and August. These vary from time to time at a given height 

 in the tree and follow no definite law. The amount of growth at a 

 definite time and place in the tree is equal to the sum of the prevailing 

 growth intensities by the time each was in force. It is very irregular at 

 different heights in the tree, but the cambium tends to even up dis- 

 crepancies as the season progresses. The irregularities of growth are 

 manifested not only in the actual dimensions of the newly formed 

 tissues, but also in the xylem elements. Wide discrepancies may occur 

 in closely neighbouring trees ; in general, larger differences may be 

 expected the greater the disparity in age. Growth is first retarded in 

 the upper portions of the tree ; it may continue vigorously below for 

 some weeks longer. 



Xylem formation goes on very sluggishly in all parts of the tree (the 



