MACDOUGAL— GROWTH IN TREES. 11 



about 400 cu. cm. of which not more than one-fifth or 80 cu. cm. 

 is attributable to variations in the wood. It is to be noted however 

 that the change in the voUnne of the wood may by no means be taken 

 to represent the water deficit in the wood. The woody mass is made 

 up of box-hke cells, which may include a bubble of gas, the water 

 forming no more than a thin film on the wall of the cell and enclos- 

 ing the gas bubble in the condition of extreme water deficit. The 

 withdrawal of water through the walls of the cells which are semi- 

 rigid, increases the surface tension of the gas bubble, which results 

 in a slight lessening of volume of the whole mass, but in an amount 

 that would constitute no more than a small fraction of the total of 

 the water loss. 



Awakening and growth of the terminal buds with resultant elon- 

 gation of leaders and branches generally begins some time before 

 enlargement of the trunk takes place in many trees. The period 

 separating the two may be no more than a week in Quercus agri- 

 folia and has been seen to be as much as ten or twelve weeks in 

 Piiiits radiata. Observations on the Parry spruce and Douglas fir 

 show that the trunks of these trees are enlarging at a time when the 

 buds are in a very early stage of enlargement. 



In the single case in which dendrographs were attached to a 

 pine tree i meter and 8 meters above the ground growth began co- 

 incidently at the two places in 1920. In the following year, however, 

 the dendrograph at the higher point on the trunk recorded enlarge- 

 ment a few days before any action near the ground was made vis- 

 ible. In February, 1921, an auxograph was brought into bearing 

 on the internode of a pine tree five or six years old which had been 

 formed in 1919. The buds had made a growth of 4 or 5 cm. but no 

 action had yet begun in the internode. A second instrument was 

 brought into bearing on the middle of the internode formed in 1920 

 on another young tree. Steady enlargement was in progress. 



The embryonic layer of a tree is in the form of an enclosing 

 sheath terminating in the cones of the growing points. Activation 

 of this tract is generally initiated in the growing points. Swelling 

 in the cambium layer may be practically coincident with this awak- 

 ening in some trees. Cases are recorded in the following paper in 

 which weeks elapsed between the awakening of the buds and the 



