MACDOUGAL— GROWTH IN TREES. 



9 



the minimum at a time after noon, dependent upon external agen- 

 cies. These variations appear to depend upon the water-balance in 

 the woody cylinder, are greatest in the seasons in which water-loss 



Fig. 3. A, dendrographic record of variations in diameter of Monterey 

 pine tree i meter from the ground for week beginning April 5, 1920. B, 

 record from instrument attached to the trunk 9 meters from the ground. 

 Daily equalizing variations with actual enlargement' beginning mid-week 

 X 8, on a scale of 10 mm. intervals. 



from the crown is greatest, are least in the cooler or damper sea- 

 sons, and are to be detected in the records even in the period of 

 most rapid enlargement of the trunk (Fig. 4). 



Fig. 4. Auxographic record of elongation of terminal internode of 

 young pine tree showing cessation of growth and shrinkage during the mid- 

 day period, coincident with the decrease in diameter of trunks of larger 

 trees, X 3, on a scale of 10 mm. intervals. 



The trunk of a tree may in fact be compared to the supply hose 

 of a fire engine coupled to a hydrant. When the pressure from the 

 mains is enough to supply water faster than it can be pumped out 

 the hose is distended. When the engine tends to take water faster 

 than it would be delivered by the system, the hose would tend to 



