12 MACDOUGAL— GROWTH IN TREES. 



enlargement of the base of the trunk. Activation of the growing 

 cells may be taken to depend upon the localized food-supply, tem- 

 perature, moisture or other factors. 



The fact that growth depends upon physical conditions largely 

 external instead of being a manifestation of a rhythm on the part 

 of the tree is well evidenced by tests in which trees which had 

 ceased to grow with the seasonal drying out of the soil were awak- 

 ened by a renewed water supply. 



Irrigation of the soil which had a moisture content of less than 

 6 per cent, around the roots of a Monterey pine was followed by 

 progressive enlargement constituting growth at the base of the tree, 

 and at a point 8 meters higher within 24 hours. The distance from 

 the absorbing surfaces of the roots through which the added water 

 supply must enter could not be less than 3 meters from the lower 

 instrument and the influence of the added supply was felt at the 

 upper instrument 11 meters from the absorbing surfaces within the 

 day. It does not seem possible that water could have been conducted 

 through the tracheids this distance within the given length of time. 



An irrigation test similar to the above was made with a small 

 California live oak {Quercus agrifolia). The results were even 

 more startling than those described for the pine. Within two hours 

 the dendrograph which had its contacts with the tree at least 3 

 meters from the absorbing surfaces showed some enlargement, an 

 action which may be directly connected with the fact that the vessels 

 in this oak are numerous and large. 



The irrigation experiments might be held to simulate the effects 

 of stream overflow, which if due to melting snows would not be ac- 

 companied by any marked higher humidity. It is seen to result in 

 the formation of a tapering shell of wood which was as thick as the 

 seasonal formation at the base of the trunk, but which had but half 

 this thickness 8 meters higher up on the trunk. The layer of normal 

 formation was of practically identical thickness at the two places. 



One of the earliest series of measurements which would enable 

 the forester to follow the seasonal variations of trunks was that 

 made by C. E. Hall at San Jorge, Uruguay, in 1 885-1 890, by com- 

 monly used methods of calibration.^ Friedrich perfected a device for 



2 Hall, C. E., " Notes on Tree Measurements ]Made ]\Ionthly at San 



