46 CARNEGIE INSTITUTION OF WASHINGTON. 



fraction of the total of the water-loss. The trunk of a tree may 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 

 takes water faster than it can be delivered by the system, the hose 

 collapses. 



9. The greatest daily equalizing variations were shown by Fraxinus, 

 Pinus, Picea, Pseudotsuga, and Juglans, and lesser variations were 

 displayed by Populus, Platanus, Fagus, Quercus, and Citrus. No 

 available facts furnish the basis of an adequate explanation of such 

 differences. 



10. Of 15 trees which were under dendrographic measurement in 

 1920, one each of Pinus scopulorum, Citrus aurantica, and Parkinsonia 

 microphylla made no enlargement during the year. Such occurrences 

 are to be taken into account in estimations of the ages of trees from 

 the annual layers. Of the 12 trees under observation in 1921, only 

 one, a small Monterey pine, showed no enlargement under the dendro- 

 graph. 



11. The final effect of rainfall shown within a few hours is to acceler- 

 ate growth, but it has been repeatedly observed that actual shrinkage 

 may take place while the rain is falling. This action can not be 

 traced to any instrumental error. 



12. Irrigation of the soil which had a moisture-content of less than 

 6 per cent around the roots of a Monterey pine was followed within 24 

 hours by progressive enlargement, constituting growth at the base of 

 the tree and at a point 8 meters higher. 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 within the day the influence of the added supply was felt at the 

 upper instrument, 11 meters from the absorbing surfaces. It does 

 not seem possible that water could have been conducted through the 

 tracheids this distance within the given length of time. 



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

 California live oak (Quercus agrifoUa). The results were even more 

 startling than those described for the pine. Within 2 hours the den- 

 dfograph, 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. 



14. The irrigation experhnents might be held to simulate the effects 

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

 panied by any marked higher humidity. It was 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 only half this thick- 

 ness 8 meters higher up on the trunk. The layer of normal formation 

 was of practically identical thickness at the two places. 



