GROWTH IN TREES. 7 



12. Estimates of the range of daily equalizing variations in a Mon- 

 terey pine taken from bearings on a thin layer of cork external to the 

 bast of a trunk which had ceased to grow for the season show that the 

 diameter might vary 1 part in 1,750. That a large share of this varia- 

 tion is due to changes in the hydration of the living cells is proved by 

 the fact that when bearings are taken on the woody cylinder of the 

 trunk internal to the growing layer the variation drops to 1 part in 

 8,750 of the diameter. The actual change in volume, in the first 

 instance calculated on the basis of a conical trunk 18 meters high and 

 35 cm. in diameter at the base, would amount to about 400 cu. cm., 

 of which not more than one-fifth, or 80 cu. cm., is attributable to varia- 

 tions in the wood. It is to be noted, however, that the change in the 

 volume 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-like cells, 

 which may include a bubble of gas, the water forming no more than a 

 thin film on the wall of the cell and inclosing 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 ten- 

 sion 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. 



13. 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. 



14. The greatest daily equalizing variations were shown by Frax- 

 inus, 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. 



15. The final effect of rainfall shown within a few hours is to accel- 

 erate growth, but it has been repeatedly observed that actual shrink- 

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

 traced to any instrumental error. 



16. 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 the influence of the added supply was within the day 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. 



